Non-destructive Micro Analysis of Cultural Heritage Materials, Volume 42 (Comprehensive Analytical Chemistry)

COMPREHENSIVE ANALYTICAL CHEMISTRY ELSEVIER B.V. Sara Burgerhartstraat 25 P.O. Box 211, 1000 AE Amsterdam, The Nether...

Author: K. Janssens | R. Van Grieken

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COMPREHENSIVE ANALYTICAL CHEMISTRY

ELSEVIER B.V. Sara Burgerhartstraat 25 P.O. Box 211, 1000 AE Amsterdam, The Netherlands

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q 2004 Elsevier B.V. All rights reserved. This work is protected under copyright by Elsevier B.V., and the following terms and conditions apply to its use: Photocopying Single photocopies of single chapters may be made for personal use as allowed by national copyright laws. Permission of the Publisher and payment of a fee is required for all other photocopying, including multiple or systematic copying, copying for advertising or promotional purposes, resale, and all forms of document delivery. Special rates are available for educational institutions that wish to make photocopies for non-profit educational classroom use. Permissions may be sought directly from Elsevier’s Rights Department in Oxford, UK; phone: (+44) 1865 843830, fax: (+44) 1865 853333, e-mail: [email protected]. Requests may also be completed on-line via the Elsevier homepage (http://www. elsevier.com/locate/permissions). In the USA, users may clear permissions and make payments through the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, USA; phone: (+1) (978) 7508400, fax: (+1) (978) 7504744, and in the UK through the Copyright Licensing Agency Rapid Clearance Service (CLARCS), 90 Tottenham Court Road, London W1P 0LP, UK; phone: (+44) 20 7631 5555; fax: (+44) 20 7631 5500. Other countries may have a local reprographic rights agency for payments. Derivative Works Tables of contents may be reproduced for internal circulation, but permission of the Publisher is required for external resale or distribution of such material. Permission of the Publisher is required for all other derivative works, including compilations and translations. Electronic Storage or Usage Permission of the Publisher is required to store or use electronically any material contained in this work, including any chapter or part of a chapter. Except as outlined above, no part of this work may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without prior written permission of the Publisher. Address permissions requests to: Elsevier’s Rights Department, at the fax and e-mail addresses noted above. Notice No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Because of rapid advances in the medical sciences, in particular, independent verification of diagnoses and drug dosages should be made.

First edition 2004 Library of Congress Cataloging in Publication Data A catalog record is available from the Library of Congress. British Library Cataloguing in Publication Data A catalogue record is available from the British Library. ISBN: 0-444-50738-8 ISSN: 0166-526X 1 The paper used in this publication meets the requirements of ANSI/NISO Z39.48-1992 (Permanence of Paper). W Printed in The Netherlands.

COMPREHENSIVE ANALYTICAL CHEMISTRY ADVISORY BOARD

Professor A.M. Bond Monash University, Clayton, Victoria, Australia Dr T.W. Collette US Environmental Protection Agency, Athens, GA, U.S.A. Professor M. Grasserbauer Director of the Environment Institute, European Commission, Joint Research Centre, Ispra, Italy Professor M.-C. Hennion Ecole Supe´rieure de Physique et de Chimie Industrielles, Paris, France Professor G. M. Hieftje Indiana University, Bloomington, IN, U.S.A. Professor G. Marko-Varga AstraZeneca, Lund, Sweden Professor D.L. Massart Vrije Universiteit, Brussels, Belgium Professor M.E. Meyerhoff University of Michigan, Ann Arbor, MI, U.S.A.

Wilson & Wilson’s COMPREHENSIVE ANALYTICAL CHEMISTRY

Edited by ´ D. BARCELO Research Professor Department of Environmental Chemistry IIQAB-CSIC Jordi Girona 18-26 08034 Barcelona Spain

Wilson & Wilson’s COMPREHENSIVE ANALYTICAL CHEMISTRY

VOLUME XLII

NON-DESTRUCTIVE MICROANALYSIS OF CULTURAL HERITAGE MATERIALS Edited by K. JANSSENS R. VAN GRIEKEN University of Antwerp Department of Chemistry Universiteitsplein, 1 B-2610 Antwerp Belgium

2004

ELSEVIER AMSTERDAM – BOSTON – HEIDELBERG – LONDON – NEW YORK – OXFORD – PARIS SAN DIEGO – SAN FRANCISCO – SINGAPORE – SYDNEY – TOKYO

CONTRIBUTORS TO VOLUME XLII Annemie Adriaens Department of Chemistry, Ghent University, Krijgslaan 281 S12, B-9000 Gent, Belgium [email protected] Michael Alram Coin Cabinet, Kunsthistorisches Museum Vienna, Burgring 5, A-1010 Vienna, Austria [email protected] Marc Aucouturier C2RMF CNRS, Centre de recherche et restauration des museés de France, 6 rue de Pyramides, 75041 Paris cedex 01, France [email protected] Dimitrios Bikiaris “ORMYLIA” Art Diagnosis Centre, Sacred Convent of the Annunciation, 63071 Ormylia, Greece [email protected] Ewa Bulska Department of Chemistry, University of Warsaw, Pasteura 1, PL-02-083 Warsawa, Poland [email protected] Thomas Calligaro C2RMF CNRS, Centre de recherche et restauration des museés de France, 6 rue de Pyramides, 75041 Paris cedex 01, France [email protected] Yannis Chryssoulakis National Technical University of Athens, Athens, Greece and “ORMYLIA” Art Diagnosis Centre, Sacred Convent of the Annunciation, 63071 Ormylia, Greece [email protected] Sister Daniilia “ORMYLIA” Art Diagnosis Centre, Sacred Convent of the Annunciation, 63071 Ormylia, Greece [email protected] Evelyne Darque-Ceretti Ecole des Mines de Paris, CEMEF rue C. Daunesse, BP 207, F = 06904 Sophia-Antipolis cedex, France [email protected]

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Contributors to volume XLII

Dalva L.A. de Faria Laboratoria de Espectroscopia Molecular, Instituto de Quimica da USP-University of Sao Paulo, Av. Prof. Lineu Prestes, 784 05508-900, Sao Paulo, Brazil [email protected] Guy Demortier Laboratoire de Reáctions Nucleaires, Faculte´s Universitaires Notre-Dame de la Paix, 61 rue de Bruxelles, B-5000 Namur, Belgium [email protected] Marc Dowsett Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom [email protected] Jean-Claude Dran C2RMF CNRS, Centre de recherche et restauration des museés de France, 6 rue de Pyramides, 75041 Paris cedex 01, France [email protected] Howell G.M. Edwards Department of Chemical and Forensic Sciences, University of Bradford, Richmond Road, Bradford BD7 1DP, West Yorkshire, United Kingdom h.g.m.edwards@bradford:ac.uk Bernard Gratuze IRAMAT CNRS, Centre Ernest Babelon, 3D rue de la Fe´rollerie, F-45071 Orleáns cedex, France [email protected] Rene´ Van Grieken Centre for Micro- and Trace analysis, Department of Chemistry, University of Antwerp, B-2610 Antwerp, Belgium [email protected] Annick Hubin Department of Metallurgy, Electrochemistry and Materials Science, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussel, Belgium [email protected] Koen Janssens Centre for Micro- and Trace analysis, Department of Chemistry, University of Antwerp, B-2610 Antwerp, Belgium [email protected]

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Teresa E. Jeffries ICP-MS Facility, Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 4BD, United Kingdom [email protected] Georgios Karagiannis “ORMYLIA” Art Diagnosis Centre, Sacred Convent of the Annunciation, 63071 Ormylia, Greece [email protected] Robert Linke Institute of Science and Technologies in Art, Academy of Fine Arts Vienna, Schillerplatz 3, A-1010 Vienna, Austria [email protected] Franz Mairinger Academy of Fine Arts, Dapontegasse 12/16, A-1030 Vienna, Austria [email protected] Luc Moens Laboratory of Analytical Chemistry, Ghent University, Proeftuinstraat 86, B-9000 Ghent, Belgium [email protected] Joseph Salomon C2RMF CNRS, Centre de recherche et restauration des museés de France, 6 rue de Pyramides, 75041 Paris cedex 01, France [email protected] Christos Salpistis “ORMYLIA” Art Diagnosis Centre, Sacred Convent of the Annunciation, 63071 Ormylia, Greece [email protected] Manfred Schreiner Institute of Science and Technologies in Art, Academy of Fine Arts Vienna, Schillerplatz 3, A-1010 Vienna, Austria [email protected] David Scott GCI Museum Research Laboratory, Getty Conservation Institute, 1200 Getty Center Drive Suite 700, Los Angeles CA 90049-1864, United States [email protected]

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Sophia Sotiropoulou “ORMYLIA” Art Diagnosis Centre, Sacred Convent of the Annunciation, 63071 Ormylia, Greece [email protected] Herman Terryn Department of Metallurgy, Electrochemistry and Materials Science, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussel, Belgium [email protected] Peter Vandenabeele Laboratory of Analytical Chemistry, Ghent University, Proeftuinstraat 86, B-9000 Ghent, Belgium [email protected] Barbara Wagner Department of Chemistry, University of Warsaw, Pasteura 1, PL-02-083 Warsawa, Poland [email protected] Heinz Winter Coin Cabinet, Kunsthistorisches Museum Vienna, Brugring 5, A-1010 Vienna, Austria

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WILSON AND WILSON’S

COMPREHENSIVE ANALYTICAL CHEMISTRY VOLUMES IN THE SERIES Vol. IA

Vol. IB Vol. IC Vol. IIA

Vol. IIB

Vol. IIC

Vol. IID Vol. III

Vol. IV

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Vol. X

Analytical Processes Gas Analysis Inorganic Qualitative Analysis Organic Qualitative Analysis Inorganic Gravimetric Analysis Inorganic Titrimetric Analysis Organic Quantitative Analysis Analytical Chemistry of the Elements Electrochemical Analysis Electrodeposition Potentiometric Titrations Conductometric Titrations High-Frequency Titrations Liquid Chromatography in Columns Gas Chromatography Ion Exchangers Distillation Paper and Thin Layer Chromatography Radiochemical Methods Nuclear Magnetic Resonance and Electron Spin Resonance Methods X-Ray Spectrometry Coulometric Analysis Elemental Analysis with Minute Sample Standards and Standardization Separation by Liquid Amalgams Vacuum Fusion Analysis of Gases in Metals Electroanalysis in Molten Salts Instrumentation for Spectroscopy Atomic Absorption and Fluorescence Spectroscopy Diffuse Reflectance Spectroscopy Emission Spectroscopy Analytical Microwave Spectroscopy Analytical Applications of Electron Microscopy Analytical Infrared Spectroscopy Thermal Methods in Analytical Chemistry Substoichiometric Analytical Methods Enzyme Electrodes in Analytical Chemistry Molecular Fluorescence Spectroscopy Photometric Titrations Analytical Applications of Interferometry Ultraviolet Photoelectron and Photoion Spectroscopy Auger Electron Spectroscopy Plasma Excitation in Spectrochemical Analysis Organic Spot Tests Analysis The History of Analytical Chemistry

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Volumes in the series Vol. XI

Vol. XII

Vol. XIII

Vol. XIV Vol. XV Vol. XVI Vol. XVII Vol. XVIII Vol. Vol. Vol. Vol. Vol.

XIX XX XXI XXII XXIII

Vol. Vol. Vol. Vol. Vol. Vol. Vol. Vol. Vol. Vol. Vol.

XXIV XXV XXVI XXVII XXVIII XXIX XXX XXXI XXXII XXXIII XXXIV

Vol. Vol. Vol. Vol.

XXXV XXXVI XXXVII XXXVIII

Vol. XXXIX Vol. XL Vol. XLI

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The Application of Mathematical Statistics in Analytical Chemistry Mass Spectrometry Ion Selective Electrodes Thermal Analysis Part A. Simultaneous Thermoanalytical Examination by Means of the Derivatograph Part B. Biochemical and Clinical Applications of Thermometric and Thermal Analysis Part C. Emanation Thermal Analysis and other Radiometric Emanation Methods Part D. Thermophysical Properties of Solids Part E. Pulse Method of Measuring Thermophysical Parameters Analysis of Complex Hydrocarbons Part A. Separation Methods Part B. Group Analysis and Detailed Analysis Ion-Exchangers in Analytical Chemistry Methods of Organic Analysis Chemical Microscopy Thermomicroscopy of Organic Compounds Gas and Liquid Analysers Kinetic Methods in Chemical Analysis Application of Computers in Analytical Chemistry Analytical Visible and Ultraviolet Spectrometry Photometric Methods in Inorganic Trace Analysis New Developments in Conductometric and Oscillometric Analysis Titrimetric Analysis in Organic Solvents Analytical and Biomedical Applications of Ion-Selective Field-Effect Transistors Energy Dispersive X-Ray Fluorescence Analysis Preconcentration of Trace Elements Radionuclide X-Ray Fluorescence Analysis Voltammetry Analysis of Substances in the Gaseous Phase Chemiluminescence Immunoassay Spectrochemical Trace Analysis for Metals and Metalloids Surfactants in Analytical Chemistry Environmental Analytical Chemistry Elemental Speciation – New Approaches for Trace Element Analysis Discrete Sample Introduction Techniques for Inductively Coupled Plasma Mass Spectrometry Modern Fourier Transform Infrared Spectroscopy Chemical Test Methods of Analysis Sampling and Sample Preparation for Field and Laboratory Countercurrent Chromatography: The Support-Free Iiquid Stationary Phase Integrated Analytical Systems Analysis and Fate of Surfactants in the Aquatic Environment Sample Preparation for Trace Element Analysis

Contents Contributors to Volume Volumes in the Series . Series Editor’s Preface Preface . . . . . . . .

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Chapter 1. Introduction and overview . . . . . K. Janssens and R. Van Grieken 1.1 Introduction . . . . . . . . . . . . 1.2 Overview of the analytical reference 1.3 Overview of the case studies section References . . . . . . . . . . . . . . . .

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Chapter 2. UV-, IR- and X-ray imaging . . . . . . . . . . . . . . . . Franz Mairinger 2.1 Scientific investigations of works of arts and crafts . . . . 2.2 Application of electromagnetic radiation for the examination of cultural heritage objects . . . . . . . . . . 2.3 Instrumental basis . . . . . . . . . . . . . . . . . . . . . 2.3.1 Light and radiation sources . . . . . . . . . . . . 2.3.2 Imaging . . . . . . . . . . . . . . . . . . . . . . 2.3.3 Sensor systems . . . . . . . . . . . . . . . . . . . 2.3.4 Sensor subsystems . . . . . . . . . . . . . . . . . 2.4 Surface examinations . . . . . . . . . . . . . . . . . . . 2.4.1 Surface examinations with ultraviolet radiation . . 2.4.2 Instrumental techniques for UV-fluorescence photography . . . . . . . . . . . . . . . . . . . . 2.4.3 Instrumental techniques for reflected UV photography . . . . . . . . . . . . . . . . . . . . 2.4.4 Application of UV-fluorescence photography . . . . 2.4.5 Application of UV photography . . . . . . . . . . 2.5 Depth examinations . . . . . . . . . . . . . . . . . . . . 2.5.1 Depth examinations with infrared radiation . . . . 2.5.2 Depth examinations with X-rays and gamma-rays . References . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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PART I. ANALYTICAL REFERENCE SECTION

15 16 16 17 25 33 41 44 45 46 46 47 48 49 50 54 66

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Chapter 3. Electron microscopy and its role in cultural heritage studies . . . . . . . . . . . . . . . . . . . . . A. Adriaens and M.G. Dowsett 3.1 Introduction . . . . . . . . . . . . . . . . . . . 3.1.1 Why use electron microscopy? . . . . . . 3.1.2 Imaging with electrons . . . . . . . . . . 3.1.3 Varieties of electron microscopy . . . . . 3.1.4 Recent developments in commercial SEM 3.2 The interaction of electrons with a solid—contrast mechanisms . . . . . . . . . . 3.2.1 Scattering . . . . . . . . . . . . . . . . 3.2.2 Secondary electron emission . . . . . . . 3.2.3 Backscattered electrons . . . . . . . . . 3.2.4 Cathodoluminescence . . . . . . . . . . 3.2.5 Core-level excitation and X-ray or Auger emission . . . . . . . . . . . . . . 3.2.6 Electron energy loss spectroscopy . . . . 3.2.7 Diffraction in TEM . . . . . . . . . . . . 3.2.8 Image contrast in TEM . . . . . . . . . 3.3 Components and optics of electron microscopes . . . . . . . . . . . . . . . . . . . . 3.3.1 Basic optics . . . . . . . . . . . . . . . . 3.3.2 The electron gun . . . . . . . . . . . . . 3.3.3 Focusing an electron beam . . . . . . . . 3.3.4 Newtonian lens model . . . . . . . . . . 3.3.5 The magnetic prism . . . . . . . . . . . 3.3.6 Detectors . . . . . . . . . . . . . . . . . 3.4 Sample preparation techniques . . . . . . . . . 3.5 Origin/ provenance studies . . . . . . . . . . . . 3.5.1 Ceramics . . . . . . . . . . . . . . . . . 3.5.2 Glass . . . . . . . . . . . . . . . . . . . 3.6 Technology and techniques of manufacture . . . 3.6.1 Seals . . . . . . . . . . . . . . . . . . . 3.6.2 Ceramics . . . . . . . . . . . . . . . . . 3.6.3 Glass . . . . . . . . . . . . . . . . . . . 3.6.4 Metals . . . . . . . . . . . . . . . . . . 3.7 Use . . . . . . . . . . . . . . . . . . . . . . . . 3.8 Degradation processes, corrosion and weathering . . . . . . . . . . . . . . . . . . . . 3.8.1 Metals . . . . . . . . . . . . . . . . . .

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3.8.2 Glass . . . . . . . . . . 3.8.3 Ceramics . . . . . . . . 3.9 Authenticity and authentication 3.10 Conclusions . . . . . . . . . . Acknowledgements . . . . . . . . . . References . . . . . . . . . . . . . .

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Chapter 4. X-ray based methods of analysis . . . . . . . . . . . . K. Janssens 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . 4.2 Basic principles . . . . . . . . . . . . . . . . . . . . . . 4.2.1 X-ray wavelength and energy scales . . . . . . . 4.2.2 Interaction of X-rays with matter . . . . . . . . 4.2.3 The photoelectric effect; X-ray fluorescence . . . . . . . . . . . . . . . . . . . . 4.2.4 Scattering and diffraction . . . . . . . . . . . . 4.2.5 X-ray absorption fine structure and spectroscopy . . . . . . . . . . . . . . . . . . . 4.3 Instrumentation for X-ray investigations . . . . . . . . 4.3.1 X-ray sources . . . . . . . . . . . . . . . . . . . 4.3.2 X-ray detectors . . . . . . . . . . . . . . . . . . 4.3.3 X-ray fluorescence instrumentation . . . . . . . 4.3.4 XRD instrumentation . . . . . . . . . . . . . . 4.3.5 XAS instrumentation at SR beamlines . . . . . . 4.4 A survey of applications of X-ray methods in the cultural heritage sector . . . . . . . . . . . . . . . . . . . . . . 4.4.1 Compositional analysis of historic glass . . . . . 4.4.2 Pigments . . . . . . . . . . . . . . . . . . . . . 4.4.3 Lustre ware . . . . . . . . . . . . . . . . . . . 4.4.4 Metallic artefacts . . . . . . . . . . . . . . . . . 4.4.5 Analysis of graphic documents . . . . . . . . . . 4.4.6 Mn oxidation in odontolites . . . . . . . . . . . 4.4.7 Therapeutic and cosmetical chemicals of Ancient Egypt . . . . . . . . . . . . . . . . . 4.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . A4.1 Figures-of-merit for XRF spectrometers . . . . . . . . . A4.1.1 Analytical sensitivity . . . . . . . . . . . . . . . A4.1.2 Detection and determination limits . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Chapter 5. Ion beam microanalysis . . . . . . . . . . . . T. Calligaro, J.-C. Dran and J. Salomon 5.1 Historical background and motivation . . . . . 5.2 Fundamentals of ion beam analysis . . . . . . 5.2.1 Interaction of radiations with matter . 5.2.2 Particle-induced X-ray emission . . . . 5.2.3 Elastic scattering of particles . . . . . 5.2.4 Nuclear reaction analysis . . . . . . . 5.3 Specific arrangements for the study of art and archaeological objects . . . . . . . . . . . . . 5.3.1 External beams . . . . . . . . . . . . . 5.3.2 Nuclear microprobes . . . . . . . . . . 5.3.3 Micro and macro-imaging . . . . . . . 5.3.4 Portable systems . . . . . . . . . . . . 5.4 Applications in the field of art and archaeology 5.4.1 Materials’ identification . . . . . . . . 5.4.2 Provenance of the materials . . . . . . 5.4.3 Alteration phenomena . . . . . . . . . 5.4.4 Authentication and relative dating . . . 5.5 Survey of worldwide IBA activity in the field of cultural heritage . . . . . . . . . . . . . . . . 5.6 Conclusion and future prospects . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . .

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Chapter 6. X-ray photoelectron and Auger electron spectroscopy Annick Hubin and Herman Terryn 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . 6.2 The basic concepts of XPS and AES . . . . . . . . . 6.2.1 Principle of X-ray photoelectron spectroscopy 6.2.2 Principle of Auger electron spectroscopy . . . 6.3 XPS and AES instruments . . . . . . . . . . . . . . 6.3.1 General set-up . . . . . . . . . . . . . . . . 6.3.2 The vacuum system . . . . . . . . . . . . . 6.3.3 The X-ray source for XPS . . . . . . . . . . 6.3.4 The electron gun for AES . . . . . . . . . . 6.3.5 Detection of electron energy . . . . . . . . . 6.3.6 The ion gun . . . . . . . . . . . . . . . . . 6.3.7 The sample holder and stage . . . . . . . . . 6.4 Sample requirements . . . . . . . . . . . . . . . .

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6.5

Information in XPS and AES spectra . . . 6.5.1 Surface analysis . . . . . . . . . . 6.5.2 Qualitative analysis . . . . . . . . 6.5.3 Quantitative analysis . . . . . . . 6.5.4 Chemical analysis . . . . . . . . . 6.5.5 In-depth analysis . . . . . . . . . 6.5.6 Data analysis . . . . . . . . . . . 6.5.7 Imaging . . . . . . . . . . . . . . 6.6 Comparison of XPS, AES and other surface analytical techniques . . . . . . . . . . . . 6.7 XPS and AES for chemical analysis of cultural heritage materials . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . .

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Chapter 7. Laser ablation inductively coupled plasma mass spectrometry . . . . . . . . . . . . . . . . . . . . . . . . Teresa E. Jeffries 7.1 Introduction . . . . . . . . . . . . . . . . . . . . 7.2 The inductively coupled plasma mass spectrometer 7.2.1 Historical account . . . . . . . . . . . . . 7.2.2 Operational rationale . . . . . . . . . . . . 7.2.3 The inductively coupled plasma . . . . . . 7.2.4 The plasma sampling interface . . . . . . 7.2.5 Ion focusing . . . . . . . . . . . . . . . . 7.2.6 Quadrupole mass analyser . . . . . . . . . 7.2.7 The vacuum system . . . . . . . . . . . . 7.2.8 Ion detection and signal handling . . . . . 7.3 Laser ablation: essential components . . . . . . . 7.3.1 Development of the laser . . . . . . . . . . 7.3.2 The association of lasers with ICP-MS . . . 7.3.3 Stimulated emission . . . . . . . . . . . . 7.3.4 Nd:YAG laser (resonator) cavity . . . . . . 7.3.5 Harmonic generation . . . . . . . . . . . . 7.3.6 Harmonic separation . . . . . . . . . . . . 7.3.7 Energy attenuation and control. . . . . . . 7.3.8 Beam delivery and viewing optics . . . . . 7.3.9 Ablation cell and sample transport . . . . 7.4 Analytical concepts and factors affecting analysis . 7.4.1 Why use the technique? . . . . . . . . . .

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7.4.2 7.4.3 7.4.4 7.4.5 7.4.6

Sample preparation and mounting . . . . . Analysis of transient signals . . . . . . . . Factors affecting analysis . . . . . . . . . Optimization and calibration . . . . . . . . Figures of merit and analytical performance targets . . . . . . . . . . . . . . . . . . . 7.5 Continuing developments and final remarks . . . . 7.5.1 Continuing developments . . . . . . . . . 7.5.2 Final remarks . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . Chapter 8. Infrared, Raman microscopy and fibre-optic Raman spectroscopy (FORS) . . . . . . . . . . . . . . Howell G.M. Edwards and Dalva L.A. de Faria 8.1 Introduction . . . . . . . . . . . . . . . . . . . 8.2 Comparison of the potential use of IR and Raman spectroscopies for the non-destructive analysis of art works . . . . . . . . . . . . . . . . . . . 8.3 Some theoretical aspects of IR and Raman spectroscopies . . . . . . . . . . . . . . . . . . 8.4 Instrumentation . . . . . . . . . . . . . . . . . 8.5 Sampling . . . . . . . . . . . . . . . . . . . . . 8.6 Resonance Raman . . . . . . . . . . . . . . . . 8.7 SERS . . . . . . . . . . . . . . . . . . . . . . . 8.8 Intensity measurements in Raman scattering . . 8.9 Raman spectroscopy with fibre optics . . . . . . 8.9.1 Sampling considerations . . . . . . . . . 8.9.2 Probe design . . . . . . . . . . . . . . . 8.9.3 Probe background . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . .

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344 345 347 350

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352 355 355 356 357

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367 373 381 387 390 390 392 392 392 393 393

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397

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397 399 399 410 420 420

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Chapter 9. Secondary ion mass spectrometry. Application to archaeology and art objects . . . . . . . . . . . . . . . . . . Evelyne Darque-Ceretti and Marc Aucouturier 9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . 9.2 Principles and equipment . . . . . . . . . . . . . . . 9.2.1 Principles . . . . . . . . . . . . . . . . . . . . 9.2.2 Equipment and choice of analytical parameters 9.3 Analysis procedures . . . . . . . . . . . . . . . . . . 9.3.1 Elemental identification, sensitivity . . . . . .

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9.3.2 Quantitative analysis . . . . . . . . . . . . 9.3.3 In-depth analysis and depth resolution . . . 9.3.4 Surface analysis . . . . . . . . . . . . . . . 9.3.5 Imaging, lateral resolution . . . . . . . . . . 9.3.6 Chemical compound analysis and distribution 9.4 Examples of applications for cultural heritage . . . . 9.4.1 Dating and/or provenance studies based on isotopic analysis . . . . . . . . . . . . . 9.4.2 Dating (not based on isotopic analysis) . . . 9.4.3 Provenance studies not based on isotopic analysis . . . . . . . . . . . . . . . . . . . 9.4.4 Surface layer analysis on artefacts . . . . . 9.4.5 Interface studies on coated layers . . . . . . 9.4.6 ToF-SIMS applications . . . . . . . . . . . . 9.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . .

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425 429 435 438 439 440

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465 467 470 474 477 480 485 488 490 490

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493

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493 493

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PART II. CASE STUDIES SECTION

Chapter 10. The non-destructive investigation of copper alloy patinas . . . . . . . . . . . . . . . . . . . . David A. Scott 10.1 A brief historical account . . . . . . . . . . 10.2 Optical examination . . . . . . . . . . . . . 10.3 Environmental scanning electron microscopy 10.4 X-ray fluorescence analysis . . . . . . . . . 10.5 Scanning X-ray fluorescence microanalysis . 10.6 XRD analysis . . . . . . . . . . . . . . . . . 10.7 FTIR spectroscopy . . . . . . . . . . . . . . 10.8 Conclusions . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . .

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Chapter 11. Precious metals artefacts . . . . . . . . . . . . . . G. Demortier 11.1 Introduction . . . . . . . . . . . . . . . . . . . . . 11.2 Non-destructive analysis of gold jewellery items . . . 11.2.1 Contribution of atomic and nuclear (but non-radioactive) methods to the analysis of ancient gold jewellery items . . . . . . . .

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11.2.2 Illustration of the analytical performances of non-vacuum PIXE for gold artefacts . . . . . . 11.3 The soldering of gold . . . . . . . . . . . . . . . . . . 11.3.1 Ancient recipes for gold soldering . . . . . . . 11.3.2 Iranian goldsmithery from the 4th century BC . 11.3.3 Tartesic gold artefacts . . . . . . . . . . . . . 11.3.4 Later Iranian goldsmithery . . . . . . . . . . . 11.3.5 Preparations of low-melting brazing alloys . . . 11.3.6 A new reading of Elder Pliny’s Natural History 11.3.7 Italian jewellery . . . . . . . . . . . . . . . . 11.3.8 Gold artefacts from Slovenia . . . . . . . . . . 11.3.9 The Guarrazar treasure . . . . . . . . . . . . 11.3.10 Merovingian and late Byzantine jewellery . . . 11.4 Pre-Hispanic gold artefacts of Mesoamerica . . . . . . 11.4.1 Archaeological context . . . . . . . . . . . . . 11.4.2 A selection of typical artefacts . . . . . . . . . 11.4.3 Differential PIXE . . . . . . . . . . . . . . . . 11.4.4 Application to the measurement of the gold enhancement at the surface of tumbaga . . . . 11.5 Characterization of complex items . . . . . . . . . . . 11.5.1 XRF induced by a g-ray source . . . . . . . . . 11.5.2 Gamma-ray transmission measurements . . . 11.5.3 Study of a composite gold jewellery artefact . . 11.6 Gold coins . . . . . . . . . . . . . . . . . . . . . . . 11.6.1 Fineness measurements of gold coins . . . . . 11.6.2 Gold coins from the ancient world . . . . . . . 11.6.3 Gold coins from the new world . . . . . . . . . 11.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . .

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496 498 498 499 502 506 509 517 520 521 525 528 530 530 534 536

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538 544 544 544 545 548 548 548 556 558 559 560

Chapter 12. Diagnostic methodology for the examination of Byzantine frescoes and icons. Non-destructive investigation and pigment identification. . . . . . . . . . . . . . . . . . . . . Sister Daniilia, Sophia Sotiropoulou, Dimitrios Bikiaris, Christos Salpistis, Georgios Karagiannis and Yannis Chryssoulakis 12.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . 12.2 The Entry of the Mother of God into the Temple . . . . . . 12.2.1 Description . . . . . . . . . . . . . . . . . . . . .

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12.2.2 The preparation of the plaster: materials and technique . . . . . . . . . . . . . . 12.2.3 The drawing . . . . . . . . . . . . . . . 12.2.4 Materials and painting techniques . . . . 12.2.5 Study of the colour palette . . . . . . . . 12.2.6 Conclusions . . . . . . . . . . . . . . . 12.3 Mother of God Hodegetria . . . . . . . . . . . . 12.3.1 Description . . . . . . . . . . . . . . . . 12.3.2 Construction and state of preservation of the support . . . . . . . . . . . . . . 12.3.3 State of preservation of the surface . . . 12.3.4 The ground . . . . . . . . . . . . . . . . 12.3.5 The drawing . . . . . . . . . . . . . . . 12.3.6 Materials and technique of the painting . 12.3.7 Conclusions . . . . . . . . . . . . . . . A12.1 Experimental details . . . . . . . . . . . . . . . A12.1.1 Non-destructive analysis . . . . . . . . A12.1.2 Micro-sampling analysis . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . .

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567 568 570 581 584 585 585

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587 588 592 592 593 601 602 602 603 604

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605 606 606 608 613 622 622 624 629 630 630

Chapter 14. Pigment identification in illuminated manuscripts . . . Peter Vandenabeele and Luc Moens 14.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . .

635

Chapter 13. The provenance of medieval silver coins: analysis with EDXRF, SEM/EDX and PIXE . . . . . . . . . . . . . . . . . . Robert Linke, Manfred Schreiner, Guy Demortier, Michael Alram and Heinz Winter 13.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . 13.2 The Friesacher Pfennig . . . . . . . . . . . . . . . . . 13.2.1 Introduction . . . . . . . . . . . . . . . . . . . 13.2.2 Experimental . . . . . . . . . . . . . . . . . . 13.2.3 Results . . . . . . . . . . . . . . . . . . . . . . 13.3 The Tiroler Kreuzer . . . . . . . . . . . . . . . . . . . 13.3.1 Introduction . . . . . . . . . . . . . . . . . . . 13.3.2 Experimental . . . . . . . . . . . . . . . . . . 13.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . .

635

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14.2 Combined method approach . . . . . . . . . . . . 14.2.1 Analysis of manuscripts . . . . . . . . . . 14.2.2 Sources of impurities . . . . . . . . . . . . 14.3 Analysis of the manuscripts from the collection of Raphael De Mercatellis . . . . . . . . . . . . . 14.3.1 Introduction . . . . . . . . . . . . . . . . 14.3.2 Pigment identification with TXRF and MRS 14.3.3 Intra-manuscript comparison of Expositio problematum Aristotelis . . . . . . . . . . 14.3.4 Analysis of Decretum Gratiani . . . . . . . 14.4 Conclusion . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . .

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636 639 652

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654 654 655

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657 658 659 660 660

Chapter 15. Provenance analysis of glass artefacts . . . . . . . . . Bernard Gratuze and Koen Janssens 15.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . 15.2 Obsidian, a natural glass used since the Paleolithic era . 15.3 Bronze and Iron age glasses . . . . . . . . . . . . . . . 15.3.1 Neolithic first artificial glassy materials and the discovery of glass during Bronze Age . . . . 15.3.2 When trade beads reached Europe . . . . . . . . 15.3.3 Middle Bronze Age plant ash soda-lime glasses . 15.3.4 Late Bronze Age mixed soda – potash glasses . . 15.3.5 Iron Age and Antiquity natron soda-lime glasses 15.3.6 Protohistoric glass trade routes . . . . . . . . . 15.3.7 Glass chrono-typo-chemical models: a dating tool? . . . . . . . . . . . . . . . . . . . 15.4 Glass trade towards and from Central Asia and the Indian world during Antiquity . . . . . . . . . . . . 15.5 Carolingian glass production: some unusual lead glass composition smoothers . . . . . . . . . . . . . . . 15.6 Late Middle Age recycled glass . . . . . . . . . . . . . 15.7 Glass technology transfer during the 16th –17th century to and from Antwerp . . . . . . . . . . . . . . . . . . . 15.8 Trade beads: the glass trade internationalization, during the Post-Medieval period . . . . . . . . . . . . . 15.9 Conclusions . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Chapter 16. Corrosion of historic glass and enamels . . . . . . . Manfred Schreiner 16.1 Introduction . . . . . . . . . . . . . . . . . . . . . . 16.2 The weathering of medieval stained glass . . . . . . . 16.2.1 SEM investigations of the corrosion phenomena on naturally weathered medieval glass . . . . 16.2.2 The determination of hydrogen in the leached surface layer by SIMS and NRA . . . . . . . . 16.2.3 Leaching studies of glass with medieval composition . . . . . . . . . . . . . . . . . . 16.2.4 IRRAS investigations on leached glass with medieval composition . . . . . . . . . . . . . 16.2.5 Weathering phenomena on glass with medieval composition studied with TM-AFM . . . . . . 16.3 The degradation of medieval enamels . . . . . . . . . 16.3.1 SEM investigations of the enamel of the medieval goblets . . . . . . . . . . . . . . . . 16.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . .

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713 716

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773

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774 775

Chapter 17. A study of ancient manuscripts exposed to iron – gall ink corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . Ewa Bulska and Barbara Wagner 17.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . 17.1.1 Iron –gall ink . . . . . . . . . . . . . . . . . . . 17.1.2 Iron –gall ink corrosion . . . . . . . . . . . . . 17.1.3 Investigated artefacts . . . . . . . . . . . . . . 17.2 Analytical methods . . . . . . . . . . . . . . . . . . . 17.2.1 Inspection by scanning electron microscopy . . . 17.2.2 Compositional analysis by X-ray fluorescence spectrometry . . . . . . . . . . . . . . . . . . . 17.2.3 Electron probe micro-analysis . . . . . . . . . . 17.2.4 Laser ablation inductively coupled plasma mass spectrometry . . . . . . . . . . . . . . . . 17.2.5 Elemental analysis by inductively coupled plasma mass spectrometry . . . . . . . . . . . . . . . . 17.2.6 Graphite furnace atomic absorption spectrometry . . . . . . . . . . . . . . . . . . . 17.2.7 Mo¨ssbauer spectrometry . . . . . . . . . . . . .

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17.2.8 Investigation of Fe(II)/Fe(III) by X-ray absorption near edge spectroscopy . . . . . . . . . . . . . . 17.3 Searching for the conservation treatment . . . . . . . . 17.3.1 Reconstitution of manuscript by model samples . 17.3.2 Requirement for conservation treatment . . . . 17.3.3 Investigation of the model samples . . . . . . . 17.4 Concluding comments . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . .

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778 780 780 782 784 784 785 785

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Series Editor’s Preface The investigation of cultural heritage materials is a field that, contrary to other fashion or curiosity-driven research topics, will always need to be funded. It becomes obvious to all of us that to achieve knowledge and progress in our society, we need to know more about our past, represented by artefacts of previous civilisations such as ancient silver and gold coins, frescoes or old manuscripts. In this respect Janssens and Van Grieken have produced a timely book on the non-destructive analysis of cultural heritage materials and it is a useful addition to the Comprehensive Analytical Chemistry Series. Although we have recently published books on molecular and spectroscopic techniques, the artefacts that comprise our cultural heritage require specific techniques and instrumentation which should preferably be non-destructive or micro-destructive and the measurements often need to be made in situ. By using non-destructive or micro-destructive techniques, the excellent ancient pieces of work that need to be examined do not suffer any damage or no visible damage, respectively. To get a good overview of the contents of this book, I strongly recommend that you first read chapter one, where the editors describe in detail the content and structure of the book. The book contains 17 chapters. In the first part the emphasis is on the techniques used in this field. There are 9 chapters, which include a concise description of non-destructive microanalytical techniques, such as X-ray based methods, electron microscopy and Raman microscopy, among others. The second part is intended to show to the reader how the different analytical techniques are being applied in the real world, and comprises 8 chapters, with applications concerning the diagnostics of different important pieces of work such as Byzantine frescoes and icons, medieval silver coins or glass artefacts among other cultural heritage materials. In summary, the book gives a state-of-the art report on the major techniques used for the non-destructive analysis of cultural heritage artefacts or materials. It is with a great deal of pleasure that I take this opportunity to present this exceptional and unique volume and I would like to thank the editors and the authors for their excellent work.

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Series Editor’s Preface

With this book we have now published 42 volumes and we appear to be on course to reach volume 50 by the year 2006. I would like to thank not only the editors and authors of past, present and future volumes but also the scientific community for the broad acceptance of the Comprehensive Analytical Chemistry series. Damia Barcelo´ Barcelona, June 2004

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Preface

During the last decade, considerable progress has been made in the instrumental and methodological aspects of microscopic analysis. These improvements are apparent in rather diverse fields, ranging from infrared reflectography over Raman microscopy and X-ray microanalysis by means of electrons, protons and photons to mass spectrometric methods that employ laser sampling. Considerable improvements in detector technology, instrument-computer interfacing, focusing optics, and in the performance of radiation sources suitable for use in various parts of the electromagnetic spectrum lie at the basis of the advances. In the present context of investigations of cultural heritage materials, which often must be performed in situ, an important recent development has been the ongoing miniaturization of components, permitting the design of compact, portable and sometimes handheld analytical instruments that are able to provide analytical data of comparable quality to those produced by ‘regular’ laboratory instruments. Since many of these technological improvements are predominantly application-driven, the above-mentioned progress has led to an effective increase in the applicability of the various methods that are described in this book. The augmented applicability is, on the one hand, realized through an increase in the sensitivity, the detection power and the lateral, spatial and/or spectral resolution of the methods, thus allowing information on more subtle variations in specific material properties to be obtained in a more accurate and/or more precise manner. In some cases, an increasing number of independent properties can simultaneously be recorded from the same location in a material. In some of the application-oriented chapters in the second part of the book, the advantages of the combined use of various atomic and molecular spectroscopies for gathering different types of information on the same material, objects or problem is already apparent. The applicability of (micro)analytical methods for the investigation of materials from the cultural heritage field is also realized through the relaxation of the boundary conditions or circumstances that have traditionally limited the application range of a number of the conventional methods of

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investigation. In earlier decades, conventional spectroscopic methods could exclusively be used for the investigation of ‘ideal’ samples (e.g., dilute solutions of previously dissolved materials, polished disks of metals or glass), usually requiring considerable sample preparation. Through the development of microscopic and/or solid-sampling variants of the original methods, complex objects, composed of different materials and having non-flat, irregular shapes can now also be analyzed with some reliability, either with minimal or no damage whatsoever to the objects. All chapters have been written by experts in their own fields, whether they be predominantly methodological and instrumental in nature or more specifically directed towards the study of a particular kind of cultural heritage problem or artifact type. The multi-author approach, although inevitably leading to a certain variability in style of presentation, in our opinion, outweighs any advantages of uniformity and homogeneity that characterize a single-author book. Since many different methods and problems are covered in this volume, it is in practice impossible for a single individual to cover a significant fraction of the relevant methodological, technical and applied aspects of non-destructive micro-analysis. From the beginning, our intention was to target two types of practitioners with this book. On the one hand, we hope that MSc and PhD level students, and spectroscopists and analysts with a background in the natural sciences, seeking to broaden their knowledge on methods of non-destructive microanalysis and how (one or combination of several of) these methods may be applied in the cultural heritage area, will profit from this book. On the other hand, by presenting in the same volume a collection of applied studies, where several methods of analysis are employed together and in a problem-oriented fashion, we also hope to have provided a resource for conservators and museum curators who are faced with questions about which combination of analytical methods or services would be optimal to shed light on a particular curatorial or conservation problem. K. Janssens R. Van Grieken

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Chapter 1

Introduction and overview K. Janssens and R. Van Grieken

1.1

INTRODUCTION

For the study, conservation and restoration of materials and artefacts of culturo-historical value, there is a well-defined need for analytical methods that are able to provide information on (see Fig. 1.1): † the chemical nature/composition of selected parts of cultural heritage artefacts and materials in order to elucidate their provenance; † the state of alteration (on the surface and /or internally) of objects as a result of short-, medium- and long-term exposure to particular environmental conditions; † the effect/effectiveness of conservation/restoration strategies during and after application. According to Lahanier et al. [1], the ideal method for analysing objects of artistic, historic or archaeological nature should be: (a) non-destructive, i.e., respecting the physical integrity of the material/ object. Often valuable objects can only be investigated when the analysis does not result in any (visible) damage to the object. Usually this completely eliminates sampling or limits it to very small amounts; (b) fast, so that large numbers of similar objects may be analysed or a single object investigated at various positions on its surface; this property is very valuable since this is the only way of being able to discern between general trends in the data and outlying objects or data points; (c) universal, so that by means of a single instrument, many materials and objects of various shapes and dimensions may be analysed with minimal sample pre-treatment; (d) versatile, allowing with the same technique to obtain average compositional information as well as local information of small areas (e.g., millimetre to micron-sized) from heterogeneous materials; Comprehensive Analytical Chemistry XLII Janssens and Van Grieken (Eds.) q 2004 Elsevier B.V. All rights reserved

1

K. Janssens and R. Van Grieken

Fig. 1.1. Interaction between cultural heritage materials, the use of analytical techniques and environmental factors.

(e) sensitive, so that object grouping and other types of provenance analysis can be done by means of not only major elements but also trace-element fingerprints; and (f) multi-elemental, so that in a single measurement, information on many elements is obtained simultaneously and, more importantly, information is also obtained on elements which were not initially thought to be relevant to the investigation. While most methods that are described in the chapters of this book fulfil several, but usually not all, requirements described above, it is obvious that in the cultural heritage field, the analytical techniques should preferably be non-destructive or micro-destructive. Non-destructive techniques allow analytical information to be obtained with no damage to the sample or (in some cases) to the artefacts in question. When micro-destructive methods are used, all visible damage is avoided and the objects under examination remain aesthetically unimpaired [2]. The possibility of using these types of methods is of enormous advantage when sampling is not feasible or when fragments used for analysis need to be put back in their original location at the end of the investigation. Among the truly non-destructive methods are the spectroscopies based on ultraviolet, visual and infrared (IR) radiations, as well as the X-ray-based methods.

2

Introduction and overview

Objects and monuments of culturo-historical significance comprise a wide variety of materials (metals, ceramics, glass, various igneous and sedimentary rocks, textile, leather, wood, horn, parchment, paper, etc.) and usually exhibit a fairly complex three-dimensional structure and a heterogeneous chemical composition. Especially in the case of artefacts of precious nature (e.g., jewellery, weaponry, religious objects), cultural heritage objects often: † are composed of various materials (e.g., Au/Ag alloy artefacts adorned with gemstones), † consist of a base material covered with one or more layers of pigmentation (e.g., polychrome wooden statues, easel paintings, illuminated manuscripts) or † show significant (surface) alteration due to burial or atmospheric exposure (e.g., bronze statues, silver coins, etc.) [3]. In addition to the requirement that the method(s) of analysis that are employed to assess the material state of such objects/materials are as non-destructive as realistically possible, in most cases, preference is then given to methods that are able to yield information on well-defined areas of the artefacts in question [4]. Sometimes, these areas are microscopically small; in other cases, a lateral resolution of 1 mm2 suffices. Techniques having a high spatial resolution are considered by some to take an intermediate place between destructive and non-destructive methods. In recent years, partially driven by the increasing importance of hightech materials of a complex micro-structural nature and mainly as a result of miniaturization of components (radiation sources, radiation guide tubes, detectors), a number of portable and /or microscopic versions of established analytical methods have come into existence. Such methods are well suited for inspection and/or analysis of objects of great cultural value as measurements can be made on site (e.g., pigment identification in frescoes) thus eliminating the need to sample or even move the objects out of their normal surroundings (e.g., a museum or an archaeological site). Thus, most of today’s major museums have at their disposal one or more (trans)portable instruments for high spatial resolution examination of the artefacts in their collection. As well as to being useful for analysing the cultural heritage artefacts themselves, such techniques are also employed in support studies, where under controlled laboratory circumstances, e.g., the deterioration of building materials (mortar, sandstone, etc.) under the influence of rain, exposure to solar radiation, pollution gases, etc., is examined.

3


Although such methods have been described in the specialized literature of the field or sub-field to which they belong (e.g., portable and microscopic X-ray fluorescence analysis (XRF) in the atomic spectrometry literature, fibre-optic IR reflectography in the molecular spectroscopy literature), the flow of information towards the larger field of potential users has been limited because: † the technical literature that pertains to each of the sub-fields from which the techniques have emerged (physics, analytical chemistry, atomic/ molecular spectroscopy) is quite fragmented; † the potential users (archaeologists, art-historians, conservators, architects, museum conservators) are not sufficiently trained in the technical/ scientific aspects to allow them to absorb/monitor the above-mentioned state-of-the-art innovations. In view of the above, we have gathered together a group of authors who combine (a) a number of concise descriptions of non-destructive microanalytical techniques that have shown their effectiveness in the cultural heritage field with (b) a number of case studies where one or a series of artefacts of particular, generic nature (e.g., bronze statues, illuminated manuscripts, glass artefacts, etc.) are studied from the provenance or conservation point of view, preferably using an interdisciplinary, multitechnique approach. Among the chapters in the first section of this volume, the methods based on the use of energetic radiation belonging to the X-ray or Ro¨ntgen part of the electromagnetic spectrum are well represented. X-ray-based methods of non-destructive analysis are very frequently employed in the cultural heritage area for various purposes. Starting in earnest in the 1950s by judicious use of the available means then, this is still very much the case today where many technological advances increase the applicability of the methods, resulting in an extensive literature. This was also apparent during the seventh edition of the international conference on “Non-destructive Investigations and Micro-analysis for the Diagnostics and Conservation of the Cultural and Environmental Heritage” (Art 2002) that was organized in Antwerp (Belgium) by the editors of this book [5]. Of the ca. 200 papers presented at this conference, 115 included the use of analytical techniques for characterization of cultural heritage artefacts or materials. Among these contributions, 63 employed one or more X-ray-based or related technique (such as XRF analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), protoninduced X-ray emission (PIXE), X-ray diffraction (XRD), X-ray absorption

4


spectroscopy (XAS), total-reflection X-ray fluorescence (TXRF) and X-ray microtomography) while 28 made use of IR spectroscopy, 14 used Raman microscopy, 12 used visible light spectrometry or reflectometry and six employed gas chromatography coupled to mass spectrometry (GC – MS). Thus, the first part of this volume can be considered to be the analytical reference section of the book, introducing and bringing together all technical information, library spectra (if any) and literature references of various methods. Each description consists of an explanation of the basic principles of the technique (keeping a cultural heritage user profile in mind), information on the availability of equipment, skills required, etc., and is illustrated with a few concrete examples of applications in the cultural heritage field. The second part of the book is intended to show the reader how these methods can be employed, either separately or in combination with each other, to solve concrete, real-world problems and provide overviews of the literature in specific application areas. Among these case studies, both investigations that aim to extract provenance information from cultural artefacts as well as studies that seek to evaluate and monitor preservation/ restoration treatments have been included. 1.2

OVERVIEW OF THE ANALYTICAL REFERENCE SECTION

The first section of the book starts with a overview (Chapter 2) of imaging and photographic techniques in the IR, visual, ultraviolet and X-ray part of the electromagnetic spectrum. Strictly speaking, the techniques discussed here are not microscopic in nature, but in practice will be very frequently employed prior to or in combination with some of the micro-beam methods that are discussed in later chapters. Chapter 3 is intended to introduce the reader to various forms of electron microscopy that are currently used to study cultural heritage materials. It outlines the principles underlying the technique and continues to discuss the role of electron microscopy in the field under study. The instrumentation, analytical possibilities and limitations of both SEM and TEM are discussed. An overview of recently published work involving the application of SEM and /or TEM analyses in the cultural heritage field concludes this chapter. In addition to X-ray radiography and tomography, discussed in Chapter 2, X-ray emission techniques are very frequently employed for non-destructive analysis of cultural heritage materials and artifacts. Most X-ray emission techniques involve irradiation of a material with a beam of X-ray photons

5


(or other particles), followed by detection of element-specific fluorescent radiation. In Chapter 4, besides a brief outline of the theoretical background of interactions between X-rays, and matter, the systematics of X-ray line spectra, instrumentation of X-ray spectrometry, the principles of quantitative analysis by means of XRF, and various specialized forms of this method that are of use in the cultural heritage sector are described, including totalreflection XRF, microscopic XRF and portable XRF for in situ investigations. Illustrative examples of their use are given. In addition, the use of more exotic X-ray techniques based on synchrotron radiation such as (microscopic) X-ray absorption near edge structure spectroscopy (m-XANES) and (microscopic) X-ray diffraction analysis (m-XRD) in combination with (m-)XRF are also mentioned. In Chapter 5, ion-beam methods of analysis (IBA) are discussed in detail. After a description of the fundamentals of the interaction of heavy charged particles with matter, attention is focused on the PIXE method and its companion techniques, particle-induced gamma emission (PIGE), Rutherford backscattering spectrometry (RBS) and elastic recoil detection analysis (ERDA). Specific attention is given to instrumentation devoted to the analysis of artistic and archaeological artifacts in so-called “external beam” facilities. Examples of the application of IBA techniques for surface and /or for bulk analysis of different materials are included in this chapter. In Chapter 6, the topic of surface analysis of materials by means of electron spectroscopy is discussed with a description of the theoretical background and the instrumentation for X-ray photo-electron spectroscopy (XPS) and (scanning) Auger spectrometry (SAM). The current state-of-theart of the method is described, with special attention to the lateral resolution currently achievable with both methods. This chapter also discusses sample preparation procedures and provides a review of SAM and XPS applications in the cultural heritage domain. Chapter 7 looks at the relatively new technique of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The chapter begins with a short background showing the major steps leading to the development of both lasers and ICP mass spectrometers. This is followed by a more detailed technical discussion of a “generic instrument,” dealing separately with the ICP-MS and the laser. A section concerning the analytical technique describes how the two components work together. The reader is guided through appropriate sample preparation techniques towards taking the right choice in setting instrument parameters and dealing with the date produced. This chapter also concludes with a brief review of the relevant literature.

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In the next chapter (Chapter 8) on IR and Raman microscopy (including fiber-optics Raman spectroscopy, FORS), first a resume´ of the requirements of IR, Raman spectroscopy and fiber-optic coupling in dealing with archaeological and art specimens is presented; this is followed by some case-type studies to illustrate the advantages and problems encountered in the experimental use of these techniques. Studies involving the combined use of XRD, SEM and GC – MS along with Raman microscopy are now becoming more frequent and some of these are discussed as examples. A description of secondary ion mass spectrometry or -microscopy (SIMS) can be found at the end of the reference section in Chapter 9. After some introductory considerations on the non-destructive nature of SIMS, and a description of the basic principles, the instrumentation for SIMS measurements is discussed together with an outline of the analytical procedures employed here. The quantitative, imaging and in-depth modes of operation are described and the chapter ends with a review of current applications in the cultural heritage sector, including the measurement of lead isotopes, determining the origin of gems and metal ingots and interface studies of coatings. 1.3

OVERVIEW OF THE CASE STUDIES SECTION

The second part of the book starts with Chapter 10, which is devoted to the analysis of corroded Cu-alloy materials, a very frequently encountered type of artistic or archaeological artefact. The combined use of methods of nondestructive analysis such as SEM, in situ XRD (with imaging system) and optical examination by bench microscopic methods for characterization of the alteration layers found at the surface of Cu-alloy objects is described. In situ examination of such artefacts by means of XRF in point-analysis and /or scanning mode is also discussed. The application of these types of methods is discussed for the examination of a gilded bronze Osiris from ancient Egypt, an important Greek-inscribed copper plaque of the 7th century BC , and small fragments of corroded copper alloy Phiale from the Southern Italian site of Francavilla Marittima, dated to the 5 – 6th century BC ; the contribution of analytical methods to the understanding of the development of patinas and their authenticity is evaluated. The use of bulk in situ XRD to study the corrosion of test coupons in assessing the museum environment is exemplified by examples of copper alloy coupons exposed to fixed amounts of formic and acetic acid pollutants. Chapter 11 extensively discusses the characterization of gold artifacts of various nature and the materials employed in various historical periods to

7


manufacture these items. The role of nuclear and atomic methods of nondestructive analysis of noble metals is described with emphasis on the ability of these methods to provide information on the bulk composition of the materials. The complementary use of RBS– PIXE and nuclear reaction analysis (NRA) for the compositional study of silver and gold artifacts is outlined, including the investigation of thick objects by means of gamma-ray transmission. Next, the study of the procedures of soldering on gold jewelry objects of different origin (Achemenide, Roman, Greek, Artesic and Etruscan artifacts) is treated, including the reproduction in laboratory circumstances of ancient soldering and brazing procedures. The discussion includes a comparison of modern analytical results with the description of ancient recipes listed in the Natural History of Pliny (1st century AD ). In this chapter, a study on the gilding of Mesoamerican tumbaga (a copper– gold alloy) artifacts is also presented. Through the application of diagnostic analytical methods for studying Byzantine iconography, the current state of preservation of works and the associated creative process of both wall-painting ensembles and portable icons may be revealed. In Chapter 12, the usefulness of this approach in the detailed evaluation of the aesthetic and historic value of the artworks is discussed. The use of stratigraphic data concerning structural painting materials and techniques as applied to two major works of art is described in detail: (a) the frescoes of the Protaton Church (end of the 13th century), attributed to Manuel Panselinos, who is considered to be the chief exponent and a legendary icon painter of the Macedonian School and (b) the icon of the Mother of God Hodegetria, Church of St Modestos, Kalamitsi, Chalkidiki, Greece, a representative portable icon of the 16th century Cretan style of Byzantine art. The artworks were examined through the application of both non-destructive and micro-analytical methods including, on the one hand, digital photography and macrophotography, stereomicroscopic photography, ultraviolet fluorescent photography, IR reflectography, X-radiography, image processing and colorimetry (measurement and representation) and, on the other hand, sampling of small fragments followed by optical microscopic observation and digital capture of cross-sections, under polarized white light and UV light, selective staining and micro-Raman and microscopic Fouriertransform IR (m-FTIR) spectroscopies of polished sections. Chapter 13 focuses on the advantages and disadvantages of analysing corroded silver coins by means of energy-dispersive (ED) XRF compared to SEM / EDX and PIXE, for the identification of the coin’s mint by its chemical composition. The objects of investigation are Austrian silver coins of the 12th and 15th centuries. As most of the coins were found in soil, where they have

8


been buried for hundreds of years, corrosion effects influence the qualitative as well as the quantitative results that can be obtained. The different information depths of all three techniques mentioned above are a main point of discussion. Comparison of the Ag-Ka/Ag-L count ratios of the EDXRF intensities with SEM / EDX measurements on cross-sections outlines the inefficiency of quantitative non-destructive analysis for the investigation of corroded objects. In Chapter 14, the identification and trace analysis of pigments encountered in illuminated manuscripts is described by means of a combination of micro-Raman spectroscopy and total-reflection XRF. First, the analysis of the various components of an illuminated manuscript, such as the inorganic pigments, the dyes and inks, the binding material and the parchment substrate, is discussed in general. In the second part, the analysis of manuscripts from the collection of Raphael de Mercatellis and of the “Brevarium Mayer van de Bergh,” an illuminated 16th century prayer book, is described. In the latter description, attention is given to a procedure that allows us to differentiate between the different workshops that were involved in the manufacture of this manuscript. In Chapter 15, different examples chosen from various historical contexts (from Protohistory to the Post-Medieval period) and from various geographical areas (mainly Europe and the Indian world) are used to illustrate the information provided by the chemical analysis of historical glasses for trade and provenance studies on one hand, and for understanding the development and the history of sciences and techniques on the other hand. A long time before artificial glass was invented, pre-historic populations were using obsidian, a natural glass, to make tools. The importance of trace analysis to understand and reconstruct the trade and exchange patterns of this material during Neolithic times is briefly illustrated. During the Bronze Age, artificial glass, as glass beads, was the object of long-distance trade. Relationships between the chemical composition of these objects, their chronology and the production area of the raw material can be used to build a distribution model of glass. In a similar manner, the Indian glass trade and manufacture at the beginning of our era is studied, with emphasis on the trade between India and the Mediterranean world. In order to investigate the European recipes used for glass making in Carolingian times, lead isotopes ratio analysis has been used to identify the birthplace of these recipes and follow the distribution of glass products through Europe. In the Post-Medieval period, the Venetian and façon-de-Venice glass became very popular in various parts of Europe. By means of information on the major to trace composition, transfer of technology and recipes in 15 – 17th century Europe in various

9


stages can be followed, and the influence of the economic and political changes in the Low Countries on the local glass technology and workshop traditions is described. During the exposure of medieval stained glass used in window panes of cathedrals, churches and other historic buildings, weathering crusts consisting of gypsum (Ca2SO4·2H2O) and syngenite (K2SO4·CaSO4·H2O) are formed as crystalline corrosion products and mainly hydrated silica as amorphous material on the external surface of such glass objects. Chapter 16 discusses the manner in which these alternation layers can be characterized by means of several methods. After a discussion of the chemical exchange reactions that cause the weathering phenomena, analytical results obtained from specimens of medieval glass objects by SEM as well as SIMS and NRA are presented. Similar investigations were carried out on specimens from medieval enamel of the Burgundian Treasure of the Vienna Kunsthistorisches Museum. Additionally, the monitoring of in situ weathering tests, carried out on sample glass similar in chemical composition to medieval glass and enamel by means of atomic force microscopy (AFM), is described. In the final chapter (Chapter 17), the use of several analytical methods (together with their advantages and limitations) for the investigation and conservation of 16th century manuscripts endangered by iron-gall ink corrosion will be described. After a discussion of the fundamental chemical interaction that causes the mechanical strength of cellulosic materials to decrease dramatically due to the presence of Fe2þ in the ink, the results obtained by means of various analytical methods are described. This includes destructive and non-destructive investigations performed by SEM and electron probe micro-analysis, XRF spectrometry, inductively coupled plasma mass spectrometry, atomic absorption spectrometry, Mo¨ssbauer spectrometry and m-XANES. Also, the use of these techniques for optimization of a suitable conservation treatment for documents suffering from damage induced by iron-gall ink is described.

REFERENCES 1

2

10

Ch. Lahanier, G. Amsel, Ch. Heitz, M. Menu and H.H. Andersen, Proceedings of the International Workshop on Ion-Beam Analysis in the Arts and Archaeology, Pont-A-Mousson, Abbaye des Premontre´s, France, February 18 –20, 1985—Editorial, Nucl. Instr. Meth. Phys. Res., B14 (1986) 1. E. Ciliberto and G. Spoto (Eds.), Modern Analytical Methods in Art and Archaeology, Chemical Analysis Series, Vol. 155. Wiley, Chichester, 2000.

Introduction and overview 3 4 5

A.M. Pollard and C. Heron, Archaeological Chemistry. Royal Society of Chemistry, London, UK, 1996. D.C. Creagh and D.A. Bradley (Eds.), Radiation in Art and Archaeometry. Elsevier, Amsterdam, The Netherlands, 2000. R. Van Grieken, K. Janssens, L. Van ’t Dack and G. Meersman (Eds.), Proceedings of Art 2002: Seventh International Conference on Non-destructive Testing and Microanalysis for the Diagnostics and Conservation of the Cultural and Environmental Heritage, 2–6 June 2002, Congress Centre Elzenveld, Antwerp, Belgium, University of Antwerp (UIA), Antwerp, Belgium, 2002, 780 pp.

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Part I: Analytical Reference Section


Chapter 2

UV-, IR- and X-ray imaging Franz Mairinger

2.1

SCIENTIFIC INVESTIGATIONS OF WORKS OF ARTS AND CRAFTS

Works of art are the result of mental processes in material shape. The realization of an artistic idea in fine arts is bound to a proper choice of an artistic technique and appropriate materials. Appearance, technique and material are closely connected. For example, the soft “sfumato” transition between different hues in baroque paintings can only be achieved with (drying) oil paints [1], whereas the precise rendering of finest details of an object (like furs, hair, jewellery) in gothic panel paintings is reserved for special distemper techniques [2]. Similar considerations hold for all other branches of arts and crafts. These considerations prove that any work of (traditional) fine arts needs a material base, so the tasks and aims of scientific examinations are easily defined: questions of the material nature in context with material history and the state of preservation of an object can be answered. This includes: – The analysis of utilized materials. – The investigation of artistic techniques. – The investigations of ageing processes of ancient materials and modern industrial products (used also in conservation). To answer such questions two types of methods are applied: area and point examinations. These terms are more or less self-explanatory. Area examinations cover the states of the total or greater parts of surface of an object (including macro- and microscopic investigations). This fact meets the holistic approach of art historians. Aims of these methods are to make invisible or imperceptible surface states or the inner structure of opaque objects visible to the naked eye. Infrared, UV and X-ray radiation can serve for this purpose. The methods are non-destructive, in a strict sense no Comprehensive Analytical Chemistry XLII Janssens and Van Grieken (Eds.) q 2004 Elsevier B.V. All rights reserved

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samples are taken. Reliable statements about the material composition of the object are possible only in special cases (e.g., in radiography). Questions of the material composition or layer structures are the domain of point examinations. Among the variety of modern instrumental techniques of analysis that are currently available [3,4] non-destructive ones are preferred. But to answer questions concerning the structure of complex paint layers or the chemical nature of binding media in paints, sampling is unavoidable, a procedure that is always destructive. 2.2

APPLICATION OF ELECTROMAGNETIC RADIATION FOR THE EXAMINATION OF CULTURAL HERITAGE OBJECTS

For area examinations of works of art, five regions of the electromagnetic spectrum are of special interest: – radiation in the visible range (400 – 780 nm) for colour and black and white documentation by photographic emulsion and digital photography. – near or long-wave ultraviolet radiation (320 –400 nm) for UV-fluorescence and reflected UV examinations. The use of middle (280– 320 nm) and short-wave (200 – 280 nm) ultraviolet radiation is only permitted in special cases (minerals, gems), since it is harmful for many art objects. – near infrared radiation (780– 3000 nm) for depth examination of paintings, objects of graphic art or textiles. – radiation in the intermediate (3 –6 mm) and far (6– 15 mm) IR are used in IR-thermography (a topic that will not be discussed here) which is useful for conservation of historic buildings and multispectral aerial surveying. – X-rays for the radiography of opaque (metallic and non-metallic) objects. They are produced by X-ray tubes. A wide range of tube voltages (5– 400 kV) is used, depending on the nature of the objects. – g-rays emitted by radioactive isotopes. They are distinguished from X-rays by their source, rather than by their nature and are mainly used for metallic objects. A disadvantage is that their intensity is not easily controlled. 2.3

INSTRUMENTAL BASIS

The generation of images of objects by visible and non-visible radiation and their registration is a complex process that involves many factors including – lighting (resp. irradiation); – imaging by optical systems;

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UV-, IR- and X-ray imaging

– sensing; and – registration. In this chapter the instrumental basis of these four steps is discussed. 2.3.1

Light and radiation sources

The choice of the proper illumination resp. radiation source is of paramount importance for the imaging of objects by photographic and electronic means. The spectral output and physical intensity distribution of the illumination sources are important. Poor lighting design may obscure important details of the object. For the choice of a proper radiation source not only the spectral content of the source (broadband, narrowband, monochromatic) is significant; other questions are also involved: availability, costs, useful life time and also questions concerning the optical components: spectral sensitivity of the sensor, blooming, lag, drift or noise of the sensor at high or low lighting levels, limits of lighting power level for objects sensitive to light or heat and in case of UV-examinations: auto-fluorescence of any optical component. In the following sections, a short discussion of the various sources will be given. 2.3.1.1 Ultraviolet radiation sources There are many light sources that emit UV radiation, but not all of them can be used for UV examinations. Sunlight contains a fairly small portion of long and middle wave UV, as compared to the amount of visible radiation. The UV intensity depends on atmospheric and seasonal conditions and is therefore not reliable. Incandescent lamps (tungsten, tungsten – halogen) need not to be considered at all since they only produce a small quantity of ultraviolet radiation. Only wire-filled flash lamps are suitable for reflected ultraviolet photography. A continuous arc discharge between carbon or metallic (Fe) electrodes provide strong UV-emissions, but it is difficult to maintain a constant output. These sources require an enclosure in a light tight box and produce harmful vapours and heat. Gas discharge lamps are at the present state of development the most preferable sources of UV. Mercury high- and low-pressure lamps, fluorescent tubes, xenon arcs, electronic flash lamps and metal halide lamps belong to this group.

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All gas discharge lamps have a so-called falling characteristic, i.e., their internal electrical resistance decreases sharply with rising temperature, so that the current flow increases rapidly; without protection the lamp is destroyed within fractions of a second. Therefore, they can be operated only with a ballast; chokes are used for this purpose (and starting gear). Nearly all of these sources also emit visible radiation with high intensity, so that the use of appropriate lamp and/or camera filters is obligatory. If these UV sources are used with reflectors, it should be kept in mind that the average reflectivity of silver coatings rapidly drops off at wavelength shorter than 400 nm (9% reflectivity at 320 nm), whereas bare aluminium (and also rhodium) retains its high reflectance down to 200 nm.

Mercury vapour lamps Mercury vapour emits in an electric discharge or arc a large number of extremely bright spectral lines in the visible and ultraviolet region (Fig. 2.1). The intensity distribution is strongly dependent on vapour pressure and temperature. Two types of mercury vapour discharge lamps are commercially available: high-pressure (long-wave) and low-pressure (short-wave) lamps. Such lamps are produced in power ratings of a few watts to many kilowatts with pressure loadings from 1 mm Hg to 200 bar. Most manufacturers produce both types. In the following sections the production palette of Philips is used as representative example.

Fig. 2.1. Spectral lines of mercury vapour.

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High-pressure lamps High-pressure lamps consist of a small quartz tube in which mercury vapour is generated under a high pressure. An outer envelope made of dark Wood glass, which absorbs the visible radiation, surrounds the burner. The main output in the UV is at 366.3 nm, some middle (334 nm) and short wave (313 nm) ultraviolet is also emitted; 20% of the total emission consist of a continuous spectrum. As an example for this lamp-type the PHILIPS HPW (125 W) is chosen. Its spectral power distribution is shown in Fig. 2.2b. The HPW lamp has the common E 27 base (Edison thread) similar to domestic lamps. A ballast gear (choke) is required for operation; there are no restrictions in burning position; a warm-up period of several minutes is required for full intensity. When the lamp is turned off, re-ignition is possible only after a longer cooling period. The average useful lifetime is 6000 h. This lamp type is advantageous in illuminating smaller objects with high ultraviolet brightness. Low-pressure lamps and fluorescent tubes The peak output of low-pressure mercury vapour lamps is in the short-wave UV region at 253.7 nm. This radiation excites a strong fluorescence in some minerals and gems, has a strong germicidal action and a very painful effect on the eyes and the human skin (use of goggles and gloves is recommended).

Fig. 2.2. Spectral power distribution of (a) a Philips HPW lamp and (b) a Philips TL(D)/08 40W “black light” fluorescent tube.

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This radiation should not be used for the examination of light sensitive art objects. It is not transmitted by glass camera lenses. Philips produces TUV germicidal lamps with power ratings from 4 to 30 W. The UV emission of such sources can be modified by special phosphors. This is accomplished in fluorescent tubes. The wall of the tube is internally coated with an inorganic fluorescent powder, which converts the short-wave radiation into a long-wave UV radiation in the range of 320 –400 nm with a peak emission at 350 nm. Their spectral power distribution is shown in Fig. 2.2a. These “black light” tubes can be obtained in various lengths from 14 to 122 cm. They can be operated in standard fluorescent light fixtures with standard starter and choke. They are used to illuminate large areas with UV radiation. Their useful lifetime is over 2000 h. No eye protection is necessary. An example of this type is the Philips TL(D)/08 (40 W) “black light” fluorescent tube; it has a dark-coloured envelope which transmits the UV-A radiation and a small amount of visible light in the violet region. Electronic flash lamps and xenon arc lamps Electronic flash lamps are primarily designed for photographic work within the visible range, but they provide also a fairly intense emission in the near UV region. They contain a mixture of noble gases like xenon, krypton and argon and the actual UV output depends on the composition of this mixture. Some tubes are coated with a yellowish lacquer layer to give a better colour rendition by absorbing the UV part. These types are less suitable for UV photography. For UV examinations, the visible part of the emission must be excluded by using an appropriate short-pass filter (e.g., Wratten #18, or Schott UG 2) in front of the flash lamp. A problem in using electronic flash lamps is that since fluorescence cannot be observed during the very brief flash-time, a preliminary inspection with a continuous source is necessary. Nevertheless, this source is quite convenient for routine UV and fluorescence work, after working conditions have been established by test exposures. For feeble fluorescence the “multiflash-technique” is a convenient method: the room is darkened, the shutter of the camera is opened and a series of flashes is fired. High-pressure, continuous xenon arc lamps produce a sun-like emission with a nearly continuous spectrum encompassing the ultraviolet, visible and near infrared regions, as the diagram of the spectral power distribution (Fig. 2.3) indicates.

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Fig. 2.3. Spectral power distribution of xenon arc lamps.

They are used as a standard light source for colour measurements. Disadvantages are the high cost of the lamps and ballast gears and the lower UV efficiency compared with mercury vapour lamps. 2.3.1.2 Infrared radiation sources All hot bodies emit infrared radiation in a continuous spectrum. So all types of incandescent lamps (tungsten, tungsten –halogen) are excellent infrared radiation sources. At 3000 K, the filament temperature of a common 100 W lamp, the peak emission is at 950 nm. For infrared examinations it is not necessary to operate the bulbs at full mains voltage; a (small) voltage reduction will protect thermal sensitive objects from unnecessary radiant heat. It has been already mentioned that electronic flash tubes are also excellent sources for infrared photography. The above-mentioned “multiflash-technique” can be applied and prevents excessive heat strains for sensitive objects. Narrow-band, but feeble, incoherent sources are IR transmitting diodes, which are used in remote controls for electronic gears and for optical communications (in optical fibres). Their wavelength range lies between 800 and 1550 nm. It is fairly easy to build a homemade diode array for the illumination of medium-sized objects. Sony makes use of such diodes for the built-in IR illumination source for the “night-shot” option in their digital still cameras and camcorders. Since the bandwidth is small, about 10 MHz, IR transmitting diodes can be used in imaging spectroscopy of small object areas as a cheap replacement of expensive narrow-band interference filters.

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2.3.1.3 X-ray and gamma sources X-ray sources The mechanism of production of X-rays is well known, so it will be discussed only briefly: they are generated in a tube by accelerating electrons onto a target material (see Chapter 4 for more details). The character and intensity of the generated polychromatic radiation (bremsstrahlung) depend on the applied tube voltage. The total X-ray intensity of the continuous radiation Icont is given by: Icont ¼ AiZV m

ð2:1Þ

where A is a proportionality constant, i, the tube current, Z, the atomic number of the tube anode, V, the tube voltage and m, a constant with a value between 2 and 5, depending on the voltage and the filter type used. Since for maximum yield, Z should be as high as possible, for radiography the high melting tungsten (W) is the best choice. There are many X-ray generators commercially available but the choice of an appropriate machine affords consideration, it depends on the type of work to be done [5,6]. An X-ray tube needs for operation two power supplies: a (low voltage) filament supply and a high-voltage anode supply. The tube current is controlled by temperature changes of the filament. This is usually accomplished by a variable-voltage transformer which energizes the primary of the filament transformer. The high-voltage supply consists of a step-up transformer, and an autotransformer for the adjustment of the primary voltage of the former and for fixed machines a rectifier. Some industrial X-ray tubes are designed for the direct application of an AC high-voltage (50 or 60 Hz); in this case, the X-ray tube acts as its own rectifier since the tube current only flows during the positive half-period. Modern mobile X-ray machines generate their own AC voltages with higher frequencies than 50 (or 60) Hz; their X-ray output is much higher than that of 50 Hz half-wave machines. Seifert (Agfa) produces such middle-frequency generators up to 260 kV. The various machines can be roughly classified according to their maximum voltages. But it should be noted, that it is impossible to cover a voltage range from 5 to 300 kV with one machine. There is not only an upper voltage limit, but also lower one. The efficiency of a 300 kV generator at a tube voltage of 20 kV is near zero. This is caused by the penetration factor (transparency): at low voltages a dense cloud of electrons (space charge) is formed around the cathode, which cannot be drawn away by the anode, so that no additional electrons can leave the cathode.

22


For that reason two different machines must be acquired if radiographs of objects of low absorption (e.g. organic materials such as paper or parchment) and high absorption (metal items of greater thickness) should be made. For the best definition in a radiograph the size of the focal spot of the X-ray tube is an important factor. To minimize the geometric unsharpness, it should be as small as possible. Fine-focus tubes usually have focal spots of 0.5 £ 0.5 – 1 £ 1 mm2. These small areas are only possible at voltages up to 100 kV. There are also micro-focus tubes with spot sizes down to 8 £ 8 mm2 available. Since nearly 98% of the electric energy applied to the tube is converted into heat, the anode resp. the target must be cooled; oil, water or air are used as coolants. The continuous operation time of oil-cooled tubes is normally limited to a few minutes, than a cooling period of twice the operation time is necessary. An other essential point is the material of the tube window; it should have a low absorbance for soft X-rays and a high mechanical rigidity. Thin (0.5 mm) beryllium sheets are the best choice. This is important for a high contrast in radiographs of object with low absorption. Tubes with a voltage range up to 300 kV have to withstand large loads, have much larger focus spots (2.5 £ 2.5 – 6 £ 6 mm2). In this case long source-film distances will aid in showing finer details (with increased exposure times). Table 2.1 is meant TABLE 2.1 Typical X-ray machines Maximum voltage (kV)

Type of objects and thickness limits

Screens

Focal point size

5–50; 8–100

Paper, parchment, textiles paintings on canvas and wood, wooden sculptures, thin metal sheets (iron, brass, bronze) Iron (up to 40 mm), bronze (30–35 mm), gold (,0.5 mm) objects, rock (marble up to 150–200 mm) Iron (50 –60 mm), bronze (30–40 mm), gold (0.6 mm) objects Iron (up to 80 mm), bronze (50–60 mm)

None

0.1 £ 0.1 –1 £ 1 mm2

Lead foil fluorescent

2.5 £ 2.5 mm2

(25–) 200

260 –300

400

Lead foil

Lead foil

23

Franz Mairinger

to be a rough guide for X-ray machines; it lists voltage ranges and some possible applications. Gamma-ray sources Radiography with gamma rays has several advantages. The radioactive sources are simple and compact and have no need for external power. The latter is important when objects such as monumental outdoor bronze sculptures are to be examined. In contrast to X-ray machines that emit a broad continuous band of bremsstrahlung, gamma ray sources emit one or a few gamma lines with discrete wavelengths, i.e., they are equivalent to monochromatic radiation sources. Their energy is given in kilo- or megaelectron volts (keV, MeV). There is no defined correspondence between monochromatic and polychromatic radiation. As a rule of thumb: the penetration of monochromatic radiation expressed in keV corresponds approximately to that of polychromatic radiation expressed in kV multiplied by 2. So the monochromatic radiation of 60Co at 1200 keV will have similar penetration properties as that of polychromatic radiation emitted by an X-ray tube operated at 2400 kV. Compared to X-ray tubes the intensity of gamma-ray sources is rather small, so that long exposure times are required. As can be seen in Table 2.2, quite a few radioactive isotopes with lines of very different energies can be used in gamma radiography. Gamma-ray sources lose activity according to their half-lives; this necessitates more or less frequent adaptation of the exposure time. Another disadvantage is the considerable hardness of their radiation which causes a rather low subject and film contrast. TABLE 2.2 Radioactive isotopes used in gamma radiography Radioactive element

Symbol

Half-life

Specific g-radiation constant (R m2/h Ci)

Energy in MeV (number of lines)

Half-valuelayer in lead (mm)

Caesium 137 Cobalt 60 Iridium 192 Selen 75 Thulium 170 Ytterbium 169

137

30 a 5.3 a 74 d 118.5 d 128 d 32 d

0.35 1.30 0.48 0.203 0.0025 0.125

0.66(1) 1.17–1.33(2) 0.3 –0.6(,10) 0.066–0.4(9) 0.052–0.084 0.063–0.308

8.4 13 2.8

24

Cs Co 192 Ir 75 Se 170 Tm 169 Yb 60

0.88


2.3.2

Imaging

In an abstract sense optical images are non-uniform 2D patterns of brightness. In the ultraviolet, visible and infrared regions of the spectrum, these patterns are generated by lens systems, bearing in mind that common camera lenses are corrected for the visible spectrum. For registration, the images are projected on the surface of a sensor. The latter can be a photographic emulsion or an electronic detector with sensitivity to radiation of the appropriate wavelength range. 2.3.2.1 UV optics UV illumination is used for surface investigations of works of art in two different ways: for reflected UV-photography and for exciting UV-fluorescence. Photographic documentation of the latter presents no problems or restrictions since the fluorescence is in the visible range. Difficulties are associated with the recording of UV radiation reflected by an object. Optical glasses, depending on their composition, feature a fairly low transmission for wavelengths shorter then 335 – 360 nm. Thus, most custom cameras can be utilized for reflected UV photography with the 365 nm Hg line. Only modern high-refracting rare-earth glasses (e.g. lanthanum flint) have a transmission that drops sharply already at a wavelength a little shorter than 400 nm. Other difficulties may arise from the anti-reflection coating and the cement of modern lenses. Some of them exhibit quite a strong fluorescence when irradiated by UV. They produce glare light that deteriorates the contrast of the image. There is a rule of thumb for checking the antireflection coating: in daylight the lens surface appears always slightly coloured. Lenses with bluish or purple reflections exhibit normally less fluorescence than tinges of green, brown or amber. Dust particles also exhibit fluorescence, so that the lens surface should be cleaned thoroughly. Multi-layer anti-reflection coatings suppress reflections only in a relatively narrow band of wavelengths. With camera lenses the coating is usually optimized for green light; for UV radiation the (internal) reflectance may increase considerably. A further point is that the chromatic aberrations of camera lenses are corrected for the visible range; in the UV region their focal length is somewhat shorter. Since the focusing is done in the visible region, however, the UV image may be slightly blurred. Decreasing the lens aperture can compensate for this.

25

Franz Mairinger

In general, before a commercial camera is used for UV-examinations, test exposures are necessary. For short-wave UV documentations down to 200 nm, true UV lenses must be employed. They are made of fused quartz or synthetic fluorite, are corrected in the range between 220 and 1000 nm and are fairly expensive. Nikon produces a UV-Nikkor 4.4/105 mm lens and Zeiss a UV-Sonar 4.3/105 mm lens. 2.3.2.2 IR optics In contrast to the UV region most optical glasses and optical cements transmit near IR up to 2600 nm freely. Only for the long-wave IR radiation, materials such as fused silica (185 nm – 4 mm with an absorption peak at 2.7 mm), germanium (1.8 – 25 mm) or sapphire (150 nm – 6 mm) must be utilized. IR thermography cameras have silicon lenses that are opaque to visible light. Similar to the long-wave UV, any camera lenses that work in the visible will work satisfactorily in the 1000 – 2500 nm band if some precautions are observed that were in part discussed in Section 2.3.2.1. Since the focal length f of a lens increases with increasing wavelength by a factor of f/200 – f/300, an image focused in the visible is slightly out of focus in the IR range. In infrared photography this can be reduced by focusing through a (deep) red filter or by using a smaller aperture, but it should be remembered, that diffraction effects increase with wavelength and plays a significant role in the IR region. Some 35 mm camera lenses have special infrared markings (red dot or line), but these are only helpful up to 1000 nm. In general the resolving power of camera lenses decreases in the infrared, since the spherical and chromatic aberrations increase. Lenses with apochromatic correction have a better IR performance than achromatic ones. Very often coated lenses feature a reduced transmission in the near IR; e.g., the Nikon Micro-Nikkor 105 mm lens transmit in region between 750 and 1000 nm only 50% of the intensity at 600 nm [7]. All these effects and the penetration of IR radiation into the surface of many objects can contribute to the often observed blurring of the IR images. 2.3.2.3 Filters In this section the various types of filtering devices for the infrared, ultraviolet and X-ray region are discussed. For the photographic and electronic recording of invisible object states, filters must be applied between object and recording device since most of the latter are sensitive to unwanted spectral regions, in

26


order to avoid a superposition of the various images. The use of filters in X-ray radiography is a special field, which is also discussed. Filters are passive devices; they can attenuate light resp. radiation wavelength-invariant (neutral-density filters) or wavelength-selective by absorption, interference, or selective scattering. They can be utilized in front of camera lenses as part of the imaging system or in front of light (radiation) sources. Different materials are employed: – Dyed gelatine filters can be used for image-forming work as well as for lighting (at low power levels) in the UV, visible and infrared region. The attenuation occurs by selective absorption. They are the least expensive filter type, but are easily scratched, sensitive to moisture, fingerprints and temperatures above 50 8C. Kodak (Wratten) produces this filter type for a wide spectral range. Coloured plastic filters have similar properties but have more defects. – Ion-coloured glass filters consist of a solid solution of inorganic salts or elements in a glass matrix. They have a much higher scratch and temperature resistance; therefore, they can be utilized in front of radiation sources at high power levels. Such filters are manufactured, e.g., by Schott [8] and Corning. Also available are glass filters that attenuate radiation by colloidal scatter; they are produced by controlled annealing. During this process microcrystalline nodes are formed, which scatter and absorb certain spectral regions. Ruby glass is an example of this type of filter. – Interference filters are produced by carefully controlled vacuum deposition of non-conducting transparent materials with different indices of refraction or metallic layers on top of a glass substrate (up to 20 layers). They are used as narrow-band or edge filters. – Infrared mirrors are a special type of interference devices. They have a wavelength-sensitive surface. There are IR-reflecting or “hot” mirrors: they reflect infrared (heat) and transmit most of the visible spectrum. Their counterpart are IR-transmitting or “cold” mirrors: they transmit IR (800 nm – 2,5 mm) and reflect the visible part of the radiation. They are used effectively for lighting purposes. As far as the spectral behaviour is concerned, a distinction between the following filter types can be made: – Band-pass filters have a single transmittance band flanked by two rejection bands.

27

Franz Mairinger

– Edge filters have a more or less abrupt border between a region of high transmission and an area of rejection. Filters based on absorption and on interference are used for this purpose. When the transmission occurs in the region of shorter wavelengths, the device is designated as a short-pass filter. The counterpart, who transmits longer wavelengths and excludes shorter wavelength is called a long-pass filter. – Narrow-band interference and line filters isolate very narrow bands (down to a few nm) of radiation. They are used for critical applications such as imaging spectroscopy. – Dichroic filters are used when the transmittance band corresponds to a spectral band within the visible spectrum. The same filter has a different colour whether the light is transmitted or reflected. Such a filter can pass, e.g. the blue band and reflects the yellow band (additive mixing of red and green light). Sets of such filters are used for colour separation (RGB or CMYK) or image segmentation work. Filters for UV-examinations In Section 2.4.1 it will be discussed in detail that there are two different methods of using UV-radiation for examinations: the reflected-UV-method, analogous to ordinary photography, and the fluorescent-light-method where objects are irradiated by UV and emit visible fluorescent light. Two different types of filtration are to be applied. 1. For the reflected-UV-method, short-pass filters resp. band-pass-filters are utilized. They transmit (long-wave) UV radiation and reject visible light. They are also called exciter filter and are applied either in front of UV sources or in front of the camera lenses. In the latter case the filter must be well polished. For the long-wave UV these filters are made of a special barium-sodium-silicate glass tinted with 9% nickel oxide (called Wood glass). A filter of this type is incorporated in the HPW-lamps of Philips. For short-wave applications special tinted quartz filters are available (e.g. Schott UG 5). Data of some UV exciter-filters are shown in Table 2.3. Figure 2.4 shows the transmittance curve of the Schott UG1 filter that is especially suitable for the selection of the 365 nm Hg-line; this filter transmits also in the infrared region, so it could be used as a long-pass filter for IR examinations. This IR transmission may be suppressed by adding an appropriate band-pass filter such as the Schott BG 39 heat reflection filter. 2. For the fluorescent-light-method, a long-pass filter, also called barrier filter, is placed in front of the camera lenses. It absorbs UV that is

28

TABLE 2.3

Manufacturer

Designation

Filter-type

Kodak Corning Glass #5840 Corning Glass 9863 Schott

18 A (2 mm) CS7-60 (2 mm) CS7-54 (5 mm) UG 1

Band-pass, Band-pass, Band-pass, Band-pass,

Schott Schott

Transmission band (nm)

Remarks

glass glass glass glass

310 –400 310 –400 250 –380 310 –400

UG 5

Band-pass, ion-coloured glass

240 –480

UG 11

Band-pass, ion-coloured glass

260 –390

Transmits IR Transmits IR Transmits IR Transmits IR (710–850 nm and 2.4 – 4.4 mm) Transmits IR (660 nm –2.7 mm), for 254 nm Hg-line Transmits IR (690–750 nm)

ion-coloured ion-coloured ion-coloured ion-coloured


Exciter filters for UV-fluorescence

29

Franz Mairinger

Fig. 2.4. Transmittance of Schott UG 1 long-wave UV exciter filter (1 mm). T is transmission; R, transmission without reflection losses.

reflected or scattered by the object. The selection of the appropriate barrier filter depends on the colour of the excited fluorescent light and on the spectral purity of the UV-source. Gelatine filters with an edge near 410 – 420 nm are normally utilized. For critical applications glass – plastic compound filters (e.g., Schott KV 418) are the better choice. Table 2.4 lists characteristics of a selection of long-pass barrier filters. Some of the ioncoloured glass filters exhibit intrinsic fluorescence, when irradiated directly by UV. This can cause blurred images. If there is no violet or bluish fluorescence, any cheap light yellow gelatine filter (e.g., Wratten #9 or 11) can serve for this purpose. It should also be kept in mind that thick glass filters are a special type of optical flat. Since they displace a light ray laterally (image shift) without changing its direction (dependent on the filter thickness and incidence angle), a degradation of the image quality can occur.

Filters for infrared examinations For infrared examinations and documentation, the visible part of the spectrum must be excluded, since some infrared imaging devices (including infrared sensitive films) are sensitive to light. Without a filter the visual and the infrared image would be superimposed and the result would resemble a normal panchromatic recording. A long-pass filter that absorbs the visible radiation must be used in front of the camera lenses. These filters have a

30

TABLE 2.4 Barrier (long-pass) filters for photographic recording of UV-fluorescence Designation

Filter-type, material

Cut-on wavelength (nm)

Remarks

Kodak Kodak Kodak Kodak Kodak Schott

Wratten Wratten Wratten Wratten Wratten GG 420

395 410 420 480 510 420

Pale yellow Pale yellow Pale yellow Yellow Deep yellow Pale yellow slight intrinsic fluorescence

Schott

GG 495

495

Yellow, slight intrinsic fluorescence

Schott Schott Schott

LP 400 LP 430 KV 408

400 430 408

Interference filter Interference filter Free of fluorescence

Schott

KV 418

Long-pass, gelatine Long-pass, gelatine Long-pass, gelatine Long-pass, gelatine Long-pass, gelatine Long-pass, ion-coloured glass, annealed Long-pass, ion-coloured glass, annealed Long-pass Long-pass Long-pass, glass – plastic compound Long-pass, glass – plastic compound

418

Free of fluorescence

2B 2A 2E 9 12


Manufacturer

31

Franz Mairinger

deep red or black appearance. In an IR photography the cut-on wavelength should correspond with the sensitization of the emulsion. The Kodak Wratten filters #87 and 87 C are perfectly suited for this purpose. But even red filters (Wratten 25) can be utilized. Table 2.5 shows the data of some suitable filters. For infrared imaging spectroscopy narrow-band interference filters with a bandwidth of 10 – 15 nm are used. These (expensive) filters are commercially available in the range between 700 and 1550 nm in 10 nm steps by Schott and Electrophysics. For heat sensitive objects such as panel paintings, this technique has a severe disadvantage; high-power sources are needed for lighting, since only a small amount of the total radiant energy passes these filters. X-ray filters In radiography with X-rays, filters are used to reduce excessive subject contrast (and hence radiographic contrast) by hardening the radiation. Although in most cases the highest possible contrast is desired, there are certain instances where too much contrast is a definite disadvantage. This holds for metallic specimens having a wide variation in thickness or for paintings on canvas with a priming consisting of substrate layers containing lead white onto which thin paint layers were applied; with unfiltered radiation the radiograph would show in this case only the texture of the weave. Figure 2.5 graphically illustrates the process of filtering. The longer wavelengths (softer radiation) do not penetrate the filter so that the beam emerging from the filter has a higher portion of the more penetrating wavelengths (harder radiation). Overall, the total intensity of radiation is TABLE 2.5 Infrared long-pass filters Manufacturer

Designation

Cut-on wavelength

Remarks

Kodak Wratten Kodak Wratten Kodak Wratten Electrophysics Electrophysics Schott Schott Schneider (B þ W) Schneider (B þ W)

#87 #87 C #88 A LPF 750, LPF 800 LPF 1000, LPF 1500 RG 780 (3 mm) RG 1000 (3 mm) #092 (¼ RG830) #093 (¼ RG1000)

740 nm 800 nm 730 nm 750 nm, 800 nm 1000 nm, 1500 nm 780 nm 1000 nm (1 mm) 830 nm 1000 nm

Gelatine Gelatine Gelatine Glass Glass Glass, annealed Glass Glass, annealed Glass

32


Fig. 2.5. Effect of a filter in front of an X-ray source. The longer wavelengths are removed; the overall intensity of the beam is reduced.

reduced. Since such filters remove most of the wavelengths that would not be able penetrate the thicker portions of the specimen, overexposure of the thinner parts is avoided and scattering is reduced. Filtering is in fact analogous to an increase of the tube voltage, which causes a decrease in contrast. The net effect is determined by the nature of the individual specimen. The choice of the filter material depends on the tube voltage. For voltages up to 50 kV, a range that is used for radiography of non-metallic specimens (wood sculptures, paintings), aluminium filters of varying thickness can be employed; copper, brass and lead foils are utilized at higher voltages. A lead foil, mounted in close contact to the film will reduce not only the effect of scattered radiation from all sources on the film; at voltages above 100 kV, it also acts as an intensifying screen that increases the photographic action on the film by the emission of photo-electrons. 2.3.3

Sensor systems

In the previous sections the basic principles of lighting and imaging by lenses in the UV, visible and IR bands were discussed. This section is concerned with the registration of these visible and non-visible images projected onto the surface of a photon detector with finite area. The detection is a threshold

33

Franz Mairinger

process: any photon of energy greater than the threshold value will give rise to a detectable signal. There are two main groups of photodetectors: – photographic emulsions and – electro-optic image sensors. In photographic emulsions absorbed photons cause silver halide grains to become developable to metallic silver. Films can be utilized in the X-ray, UV, visible and near-infrared bands. Electro-optic devices rely on the photoelectric effect. The photon images, which have been focused onto the photosensitive surface, are converted by a vacuum tube or by a solid-state sensing array to electron images. These electronic images are reconverted to visible images. 2.3.3.1 Photographic materials The basis of common photography is the decomposition of the silver halides AgCl, AgBr and AgI by photons of appropriate energy to metallic silver. For photographic films the use of silver bromide suspended in gelatine as protective colloid greatly predominates. The spectral sensitivity of silver bromide ranges from the shortest X-ray wavelengths to the blue region of the visible spectrum. With the addition of some silver iodide the sensitivity is further extended into the blue-green. Yellow or red light is not sufficiently energetic to activate AgBr, although the heat of formation of silver bromide is DHf ¼ 2100:44 kJ; which would correspond to a luminosity factor of photons of a wavelength at 1390 nm. The yellowish colour of silver bromide indicates an absorption in the blue part of the visible region only. Vogel discovered in 1873 that certain dyestuffs, when added to photographic emulsion, extended the sensitivity to longer wavelengths. Research for sensitizing dyes stimulated this discovery and in the 1930s the first colour films and infrared-sensitive emulsions became commercially available. The theoretical sensitivity limit of 1350 nm was achieved with special dyestuffs of the cyanine-group. Photographic emulsions are still the standard of comparison for many other detector devices in terms of sensitivity (1– 10 photons), dynamic range (106) and resolution (pixel size 10 mm). In the following sections the properties of photographic materials for the different regions of the spectrum are discussed. X-ray films The use of X-ray films in radiography is still a very appropriate way to obtain and store a maximum of information. Modern X-ray films consist of a flexible

34


blue-tinted cellulose triacetate base, coated on both sides with thin (,20 – 25 mm) gelatin layers containing fine-grained silver halides in high concentration. This measure is due to the fact that X-rays are absorbed in such thin layers only to a very small amount; this holds especially for hard radiation. Thus the application of two layers increases film speed. When highest detail visibility is essential (such as in microradiography), films coated only on one side of the base are used. There are two types of X-ray films: medical and industrial. Medical films have a high speed (thick emulsion layers) to keep the necessary radiation dose for exposure low, but have much lower resolution than the low-speed industrial films. There are three different forms of packaging for X-ray sheet films: – sheet films: the sheets are interlaced or non-interlaced and must be loaded in cassettes or film holders. The blank films are used (in a darkened room) for radiography at very low tube voltages and for electron radiography; – envelope (day – light) packing: each sheet is enclosed in a light tight packing, having the advantage that no cassettes or film holders are necessary. – envelope (vacuum) packing between two fluoro-metallic foils, mostly containing lead (of 27 mm thickness, when manufactured by Agfa) or lead oxide foils (when manufactured by Kodak) for tube voltages .120 kV. The foils act as intensifier screens and as protection against scattered radiation. These films are available at different speeds; a lower speed corresponds to a higher resolution. Roll films (with widths up to 1 m and length up to 10 m) and large format X-ray sensitive paper are also available.

Films for reflected-UV-photography All photographic materials are sensitive to UV radiation down to as far as 230 nm, but the sensitivity of photographic emulsions is in most cases much lower than the rated speed, because the gelatin binder and the overcoat absorb UV, so that test exposures are necessary. Below 230 nm the speed is further reduced drastically [9]. For long-wavelength UV photography with common glass lenses, any black and white ortho- or panchromatic sensitized film can be utilized; the use of colour film has no advantage.

35

Franz Mairinger

Black and white and colour films for the visible wavelength range For the visible range, photographic materials of different sensitization are available. Unsensitized, orthochromatic (red-blind), panchromatic colournegative and colour-reversal films in all formats and sizes are commercially available. For the registration of UV-fluorescence in the visible wavelength range, black-and-white or colour films can be utilized, but colour films are the first choice, because they allow for the visual differentiation between colours of equal brightness but different hue that would be rendered by means of identical grey tones.

Infrared emulsions Two decades ago a great variety of infrared films with a sensitivity range between 850 and 1350 nm were commercially available. With the advent of electronic IR-sensors the assortment has been reduced to a few emulsions listed in Table 2.6. A material worthy of note is the Kodak Ektachrome Infrared Film (Type 2236). It is a 35 mm false colour daylight type reversal film, with one of the three layers sensitized to infrared up to about 900 nm. It was mainly used for aerial cameras to detect camouflage of military objects, in various crop surveys for mapping infected areas and in archaeology for photoarchaeological records. The film is very contrast-rich. It has three layers sensitized to green, red and infrared. Since all of them are sensitive to blue, use of a yellow filter (e.g., Wratten #12) is obligatory. Upon processing of the green sensitive layer, a positive yellow image is formed. The red sensitive layer yields a positive magenta and the infrared a positive cyan image. This film adds an infrared component to the visible record; thus, visually equal or similar colours are rendered in different hues, depending on their infrared behaviour. Table 2.7 lists the colour rendition modified by IR reflection of the object. It is also possible to sensitize common black-and white films to IR by immersing them in a highly diluted solution of IR-sensitizers such as xenocyanine followed by a rapid drying. The procedure is described in detail by Bru¨gel [10].

2.3.3.2 Electronic imaging detectors The input image for electronic imaging detectors may be formed in any band of the electromagnetic spectrum from the UV to the far-IR region.

36

TABLE 2.6 Commercial infrared films Kodak infrared high-speed type 4143

Kodak Ektachrome infrared type 2236

Konica infrared 750 black and white film

Film format

35 mm, Sheets 4 £ 500 ,þ138C or deep freeze With Wratten # 87C 200/248 (Incandescent l.3400 K) 300 –920 nm (,820 nm)

EU: 135, USA: 135, 4 £ 500 ,þ138C or deep freeze With Wratten #12: 100/218 (electronic flash lamps) 360–900 nm (,750 nm)

35 mm, rollfilm

Daylight, electronic flash, tungsten Wratten #25, 29, 70 (red filters) IR: 87, 88A, 87C 1 s (1/2 stop)

Daylight, electronic flash, tungsten Flash: Wratten #12 Tungsten: Wratten #12 þ CC20C þ Schott BG22 1/10 s (1 stop)

1 s (?)

D 76, D19, HC 110

Kodak Laboratories

D 76, DK 20, ID19

Storage Speed (ISO) Spectral sensitivity (maximum IR) Recommended light source Recommended filters

Reciprocity failure starts at (correction) Development

,þ138 or deep freeze With Wratten #25 32/168 (daylight) 400 –820 nm (750 nm) Daylight, electronic flash, tungsten Wratten #25, 29, 70


Film type

37

Franz Mairinger TABLE 2.7 Object-colour rendition of Ektachrome infrared film Object colour

Rendition (no IR reflection)

Rendition (with IR reflection)

Red Magenta Green Yellow Blue White Black (Grey) Infrared

Green Green Blue Cyan Black Cyan Black –

Yellow Yellow Magenta White Red Red Red Red

Their electric response varies with the level of irradiance and is proportional to the radiant input. There are four types of photoelectric sensors : – Photoemissive surfaces, used in vacuum tube cameras and image intensifiers. Photons cause the emission of secondary electrons, which can be accelerated, collected and measured or used to form an image on a screen. – Photoconductors, are light-dependent resistors. Their resistivity varies with irradiance. For example, the photosensitive surface of a vidicon camera tube is photoconductive. Photoconductors are passive devices that cannot produce an output signal directly; they only influence the electrical current of an external bias source. – Photovoltaic cells (photodiodes) generate (small) voltages on incidence of radiation. They do not need an external voltage source for their operation. – p– n junction devices are the most recent type of photodetectors and are based on photoconductive and photovoltaic phenomena. An electronic read-out device is integrated. UV detectors The basic principles of ultraviolet detectors are very similar to those for the visible range. They are based on the principles of photoconductivity or of photoelectric emission. Imaging detectors need additional components to record the spatial intensity distribution information. The response of silicon detectors decreases quite rapidly towards shorter wavelengths, as can be seen in Fig. 2.6. This is caused by a quick recombination of electron– hole pairs near the surface. Applying a thin fluorescent film of polycyclic organic phosphors (coronene, lumogen) to the surface of a CCD, similar to fluorescent tubes, can increase the sensitivity in

38


Fig. 2.6. Spectral response (Sl) and quantum efficiency (QE) of a silicon-CCD sensor.

the UV region [11]. Special UV-CCD arrays (thinned backside illuminated) as large as 4096 £ 4096 pixels are available, next to UV sensitive targets down to 200 nm for vidicon tubes. A wide variety of imaging photoemissive UV detectors has been developed. They consist of a photo-cathode (mostly alkali metals and their compounds) onto which the image is projected, and a phosphor screen that emit light when the accelerated photoelectrons impact on it. The resolution of such devices is rather poor. IR detectors Imaging (quantum) IR photodetectors operate similar to visible spectrum photo-detectors on the basis of the photoelectric effect. But in contrast to the visible range (390 –780 nm) where photodetectors are almost exclusively based on silicon, a great variety of semiconductor materials are used, since the IR comprises a range from 0.78 to 500 mm (with decreasing photon energies). Two classes of materials employed for IR quantum detectors: up to 8 mm intrinsic and for longer wavelengths (up to 150 mm) extrinsic semiconductor materials are used [11]. The spectral behaviour of intrinsic IR detectors is determined by their inherent bandgaps; outside this limit their sensitivity drops drastically. Two primary types are in use: bulk photoconductive sensors such as PbS, PbSe, HgCdTe (also called MCT—mercury cadmium telluride) and junction sensors made from Ge, GaAs, InAs, InSb or MCT (Table 2.8).

39

Franz Mairinger TABLE 2.8 Intrinsic IR semiconductor detectors Material

Typical operating temperature (K)

Peak wave-length (mm)

Usable range

Si GaInAs InGaAsP Ge PbS InAs Hg0.7Cd0.3Te PtSi

300 300 300 300 300 77 195 90

0.9 1.6 1.3 1.5 2.4 3.1 4.5 0.9

0.6 –1.1 0.9 –1.7 1.0 –1.6 0.9 –1.6 1.1 –3.5 1.8 –3.8 1.0 –5.5 0.8 –5.0

The performance of extrinsic quantum IR detectors is not so much governed by band gaps of the materials used, by rather by the type of the added doping compounds for the Ge or Si bulk substrate. Si:Ga, Si:As, Ge:Cu, Ge:Zn combinations are used. The extrinsic quantum detectors have their peak wavelength in the far IR region; they are used mainly in IRthermography and will not be discussed further here. Silicon detectors can be used in imaging applications in the spectral range 0.6 –1.3 mm with a sensitivity peak at 0.9 mm, corresponding approximately to that of IR-sensitized photographic emulsions. Lead sulfide (PbS) is a reliable photoconductor material. It is sensitive from 1.0 to 4.0 mm with a peak at 2.2 mm (Fig. 2.7). It becomes more sensitive when cooled, but also allows for stable operation at room temperature; it is employed for targets in IR-vidicons. Gallium –indium –arsenide (GaInAs) is used for imaging detectors in IR video cameras with a good sensitivity up to 1.6 mm. Sensors in this material can be operated at room-temperature. An important class of IR detectors are Schottky-barrier photodiodes. These devices are formed by depositing metals such as Pt or Pd onto the surface of a p-type silicon substrate, which (after a heat treatment) is mounted on a CCD. These platinum silicide (PtSi) focal-plane arrays are useful between 1.1 and 5.0 mm; however, they must be operated at low temperature (77 – 90 K). They have a linear response, but their quantum efficiency is quite low. X-ray detectors Real-time electronic X-ray imaging presented a difficult problem because most video cameras cannot be exposed to intense X-ray radiation without a

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Fig. 2.7. Spectral response of IR vidicons with PbS/PbO targets, manufactured by Hamamatsu.

severe degradation or even destruction of the image sensors. To avoid this, an X-ray conversion process with an X-ray sensitive phosphor that fluoresces in the visible and fibre-optics must be used. For X-rays with an energy above 20 keV, scintillation devices on the basis of NaI or CsI doped with thallium are used (in connection with video cameras). 2.3.4

Sensor subsystems

In this section the selection and the key parameters of solid-state and vacuum tube cameras for UV and IR documentation are discussed. For the choice of a particular camera not only the imaging performance is important, but also its physical and electrical characteristics. A key parameter is the video signal format; it provides a standardized transfer to equipment outside the camera by appropriate interfaces. There are two mutually non-compatible (colour) TV standards: PAL (CCIR) in Europe and NTSC (RS-170) in the US. They have different frame rates, scan lines, bandwidths and scanning frequencies. This is important to know for the use of monitors and A/D converters. Nowadays solid-state devices are more popular than tube cameras. There are many reasons for this: they are more or less maintenance-free, are free of geometric distortions and image drift or lag, do not suffer from image burn-in (in normal circumstances), have a better temporal stability and are

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lightweight and compact. However, vacuum tube cameras sometimes offer a degree of versatility that digital solid-state devices cannot match. Since camera tubes such as vidicons are analogue devices, their photo-surfaces and targets are nearly continuous in nature; the sampling is done only in one direction by the line raster, so the raster line number can be freely chosen for better vertical resolution, whereas solid-state imagers have well-defined pixels, which are sampled in both directions. Exposure and sensitivity control of tubes are also easier to manage. 2.3.4.1 UV video cameras Solid-state video cameras based on standard silicon CCD sensors have quite a low sensitivity in the UV region even in the long-wave band (Fig. 2.6); the quantum efficiency at 400 nm is only about 2%. As has been already mentioned, applying a thin fluorescent layer to the surface of the imaging CCD chip can increase the UV sensitivity; the film acts as a “converter” of UV into visible light. However, all these methods are rather cumbersome and costly. Vidicons of tube cameras for the visible with a normal Sb2S3 target are the better choice for work in the long-wave UV band. For the middle and short-wave ultraviolet special UV-vidicons with quartz faceplates are available (e.g., Hamamatsu N 983); their range extends from 200 to 700 nm. The horizontal resolution at centre is up to 900 lines. 2.3.4.2 IR video cameras Imaging silicon CCD chips in commercial digital still cameras and camcorders have their maximum sensitivity at 900 nm (Fig. 2.3) that would correspond approximately to IR photographic emulsion. This responsivity causes problems in the rendition of colours and grey tones in common photography. Hence most digital cameras are equipped with a builtin IR (cut-off) filter (e.g., Schott BG-38). Therefore, it is not possible to use these cameras for IR documentation. Some digital cameras (e.g., SONY DCS 828 with an 8 Mpixel sensor, and older models like DSC 707 and 717) and camcorders (SONY Mini DV and Digital 8) are provided with a special feature called night-shot. The IR-cut-off filter in front of the sensor can be mechanically retracted, so that near infrared recording (up to 1.1 mm) is possible. The image quality of the digital still cameras is quite good, that of the camcorders fair, so that this technique can be used as an alternative to infrared emulsions. An example of such an application is shown in Fig. 2.8. Hamamatsu, Oriel and Sony manufacture monochrome CCD cameras with increased IR sensitivity up to 1.3 mm.

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Fig. 2.8. Friederichsmeister, Wiener Neusta¨dter Altar, detail of the Predella. (a) Normal recording, (b) IR recording with Sony DRV 9, “night shot”, IR filter B þ W 092, captured with Sony DVBK-2000E Board.

For IR work up to 2.2 mm video cameras with special IR imaging sensors must be used. Following Van Asperen de Boer [12 – 15] this technique is called infrared reflectography. Both vacuum tube and solid-state cameras can be employed for this purpose. Infrared vidicons with a lead oxisulfide (PbS/PbO) target (photosensitive front plate) are still quite popular for the examination of paintings in musea. Their (IR) sensitivity maximum is situated around 1.9 mm and ends at 2.2 mm. A typical example for this type is the N2606-06 vidicon made by Hamamatsu. Its spectral response together with other IR vidicons is shown in Fig. 2.7. The horizontal resolution of vidicons depends on several factors such as object contrast and lighting level; it varies between 250 and 500 lines, i.e., rather low. These imaging tubes can be utilized after some adjustments in regular control units, but semi-professional ones with auxiliary electronic circuits (automatic gain control, video booster, contrast enhancement) give a better image quality [16]. Several manufacturers such as Hamamatsu, Ikegami, Quantex or Sony offer such devices. Vidicon cameras have several disadvantages. Features such as image lag, blooming or even burn-in at higher lighting levels, geometric and radiometric distortions, or thermal instability with a loss of image contrast are well known and frequently encountered phenomena. In the early 1970s IR sensitive solid-state imaging devices for military applications were developed [17,18]. On the basis platinum silicide (PtSi) focal plane photodiode arrays (FPA) with silicon CMOS read-out devices became available. When the sensor is cooled to liquid nitrogen temperature

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(77 K), radiation in the spectral range between 1.1 and 5 mm can be detected. When such cameras are equipped with a 1.1 – 2.2 band-pass filter, they are an excellent tool for the study of underdrawings in paintings. Sterling and thermo-electric cooling of the sensors is used (see Section 2.3.4.3). These camera’s are manufactured by Mitsubishi (Japan), Infratec, Thermosensorik (both Germany) and Inframetrics (USA) and are quite expensive. Cameras with InGaAs sensors are also quite attractive. They can be operated at room temperature (18 8 C) and are sensitive in the 0.9 – 1.7 mm band and are offered by Sensors Unlimited (USA). Rockwell (US) manufactures high-priced cameras with a MCT (HgCdTe/Al2O3) sensor with a cut-off wavelength near 2.5 mm. 2.3.4.3 Sensor cooling Several parameters of IR-sensors such as random noise, responsivity or spectral sensitivity are associated with temperature. Thus, cooling provides a better performance and is even obligatory for focal plane arrays based on PtSi-sensors. This can be accomplished by means of several methods. The simplest and cheapest way is the use of a heat sink in connection with forced air-cooling. But this method is not very effective and can be a source of noise. An other possibility is the use of dewar vessels filled with liquefied gases (He 4.2 K, H2 20 K, N2 77 K, dry ice 195 K). The primary disadvantage is that the dewar must be refilled quite frequently and that the system is bulky. A Joule – Thompson cryostat makes use of the sudden expansion of a highpressure gas through an expansion valve. Similar in function to an ordinary refrigerator is the Sterling cycle refrigerator. This device makes use of the compressing and expanding action of two coupled opposed pistons (moved by linear motors). This device is very efficient; it can cool sensors down to near liquid nitrogen temperatures rapidly. It is frequently used for cooling portable thermographic cameras. Thermoelectric (TE) cooling makes use of the Peltier effect. Modern Peltier modules utilize semiconductor materials (such as BiTe); the elements are formed into large arrays (TE-modules) that can be mounted thermally in series. Such multi-stage devices can cool down sensors to about 2 1008C and allow a precise electronic temperature control. They are very reliable and are also used for cooling CPUs in computers. 2.4

SURFACE EXAMINATIONS

This group of examinations comprises methods, which reveal non-destructively surface states of an object invisible to the naked eye. Natural and

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Fig. 2.9. Methods of surface examination.

monochromatic lights as well as UV radiation are used [3]. Figure 2.9 provides an overview of these methods. The registration of the results is achieved by photographic or electronic means. Since the methods and possibilities of macro- and micro-examinations are well known they will not be discussed any further, apart from a few remarks about the application of raking light. By choosing a large angle of incidence, the rendition of the surface relief can be exaggerated. This is a simple method to increase the legibility of engraved or embossed patterns or inscriptions. Surface imperfections of paintings such as cracks fissures or lacunae show up quite clearly, but also brush marks, impastos, primary cusps (scalloped weave deformations of canvas) and blocked areas can become (more) visible. 2.4.1

Surface examinations with ultraviolet radiation

The application of UV radiation for the examination of works of art and cultural heritage materials is an important and well-known tool [3,4,9, 19 – 21]. Since this radiation is in most cases reflected or absorbed already in the top layers of an object, mainly surface states invisible to the naked eye can be observed. This is due to two properties of UV: the excitation of visible fluorescence and the different reflectivity resp. absorption behaviour of UV compared to that of visible radiation. As already stated, here are two different methods of using UV: the photographic or electronic recording of the radiation reflected by the object and the photographic recording of the (feeble) visible fluorescence. Both methods are easy to perform with common photographic equipment; only some special filters are required. Since UV photography and fluorescence are complementary, both should be carried out in course of an investigation to facilitate the interpretation.

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2.4.2

Instrumental techniques for UV-fluorescence photography

Many inorganic and organic materials subjected to UV irradiation will give emit fluorescent light (of low intensity) in the visible range; it can be registered by common photographic emulsions. Its colour may comprise the entire visible spectrum; violet, blue and green hues are frequently observed. The object reflects also a considerable part of the incident UV radiation. For registering the visible fluorescence only, barrier (long-pass) filters such as Wratten 2B, 2E or Schott KV17 that exclude the reflected UV are placed in front of the lens. The registration is done in a darkened room. Since many common materials such as cotton textiles, human skin, teeth, and even eyes fluoresce quite strongly, the operator should leave the room during the long exposure. Colour films are the first choice for registration of UV-fluorescence. A differentiation between colours of equal brightness (e.g., red-green) is possible, which a black and white emulsion would render in equal grey hues. The intensity of the fluorescent light is very low, so that excessively long exposures (up to 5 or 10 min) result. The long exposure times cause a reciprocity failure: the film speed goes down. For black and white films, this causes no difficulties since for professional films correction factors are listed. For colour (reversal) films, this does present a problem. The spectral composition of the fluorescence is similar to daylight; therefore, a daylighttype film should be used. Colour films have three emulsion layers with different reciprocity failure characteristics. At exposure times longer than 0.1 s an erroneous colour rendition results. At longer exposures, no correction is possible. There are two possibilities to solve this problem: either by employing a high-speed daylight film or by using a tungsten light balanced colour film with a conversion filter (e.g., Wratten 85B, the barrier filter; e.g., Wratten 2E in front of it). These films provide a nearly correct colour rendition even at longer exposures. 2.4.3

Instrumental techniques for reflected UV photography

The reflected-ultraviolet-method works analogous to ordinary photography: the UV radiation reflected from the specimen is registered. To achieve this, the camera lens is covered with an exciter (short-pass) filter that transmits UV only and rejects visible light; Wratten 18A or Schott UG 2 (2 mm) filter are recommended. The object is evenly illuminated with mercury vapour lamps or fluorescent tubes. The correct exposure time is determined by a series of test exposures. With these results, it is possible to

46


calibrate a conventional exposure meter, if the filter that is used for the recording is placed in front of the meter-window. Electronic flash lamps can also be used. If one flash is not enough, the multi-flash-technique is applied; the shutter is opened and a series of flashes is fired. 2.4.4

Application of UV-fluorescence photography

Although many inorganic and organic materials exhibit a specifically coloured fluorescence, the application of this method for identification is not reliable. Traces of active impurities (even dirt) may cause strong fluorescence. An example of such behaviour is the colourless mineral calcite. Its fluorescence can vary from red or orange to blue or violet depending on the deposit. The same holds for natural chalk. In paint layers the characteristic fluorescence of pigments [19,21,22] is masked by the binding media and varnish layers There are also substances that can quench the intrinsic fluorescence of materials. Among pigments, the material verdigris, a basic copper acetate, which was used in gothic paintings as a green glaze, quenches the fluorescence of the natural resins mastic and dammar [22]. The same holds for earth pigments such as ochres, siena earths or umber. Thus, the identification of a material on the base of fluorescence colours is not reliable. UV fluorescence is frequently used to examine the state of preservation of paintings and polychromed sculptures. Very often later additions such as retouches or over-paints appear as dark spots or areas on the greenish fluorescence of old varnish layers. Since with increasing age also these additions gradually develop primary fluorescence, after 100 years they are difficult to detect. Similar considerations are valid for the examination of signatures and datings on paintings. If they are under an aged varnish layer, they will not show up in fluorescence examinations. If they do show up, they are on top of the varnish layer and are later additions or gone over by a restorer. Even local cleaning of such regions with organic solvents is easily detected [23]. Generally speaking, the fluorescence of paint layers, which are always complex mixtures, is mainly determined by the binding media that were employed (drying oils, resins, egg) or by the (aged) varnish layers and not by pigments. This is due to the poor penetration ability of UV radiation, which is already strongly absorbed in the top layer. Only in watercolours, lean tempera binders or wall paintings the primary fluorescence of pigments can be observed.

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There are many other applications of UV fluorescence. Mechanical erasure, chemical bleaching, fading of inks or paints or bio-deterioration on graphic documents can be detected quite clearly. Microscopic cracks and other defects in bronze objects can be detected by spraying a fluorescent dye to the surface. The excess is wiped off carefully; the defects show up in longwave UV quite clearly. Examinations of porcelain [24], gems [25], minerals, dyed textiles, resin coatings or pigmented glazes on gilded grounds of paintings or on metal sculptures or the state of preservation of amber objects are other examples of applications of UV fluorescence. 2.4.5

Application of UV photography

When UV radiation strikes the surface of an object, three kinds of interaction are possible: – the incident radiation is reflected or scattered on the surface while the wavelength is unchanged. UV photography renders such areas, depending on their reflectivity, in light shades. In fluorescence recordings they appear black (or dark blue), since UV is absorbed by the barrier filter. – UV radiation is absorbed and transformed into heat. Such regions reproduce in UV and fluorescence photography as black or greyish, depending on the grade of absorption. – UV radiation is absorbed by the object and excites the emission of visible (coloured) fluorescence light. Such areas appear in UV photography as black. In this manner, UV photography allows for an additional discrimination between areas that appear black in fluorescence, i.e., between UV absorbing and reflecting regions. However, the interpretation of such recordings is very often difficult. There are fewer applications [26 – 28] for this method compared to UV fluorescence photography, but it is a useful technique for the examination of graphic documents. Iron-gall ink absorbs long-wave UV strongly without generation of fluorescence, so the legibility of faded, bleached or erased parts of handwriting can be improved considerably. Regions on paper or parchment where biodeterioration, caused by bacteria or fungi, has taken place, which seem to the naked eye untouched, show up as more or less greyish spots. Figure 2.10 illustrates such applications for a fragment of a manuscript [29].

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Fig. 2.10. Fragment of a Syrian manuscript Syr. 5 (Marc 6, 20–26, 27–35, Austrian National Library). (a) Normal recording, (b) UV-fluorescence, (c) reflected UV.

2.5

DEPTH EXAMINATIONS

Depth imaging investigations reveal in a non-destructive manner the internal structure of opaque objects or of complex strata under an opaque surface layer. They can also be used to test the structural integrity of components and assemblies. Two techniques are commonly applied: examinations with infrared radiation and with X- or g-rays (Fig. 2.11). The physical bases of these methods are quite different: the penetration ability of infrared radiation is based on its reduced scatter in turbid media, whereas the great penetrating power of X-rays is due to the high energy of the photons. Both procedures generate images that are invisible; they must be transformed by photographic emulsions, electronic detectors or fluoroscopic devices into visible pictures.

Fig. 2.11. Methods of depth examinations.

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Again, similar to UV and UV fluorescence investigations, both procedures are complementary: infrared examinations reveal details of the object that cannot be seen by radiography and this also holds true for radiographic investigations. 2.5.1

Depth examinations with infrared radiation

In contrast to UV radiation that is absorbed or scattered in the surface layers of an object, IR radiation quite frequently can penetrate layers or materials that are opaque in the visible region of the spectrum. This property makes such IR examination a valuable tool in archaeology, art history and conservation [4,19,30]. There are two possibilities of use: – The absorption of infrared radiation of (coloured) substances and materials differs very often from their behaviour in the visible, so that two pigments in a paint layer of the same hue are rendered differently by means of infrared registration. The same holds for optically alike additions applied to an object during restoration. – Quite a few opaque materials such as paper, parchment, wood, human skin and turbid media such as fog, haze, paint layers and printing inks transmit in the near-IR region and become transparent. This ability permits, e.g. in certain cases the visualization of underdrawings executed on the (white) ground of paintings. The mechanism of these phenomena will be discussed briefly in the next section. The obtained invisible IR images are registered by photographic or electronic means, according to the wavelength region. 2.5.1.1 Interaction of near-IR radiation with turbid media A turbid medium consists of a homogeneous, transparent matrix and small particles dispersed within. Fog, smoke, paints, and colloidal liquid or solid solutions belong to such systems. The interactions of near-infrared radiation with these turbid media are basically the same as those of visible light. Reflection, refraction, dispersion, scattering and specific absorption are the determining factors for the transmission resp. the transparency of these media [10,12]. In case of paint layers, the opacity strongly depends on the ability for scattering (and the specific absorption of coloured pigments). An example of a matrix where scattering is the dominating factor is white paint layers. Their hiding power in the visible is exclusively due to scattering (and the thickness of the layer).

50


Scattering in paint layers is described quantitatively by the Kubelka – Munk theory. For white pigments the dominating factors are the scattering coefficients and the geometric shape of the particles. Apart from the geometric factors, the scattering coefficients depend in an intricate way on the wavelength of the incident radiation and on the difference of the refraction indices of pigment and medium [31]. Generally speaking, the transparency increases with the wavelength of the incident radiation and the decreasing difference of the refraction indices. Van Asperen de Boer [12] showed that for most pigments the maximum of transmittance occurs in the spectral region between 1.8 and 2.2 mm, which is inaccessible with photographic emulsions. Images within this range can only be made visible by video cameras equipped with imaging IR detectors. 2.5.1.2 Infrared luminescence The phenomenon of fluorescence occurs not only by interaction of UV radiation with matter; blue-green light can stimulate in many inorganic and organic materials, such as natural resins, parchment, leather, minerals, pigments and wood, a invisible near-infrared luminescence. The intensity is very low but it can be registered by IR photography. Bridgeman and Gibson [32] used this method for examinations of pigments, paintings, stamps and dyed textiles. 2.5.1.3 Instrumental techniques of infrared photography Infrared photography works similarly to ordinary photography. The IR radiation reflected (or transmitted) by the object is registered on a film sensitized up to a wavelength of 900 nm (e.g. Kodak High Speed Infrared). The recording is achieved by covering the camera lens with a long-pass filter that only transmits IR radiation and rejects visible light. Gelatin filters such as Wratten 87 and 87 C or glass filters such as Schott RG 780 are recommended. The focusing should be done with a red filter, since the focal length f increases in the IR approximately by 1/200f. The best radiation sources are incandescent lamps or electronic flashes. The lighting of the objects should be as uniform as possible in order to avoid hard, pitch-dark shadows, bare of contours. The reduced scatter of IR radiation is the cause of this phenomenon. For 3D objects, diffuse lighting must be applied. For small objects a light tent is recommended [33]. The correct exposure can be determined by conventional exposure meters after test exposures. It has been stated already that layers or thin sections of opaque materials such paper, parchment, paintings (on canvas), wood (up to 5 mm), textiles,

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minerals (rocks) are often translucent in the near-IR region; transmission lighting is possible. The set-up for this technique is fairly complicated. Since only the transmitted IR is allowed to enter the camera, no stray light is permitted. A light-tight box must be used in a darkened room. The exposure times are extremely long (30– 40 min) [34 –36]. For the Kodak Ektachrome Infrared film, an electronic flash must be used for lighting, since the reciprocity failure starts at exposure times longer than 1/25 s. A yellow filter (e.g. Wratten No. 12) must be placed over the camera lens to absorb the UV, violet and blue parts of the spectrum. The film has a very small latitude ð 12 stop). So at least three exposures of one object (varied by a 12 stop) should be made. For excitation of infrared luminescence, a source of blue-green light is needed. A tungsten or an electronic flash lamp are used, equipped with a blue-green (glass) band-pass filter that has no transmittance in the infrared region (e.g. Schott BG 39 or DMZ 20). To register the infrared emission of an object the camera is loaded with a high-speed infrared film and an IR longpass Filter (Wratten No. 87) is placed in front of the lens. The exposures are about 6 min at f/5.6 when the object is illuminated by two 500 W light sources at a distance of 1 m. For small objects an (filtered) electronic flash lamp at a distance of 10 cm is the better choice. A single flash at a lens opening of f/2.8 is enough for most objects. 2.5.1.4 Instrumental techniques for infrared reflectography It has been stated that photographic emulsions are incapable of registering the important spectral region between 1.8 and 2.2 mm; instead, video cameras equipped with IR imaging detectors are employed. Both types— vacuum tube cameras and solid-state cameras—are used [11,12,37 – 41]. The advantages and disadvantages of vidicon cameras were already discussed. The main problem of all video cameras is their fairly low resolution. In order to detect fine structures such as drawn lines, only small, overlapping (100 – 150 pixels) sections of the object should be registered. For large paintings their number may go up to 100 (and more). Thermal instability and image lag and burn-in of vidicons present quite a problem for this process. In this respect solid-state cameras have the much better performance. At the start the optimum camera settings should be adjusted on places of highest contrast of the object and should not be changed during the entire registration process. The recording of so many overlapping frames affords a precise positioning system for the camera [42 – 44]. The single frames are digitized by a frame grabber and stored in a computer. After geometric and radiometric corrections

52


a mosaic program assembles them. Such programs were published by Wecksung et al. [45], Billinge et al. [42] and Mairinger and Papst [46]. 2.5.1.5 Applications of infrared examinations Infrared examinations are employed in many fields of research and conservation of cultural heritage objects; only a few applications in graphic arts, paintings and related fields will be mentioned here [47 – 49]. 2.5.1.6 Graphic arts The non-destructive infrared examination of works of graphic arts as drawings, prints and illuminated manuscripts on paper, papyrus and parchment answer many questions of art historians, historians and conservators. The legibility of manuscripts, documents, palimpsests and papyri can be improved considerably. Very useful tools for such tasks are the already mentioned digital still or video cameras with the night-shot feature. Inks and pigments that appear identical in the visible are rendered differently by IR (false colour) photography. Thus, brownish writing and drawing fluids such as sepia, bistre, (aged) iron-gall and bark inks, of which the differentiation is cumbersome, can be discriminated [48]. In this manner, texts rendered illegible due to mechanical erasure, chemical bleaching, charring, obliteration or other deteriorating procedures can be made at least partially visible. Especially of soot inks, even remains of it, are easily detected [50,51]. As in other cases, complementary examinations by reflected UV and UV-fluorescence must be carried out. The application of infrared reflectography for such tasks has severe restrictions. The sensitivity of the usual solid state sensors (GaInAs, PtSi) in the spectral range between 0.78 and 1.2 mm is very low (for PtSi , 0) and in addition all common inks, except soot and pigmented inks or paint layers, vanish completely at wavelengths greater than 1.3 mm. Nevertheless, a discrimination between the various writing fluids is possible by using a tube camera (equipped with an IR vidicon) and appropriate narrow-band filters. 2.5.1.7 Paintings Nowadays infrared examinations of paintings have gained nearly the same importance as radiography for the stylistic approach of paintings and for conservation problems. Two fields of applications can be mentioned: – the detection and differentiation of compositional alterations by the author and of later addition, as over-paintings, retouches and reconstructions, by another hand. This can be accomplished by infrared (digital) photography. IR reflectography is less suitable, since a great number of

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paint layers become transparent near 2 mm. The base for it is the different infrared absorption or reflectivity of materials, which look alike in the visible. This information is quite interesting for art historians and restorers. Infrared photography provides also a possibility to locate old retouches and over-paintings under old, discoloured varnish layers, which are hard to detect by UV fluorescence examinations, because they are masked by the strong luminescence of the varnish. – the revealing of underdrawings and the use of stencils in paintings [12,41, 52 – 56]. Most artists made a preparatory drawing of the intended composition with brush, pen, charcoal or black natural chalk on the grounded support of a painting. The character of such underdrawings varies between rough sketches and detailed drawings. Opaque paint layers cover the underdrawings in later phases of the work. Sometimes parts of the underdrawing are even visible for the (trained) naked eye due to saponification of the basic white lead, which was the only important white pigment till 1835. For white grounds (chalk, gypsum) and black colouring matters, these drawings can be made visible again by infrared reflectography and with some restrictions (blue and green areas) by IR photography. These drawings, untouched by later hands give vital information about the author, his workshop and the optional use of stencils for contemporary or later copies. The infrared visibility of underdrawing techniques has been discussed by Jennings [57]. 2.5.1.8 Other cultural heritage materials Infrared radiation penetrates quite often saline incrustations, resinous crusts and other forms of patina on pottery or glass and allows to see the buried patterns. This holds for other materials too. Dyestuffs used on textiles exhibit quite often a characteristic infrared reflectance or luminescence; e.g. restorations on tapestries are easily detected by means of infrared colour film. 2.5.2

Depth examinations with X-rays and gamma-rays

The ability of X-rays to penetrate opaque objects and to blacken photographic emulsions was used quite early after their discovery by Ro¨ntgen in 1895 [58,59] in various fields. In 1913 the physician Faber [60 – 62] made the first radiograph of a painting and obtained a patent for this method, which proved to be a hindrance for the general application in examinations of works of art.

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A further obstacle was a heated discussion about the alleged destructive actions of X-rays on paintings in the twenties of the last century. But soon after this debate radiography became an important, indispensable tool for art historians and archaeologists. In 1938 Wolters [63] published a book about the possibilities and importance of X-ray examinations in art history, where the methodology of reading and interpreting radiographs was developed and refined. So today X-ray examinations are among the most important methods for depth examination of art objects [6,19,63 – 67]. In the following sections the interaction of X-rays with matter is briefly described (see Chapter 4 and 5 for more detailed information), followed by practical and technical considerations and finally the application of radiographic investigations of paintings are discussed. It is of course impossible to present a detailed instruction for the production of radiographs and their interpretation; only a few general rules will be outlined. 2.5.2.1 Interaction of X-rays with matter—the attenuation laws When X-ray or gamma ray photons interact with any form of matter, some are transmitted, some are absorbed and some are scattered from their path of incidence; as a result of these processes, the incident beam is attenuated [68]. Behind the object a shadow image is generated that can be registered by means of fluorescent screens or photographic emulsions. The attenuation depends on the quality of the applied radiation, the thickness and the material composition of the object. The understanding of laws of attenuation is of paramount importance for the interpretation of radiographs. Quantitatively the attenuation is described by the equation: I ¼ I0 e2mL x where I0 is the primary intensity of the incident beam, I, the intensity of the transmitted beam, mL ; the linear attenuation coefficient (given in cm21) and x, the thickness of the material. mL is very often replaced by the more fundamental mass attenuation coefficient m, which is obtained by dividing mL by the density r: m ¼ mL =r Attenuation coefficients vary not only by the nature of the materials, but also by the wavelength of radiation (i.e. photon energy). This dependence can be expressed by the approximate equation: m ¼ cl3 Z4

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where c is a universal constant, l; the wavelength (of the monochromatic radiation) and Z, the atomic number of the absorbing material. For complex mixtures such as paint layers, m is calculated by adding the contributions according to the percentage of the present atomic species. The equation implies also that organic pigments and the common binding media (oil, distemper, glue, gums) in paints make only small contributions to the overall attenuation, whereas pigments containing lead, mercury or tin attenuate strongly already in thin layers. The absorption ability of various materials can be depicted by the size of their half-value layer d 12 : This is the layer thickness of materials that diminishes the intensity of the incident radiation by 50%. Figure 2.12 shows this value for several pigments and metal foils used in paintings and for ˚ gilding up to the 19th century. Values are given for three wavelengths: 0.2 A ˚ (for silver) and 0.71 A ˚ (for molybdenum). (Ka radiation of tungsten), 0.56 A An important factor for the quality of a radiograph is the radiation contrast behind the specimen, which governs the film contrast and thus the readability of the recording. Figure 2.13 illustrates the correlation between utilized wavelength, linear attenuation coefficient and contrast. The above considerations are strictly valid only for monochromatic radiation, but X-ray tubes produce polychromatic radiation that complicates the attenuation processes. A defined correspondence between these two types of radiation does not exist. This is partly due to the fact that for thicker specimens a hardening of the incident radiation occurs. After the soft

Fig. 2.12. Half-value layers d 12 (in mm) of metal foils and pigments. 56


Fig. 2.13. Attenuation of X-rays by a softwood step-wedge.

components are absorbed in the upper layers of the object, the radiation becomes more and more homogenized and gains a greater average penetration ability. For hard radiation (.100 kV) and thick specimens, scattering (incoherent and coherent) becomes the dominant factor for attenuation and is a problem in radiography with both X-rays and gamma rays. The intensity of the scattered radiation increases with thickness of the specimens, but is nearly independent of wavelength. The greater portion of scattered radiation that is blackening the film originates from the specimen under examination, but any part of the surroundings (e.g., walls, the floor, even parts of the film cassette) that is in the path of the direct radiation from the X-ray tube becomes a source of scattered radiation, which can affect the radiography and its contrast. It is, therefore, recommended that the diameter of the beam is adjusted to the dimensions of the object by the use of cut-out (lead) diaphragms and that the film is covered by a thin lead foil (see filters). In view of the above, it is possible to draw the following conclusions for practical work: – The attenuation depends on: (a) thickness of the object, (b) the quality of radiation (third power of l), and (c) the atomic number (third power of Z) of the absorbing material; – The attenuation is caused by absorption and scattering. Both depend on the atomic number, but the latter is nearly independent of wavelength. – The quality of radiation, the chemical composition, the thickness and the density of the object determine the radiation contrast.

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– Since the readability of a radiograph depends strongly on contrast and scatter, the softest radiation is preferred, which must, of course, penetrate the specimen to a certain amount. For hard radiation diaphragms and masks are used to minimize the scattered radiation. – For best definition and sharpness of radiographs, tubes with small focal spots are preferred; for soft radiation the best window material is beryllium. The film must be in close contact with the object. If this is not possible the tube-object distance should be increased. 2.5.2.2 Special radiographic techniques There are art objects where standard radiographic techniques give unsatisfactory results or fail. Examples are paintings where the support (mostly wood panels) bears on both sides paint layers so that the two radiographs are superimposed, or paintings on metallic supports (copper, iron). Other examples are radiographs of 3D objects such as sculptures with complicated internal structures and differences of materials, which are very difficult to interpret, the detection of watermarks in illuminated manuscripts or radiography of very small archaeological objects (jewellery). For such objects special radiographic methods must be applied.

Stereoscopic techniques in radiography Visual 3D depth sensation is perceived because the two human eyeballs are laterally separated by about 67 mm. So two slightly different images are produced, which are fused into a 3D impression by the visual process. A similar impression is obtained when a binocular viewer presents a pair of stereoscopic photographs to the eyes. The same technique can be used in radiography of 3D objects. The technical layout for it is presented in Fig. 2.14. Two slightly different images are taken either by moving the source or the object relative to each other. Three conditions must be fulfilled for a 3D perception of the object: – Both film sheets are in the same plane. – The central ray is always orthogonal to the film plane. – The film-focus distance must be the same for both exposures. The two radiographs are posed on two separate illuminated screens and viewed with a mirror stereoscope.

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Fig. 2.14. Technical layout for stereoscopic radiographs.

Tomographic techniques Tomography is a special technique that provides a (relatively) distinct image of a selected plane in a 3D object. Originally, it was developed for medical radiography, often termed “body-section radiography”. Since about 1980 it was replaced by computerised tomography (CT). Hounsfield [69] developed in the 1970s a new tomographic technique, which he called computerised transversed axial tomography (CAT scanning). In the first generation of these instruments a rotating X-ray pencil beam, which was collinearly connected with a detector, was scanned in small steps over the object to be examined. In this arrangement, source and detector are mounted on a circular frame, called the gantry. The attenuation data of each step are stored in a computer and special algorithms generate an image of the chosen section.

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CT examinations permit to obtain radiographs of any selected plane of 3D objects without any interference from other layers. A basic requirement is that the beam must penetrate the specimen with sufficient intensity, since otherwise artefacts result. Medical CT instruments use tube voltages between 120 and 130 kV. They can be utilized for non-metallic art objects [70 –72]. For technical examinations CT instruments with tube voltages up to 420 kV are available, which can even be used for study of bronze sculptures, e.g. to determine their wall thickness and casting defects [73]. Since the range of grey values in the image can be chosen, even a core, if still present, can be made visible [74,75].

Microfocal radiography In standard radiography the film should be as close as possible to the object (scale 1:1) and the source-film distance comparatively long in order to minimize geometric unsharpness caused by the (effective) size of the focal spot of the X-ray source. This is not a very satisfying technique for small archaeological objects such as earrings, necklaces or needles [76]. Since about two decades, commercial microfocal tubes are available. Their focal spot has an effective size in the range down to 4 – 8 mm, which makes these tubes the equivalent of point sources. This allows a direct enlargement of the radiographic shadow picture by placing the object near the focal spot and making the distance of the film (or the detector system) as large as possible. Additionally, the intensity of scattered radiation is reduced in this manner. Tube voltages up to 200 kV are possible in some microfocal tubes, so that metallic objects can be examined [77].

2.5.2.3 Electrons as imaging medium Electrons also are able to blacken photographic emulsions and can be used for radiographic examinations. There are two groups of methods, which can be combined: – Electron radiography and beta-radiography. Both methods apply an external source of electrons, which can be transmitted through thin objects and are registered by photographic emulsions. – Auto-electron-radiography. Here, the object itself is the source of the electrons; they are generated by irradiating an object with hard radiation. This method works only when elements with a high atomic number are present.

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Electron- and beta-radiography For electron radiography the object is brought into close contact between a thin lead foil (20 mm) and an X-ray film having an emulsion only on one side of the base. This “sandwich” is wrapped in a light tight envelope [78] (Fig. 2.15a). The radiation is applied to the foil side where photo- and Comptonelectrons are produced. The kinetic energy of the electrons that determines their penetration power, depend on the applied tube voltage. Typically voltages in the 100 – 400 kV range are used. The electrons are able to penetrate thin objects such as paper or parchment and blacken the film. This is an excellent method to register watermarks of paper on tightly printed pages where transillumination gives unsatisfactory results. For beta-radiography a foil coated with a beta-emitter (mostly 14C) serves as (low intensity) electron source. A limitation in this respect is that the energy of the emitted electrons cannot be varied. The foil is brought in close contact with the object and the film, after which exposures taking many hours or days commences. Mundry et al. [79] used this technique for the recording of watermarks. Auto-electron-radiography It has been already discussed that the absorption of X-ray photons is accompanied by an ejection of electrons from the inner shells of the atoms in the irradiated material. The probability of ejection of photoelectrons increases with the atomic number of the elements, corresponds to the increasing mass absorption coefficients of the heavy nuclei such as lead, mercury or tin in contrast to calcium, aluminium or silicon. All these elements are components

Fig. 2.15. The use of electrons as imaging radiation. Technical layout for (a) electron radiography and (b) auto-electron radiography.

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of inorganic pigments, have different emission properties and therefore result in different blackenings on photographic emulsions. The technical layout for such examinations of paintings is rather simple (Fig. 2.15b). Hard radiation is produced by an X-ray generator (150– 400 kV). The soft parts of the radiation, which would strongly blacken the film, are absorbed by thick copper or lead filters. The bare film is pressed tightly onto the surface of the painting, so that examination must be done in a darkened room or under red safelight. This method permits the radiography of paintings on copper or stone slabs. For very thin or weakly absorbing objects such as stamps, documents, illuminated manuscripts or engravings a combination of electron- and autoelectron-radiography is possible. The technical layout is the same as shown in Fig. 2.15a (lead foil/object/film). The registration (i.e., the film blackening) of paint layers may look quite differently, depending on the layer thickness and tube voltage, since the effects of absorption and emission processes are superposed. Paint layers may also absorb photoelectrons generated by the lead foil. Such areas are rendered transparent on the film [80]. These methods are very interesting for the non-destructive examinations of valuable illuminated manuscripts [79]. They can reveal important information on painting techniques and provide hints about pigment usage and later additions. 2.5.2.7 Compton X-ray backscatter techniques Conventional X-ray testing techniques require access to both sides of an object. For thick or strongly absorbing specimens this can be disadvantageous, since high tube voltages must be applied and the image contrast is low. The Compton X-ray backscatter technique requires only access to one side of the object. It is based on the detection of the radiation that is scattered by the specimen [80]. An X-ray pencil beam scans the chosen area in a raster fashion while solid-state detectors, located on both sides of the scanning beam record the generated Compton backscatter radiation. A series of 22 slices is generated, each made at a constant depth below the surface of the object. The depth resolution of each slice is 0.4 mm, almost free of the influence of the other slices while the maximum depth is 50 mm. After amplification and digitisation of the signals, a computer generates the images. Such an instrument is manufactured by Philips named ComScan. This technique has several advantages: – It is possible to examine wall paintings, since only access to one side of the object is needed.

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– Radiographs of paintings on metal or stone slabs are possible. – 3D sets of data are obtained without complicated reconstructions as in CT. – The image contrast is the result of local changes of radiographic density in the object, whereas in conventional radiography the mean value of the attenuation coefficient is determining for the contrast. Niemann and Roy [83] have shown the application of this method for the examination of sculptures. 2.5.2.4 Application and interpretation of radiographic examinations The trivial fact that any variation in image grey values (i.e., blackening on a film) is caused by a variation in layer-thickness or a change in chemical composition is a physically correct statement, but it is not very helpful for the interpretation of the structure of complex art objects, since a radiograph is a summation image of all (absorbing) layers. Without a thorough knowledge of material history, art technology, conservation techniques and the permanent comparison with the examined object, the possibilities of interpretation are limited. So the following considerations should not be understood as a foolproof guide for reading radiographs. Examples are presented for paintings and graphic arts, sculptures (wood, ivory, metal) and archaeological objects. A sophisticated systematic approach has been developed for the interpretation of radiographs of paintings [63]; the latter are relatively easy to produce so that there is a vast stock of recordings. Additionally, there are many reliable historical sources on technical aspects of painting available which assist the interpretation. For radiographs of sculptures the situation is worse, due to a more difficult technical layout and less knowledge about historical carving and casting techniques. On the other hand, for archaeological objects the situation is much better. The refined technical methods of non-destructive testing of materials are quite helpful for the interpretation of radiographs of historical arms, tools and other objects of daily life. Paintings Radiographs of paintings supply information about the materials used (e.g., support, pigments), the techniques employed, including peculiarities of specific artists and their workshops, compositional and dimensional changes, temporal changes and damages such as the effects of aging processes, cracks, paint losses, later additions by restorers, etc. Information on support materials. The structure and construction of wooden supports, like growth rings, number of boards, textures

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(puttied knots, cracks), tool marks, joining techniques, worm tunnelling and later accretions can be seen quite clearly even on panel paintings that bear paint layers on both sides. In gilded panels the gold foil is invisible in a radiograph because the thickness of the gold leaf is around 1 mm. The same holds for silver foils. The structure of textile supports can only be depicted when a ground layer was applied on it, since the X-ray absorption of vegetable fibres (such as flax, hemp, cotton) is too low. In most cases, a very clear (negative) impression of the ground layer will be visible. This is important for relined paintings, where the original support is covered. The structure and diameter of the yarn (hand- or machine twisted) and the weave and seams become visible. These properties can be characteristic for an artistic period or landscape. Other features that may show up are primary cusps, i.e., scalloped weave deformations near the borders of the canvas. Before application of the ground layer, the canvas is fastened by nails on a strainer. Thereby the weave is slightly distorted and the dried ground fixes these distortions. If the cusps are still visible in the radiograph, the dimensions of the painting were unaltered. Information on painting technique and layer structure. The interpretation of these aspects of a radiograph is a complex task that cannot be covered by brief considerations. Very often the radiographs are quite different from the surface image with respect to contours, distribution of lights and contrasts and even details of the composition (pentimenti). There are many reasons for this: the superposition of all layers, the difference of X-ray and optical absorption of pigments or the presence of later additions. Thus, the dark blue veil of a gothic Madonna where a blue pigment with low X-ray absorption was applied, may be rendered very light in a radiograph, when the artist has used a greyish underpaint consisting of a mixture of white lead and bone black, due to the strong absorption by the former pigment [84,85]. Optically very light coloured flesh parts (carnation) are rendered dark grey when gesso was used as a source of light and the colouring is done with thin organic glazes. The detection of damages and (puttied) losses in paint layers can be fairly easily performed. In contrast to UV fluorescence examinations the true size of the damage can be recognized. Cracks in the paint layer are normally rendered black; if they are white the area was over painted. On the other hand, the loss of glazes in the surface layers is very difficult to detect; for this purpose, a microscopic examination of the structure of cracks is better suited.

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Graphic arts The examination of painting techniques in illuminated manuscripts is quite similar to that of paintings. Soft radiation or electrons must be used for radiographs [29,86]. Sculptures Whereas radiographs of paintings and their art-historical interpretation are nearly ubiquitous, radiographs of sculptures are less numerous [87 –91]. There are several reasons for this: – Many and very different inorganic and organic materials have been used for sculptures: different kinds of wood (polychromed and un-polychromed), ivory, amber, wax, papier-maˆche´ (carta pesta), rocks (limestone, marble, sandstone, granite, alabaster, jade, artificial stone and others), burned clay, porcelain, metals and their alloys, composite materials (metals – enamel, Gold – ivory and others) and since the last century organic polymers casts. For all these materials quite different techniques (carving, modelling, casting, embossing) and various tools are applied so that only a limited number of historical sources are available compared with reports on painting techniques. – For radiographs of these materials, quite different types of X-ray sources are needed and the accompanying measures for radiological protection can be costly. – Sculptures are 3D objects with complex inner structures; the 2D projections and the inverse perspective make the interpretation complicated. Traces of tooling are difficult to detect in radiographs. This situation is now gradually changed by the application of modern industrial CAT-scanners. With medical scanners only sculptures made of organic materials (tube voltages usually are restricted to about 130 kV) and limited size (determined by the diameter of the gantry) can be examined. In the absence of systematic studies on the subject, the complex of answerable questions is limited. For wooden sculptures, information about the method of construction, the presence of cavities, the joining techniques employed for the various parts, the preservation of the polychromy and the presences of restorations is fairly simple to obtain. Even the depth of penetration of solidifying polymer solutions for damaged wood can be detected by adding (radiopaque) contrast media that are used for medical radiography to the solutions. Goebbels et al. [91] successfully made precise measurements of wall thickness of antique

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great-bronzes by CAT scans. Casting defects, residual cores, welding processes, repairs cracks and other flaws are also detectable. Archaeological objects The technical layout and the problems of radiography of archaeological objects are very similar to those described in the section above. The radiological examinations of archaeological specimens are more difficult than those of modern (metallic) materials. Radiographs of very small objects are difficult, microfocal techniques must be used [76]. Good radiographs of ceramics provide also difficulties, because voids and other faults are very small. But the main problem is always the interpretation of the obtained radiographs. Usually long experience and the examination of larger series of similar objects are necessary to gain reliable results and statements. For example, there exist very few published results on braze welding of nonferrous metals; the book by Driehaus [67] provides detailed instructions for such examinations. REFERENCES 1 2 3 4

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A.P. Laurie, The Painters Methods & Materials, Chapter XI: The Optical Properties of Oil. Seeley Service & Co., London, reprinted 1960. P.B. Coremans, La technique de Primitifs Flamands, Stud. Conserv., 1 (1952) 1–2. see also pp. 8–29, 145 –161. D.C. Creagh and D.A. Bradley (Eds.), Radiation in Art and Archeometry. Elsevier, Amsterdam, 2000. M. Matteini and A. Moles, Scienca e restauro, metodi di indagine. Firenze 1986. German translation (A. Burmester), Naturwissenschaftliche Untersuchungsmethoden in der Restaurierung. Mu¨nchen, 1990. R.A. Quinn and C.C. Sigl (Eds.), Radiography in Modern Industry. Eastman Kodak Company, Rochester, NY, 1980. A. Gilardoni, A.O. Orsini and S. Taccani, X-rays in Art. Gilardoni SpA, Mandello Lario (Lecco), Italy, 2nd ed., 1994, Realizzazione editorale Grafica & Arte Bergamo. E. Walmsley, C. Fletcher and J. Delaney, Evaluation of system performance of near-infrared imaging devices, Stud. Conserv., 37 (1992) 120 –131. Schott, Optische Filter—Glasfilter, Interferenzfilter und Spezialfilter (No. 3555d). Also available on floppy disk. Kodak Publication No. M-27, Ultraviolet & Fluorescence Photography. Eastman Kodak Company, Rochester, NY, 1968. W. Bru¨gel, Physik und Technik der Ultrarotstrahlung. Curt R. Vincentz Verlag, Hannover, 1961.

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M.W. Burke, Image Acquisition. Handbook of Machine Vision Engineering, Vol. 1. 1996, 861 pp. J.R.J. Van Asperen de Boer, Infrared Reflectography. A Contribution to the Examination of Earlier European Paintings, Thesis, Univ. of Amsterdam, 1970, Amsterdam. J.R.J. Van Asperen de Boer, Reflectography of paintings using an infra-red vidicon television system, Stud. Conserv., 14 (1969) 96–118. J.R.J. Van Asperen de Boer, Infrared reflectography: a method for the examination of paintings, Appl. Optics, 7 (1968) 1711–1714. J.R.J. Van Asperen de Boer, Infrared reflectograms of panel paintings, Stud. Conserv., 11 (1966) 45–46. Nowadays these expensive additions can be replaced by using image processing software. F.D. Shepherd, Silicide infrared staring sensors, Proc. SPIE Infrared Detectors Arrays, 930 (1988) 2–10. J. Silvermann, J.M. Moony and F.D. Shepherd, Infrared video cameras, Sci. Am., 3 (1992) 58–63. F. Mairinger, Strahlenuntersuchung an Kunstwerken. E.A. Seemann, Leipzig, 2003. E.R. De la Rie, Ultraviolet Radiation Fluorescence of Paint and Varnish Layers, PACT 13, 1986, pp. 91 –108 (Scientific Examination of Easel Paintings). H.P. Autenrieth, A. Aldrovandi, P. Turek, Die Praxis der UV-Fluoreszenzfotografie Z. Kunsttechnol. Konserv., 4(2) (1990) 215 –234 and Nachtra¨ge zur Praxis der UV-Fluoreszenzfotografie. 6(1) (1992) 195 –196. Both papers are concerned with the UV-fluorescence of wall paintings. R.L. Feller, Artist’s Pigments—A Handbook of Their History and Characteristics, Vol. 1. National Gallery of Art Washington, Cambridge University Press, Cambridge, 1986. E.R. De la Rie, Fluorescence of paint and varnish layers. Part 1: Stud. Conserv., 27 (1982) 1–7 (1), Part 2: 27 (1982), 65–69 (2), Part 3: 27 (1982), 102 –108 (3). J. King, The examination of porcelain etc. by ultraviolet light, Apollo, 58 (1953) 74. J. De Ment, Handbook of Fluorescent Gems and Minerals: An Exposition and Catalogue of the Fluorescent and Phosphorescent Gems and Minerals Including the Use of Ultraviolet Light in the Earth Sciences. Mineralogist Pub. Co., Portland 15, OR, 1949. I.N. Gilgendorf, Study and restoration of lost ancient inscriptions on the dry plaster by the method of infrared and ultraviolet photography, ICOM Comm. f. Conserv., Fourth Triennial Meeting Venice (1975), Preprints 1, 75/4/8, pp. 1–9. C.S. Holliday, The application of ultra-violet light to prehistoric rock art, SAMAB, 7 (1961) 179 –184. B. Hallstro¨m, The use of UV-reflectograms for the examination of paintings, ICOM Comm. f. Conserv., Fourth Triennial Meeting Venice (1975) 75/VI/10 F. Mairinger, Physikalische Methoden zur Sichtbarmachung verblasster oder getilgter Tinten, Restaurator, 5(1–2) (1981/82) 45–56.

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F. Mairinger, The infrared examination of paintings. In: D.C. Creagh and D.A. Bradley (Eds.), Radiation in Art and Archaeometry. Elsevier, Amsterdam, 2000, pp. 56–75. G. Kortu¨m, Reflexionsspektroskopie. Verlag Chemie, Weinheim, 1968. C.F. Bridgeman and H.L. Gibson, Infrared luminescence in the photographic examinations of paintings and other art objects, Stud. Conserv., 8 (1963) 77 –83. C.E. Engel, Ultraviolet and fluorescence recording, Chapter 8, in Photography for the Scientist, 1968, pp. 363–382. Academic Press, London. D.A. Kushel, Application of transmitted IR radiation to the examination of artefacts, Stud. Conserv., 30 (1985) 301– 310. L. Lazzarini, Studio tecnico-scientifico di un dipinto di Guiseppe Porta detto II Salviati. Notizie da Palazzo Albani (Univ. Urbino) Anno III, no. 2/3 (1974) 38 –47. C.F. Bridgeman, in: S.A. Pollack and H.C. Pamphlet (Eds.), Uses of Radiation in Philately and Examination of Paintings. Eastman Kodak Co., Rochester NY, 1965, 28pp, Plates (Reprint: “The Science of Ionising Radiation” ed. L.E. Etter, 1965, Chapter 27). D.L. Clark, et al., Design and performance of a 486 £ 640 pixel platinum silicide IR imaging system, SPIE—The International Society for Optical Engineering, 1590 (1991) 303 –311. M. Cohen and G. Olsen, Infrared detectors. Tuned in the near infrared. InGaAs detector arrays offer silicon performance in infrared applications from blood testing to industrial inspections, Photonics Spectra, 28(7) (1994) 132 –134. D.P. Leech and I. Gutmanis, The U.S. infrared detector industry. Prospects for commercial diversification, SPIE—Int. Soc. Opt. Engng, 1683 (1992) 2– 12. E. Walmsley, C. Metzger and J.K. Delaney, Evaluation of platinum silicide cameras for use in infrared reflectography. ICOM Com. Conserv. 10th Triennial Meeting Washington, DC, 1993, pre-prints 57– 62. E. Walmsley, C. Metzger, J.K. Delaney and C. Fletcher, Improved visualisation of underdrawings with solid-state detectors operating in the infrared, Stud. Conserv., 39 (1994) 217– 231. R. Billinge, J. Cupitt, N. Dessipris and D. Saunders, A note on an improved procedure for the rapid assembly of infrared reflectogram mosaics, Stud. Conserv., 38 (1992) 92–98. A. Burmester, J. Cupitt, H. Derrien, N. Dessipris, A. Hamber, K. Martinez, M. Mu¨ller and D. Saunders, The Examination of Paintings by Digital Image Analysis, Conference Volume, Third International Conference on Non-destructive Testing of Works of Art. Italian Society for Non-Destructive Testing, Siena, 1992, pp. 201– 214. I. Sandner, FH Ko¨ln (1998). Private Communication. G. Wecksung, R. Evans, J. Walker, M. Ainsworth, J. Brealy and G. Carriveau, Assembly of infra-red reflectograms by digital processing using a portable data collecting system, ICOM Comm. Conserv. Eighth Triennial Meeting, (1987) pp. 107–109. F. Mairinger and A. Papst, Die Erstellung von Infrarot-Reflektogrammen mittels des Programmpakets IREI¨KON, Fourth International Conference on


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Non-destructive Testing of Works of Art. Deatsche Gesellschaft fu¨r Zersto¨rungsfreie Psu¨fung, Berlin, Vol. 1 (1994) 175 –182. J.M. Cabrera and M.C. Garrido, Estudio tećnico del Guernica, Boletin del Museo del Prado, 2(6) (1981) 147 –156. A. Burmester and K. Renger, Neue Ansa¨tze zur technischen Erforschung von Handzeichungen: Untersuchungen der ‘Mu¨nchner Rembrandt-Fa¨lschungen’ im nahen Infrarot, Maltechnik-Restauro, 92(3) (1986) 9–34. 21 ill., 55 ref. N. Goethghebeur and L. Kockaert, Le Grand Calvaire d’Albert Bouts au Museé des Beaux-Arts de Bruxelles, Bull. IRPA XVIII, Vol. 18 (1980/81) 5 –20. S. Fletcher, A Preliminary study of the use of infrared reflectography in the examination of works of art on paper ICOM Comm. f. Con.; Preprints 7th Triennial Meeting Copenhagen 1984, 84.14.24-28. H. Ku¨hn, Die Zeichenmaterialien des Niklaus Manuel, Maltechnik-Restauro, (1982) 156–157. D. Hollanders-Favert, M. Lietaerd-Parmentier, R. Van Schoute and H. Verougstraete-Marcq, Le dessin sous-jacent chez Albert Bouts, Acta Lovanensis, 4 (1975) 41–135. E.D. Bosshard, Fortschritt in der naturwissenschaftlichen Gema¨lde-untersuchung. Die Erforschung der Unterzeichnung mit dem Infrarot-Fernsehgera¨t, Z. fu¨r Schweizerische Archaöl. Kunstgeschichte, 39 (1982) 76–80. H. Irgang, Introduction to the study of underdrawing in early Netherlandish paintings. In: B. Cory (Ed.), Student Papers Presented at the Art Conservation Training Programs, 11th Annual Conference, May 2 & 3, 1985; Art Conservation Program, Winterthur Museum/Univ. of Delaware, New York, 1986, pp. 85 –101. K.H. Weber, Die sixtinische Madonna, Maltechnik Restauro, 90(4) (1984) 9–28. Bibliographie de l’Infrarouge et du de dessin sous-jacent dans, Le Dessin SousJacent dans la Peinture, Van Schoute, R. et Verougstrate-Marcq, H., Louvain-la Neuve Colloques, 1975–2003. J. Jennings, Infrared visibility of underdrawings techniques and media. In: R. Van Schoute and H. Verougstrate-Marcq (Eds.), Le Dessin Sous-Jacent dans la Peinture, Colloques IX 12 –14 Septembre 1993. Colle`ge Erasme—Place Blaise Pascal, Louvain-la-Neuve, 1993, pp. 241 –252. W.C. Roentgen, Eine neue Art von Strahlen. Sitzungsbericht der physikal.medizinischen Gesellschaft, Wu¨rzburg 137 (Dez 1895). ¨ ber eine neue Art von Strahlen W.C. Roentgen, in: W. Gerlach u. Krafft (Eds.), U Essay. Kindler Verlag, Mu¨nchen, 1972, Naturwissenschaftliche Texte bei Kindler. A. Faber, Eine neue Anwendung der Ro¨ntgenstrahlen. Die Umschau, 18 12 (21. 3. 1914). ¨ lbildern. Umschau 15, A. Faber, Die Ergebnisse der Ro¨ntgenuntersuchung von O 24 (11.6.1921), 325– 327. C.F. Bridgeman, The amazing patent on the radiography of paintings, Stud. Conserv., 9 (1964) 134 –139. Ch. Wolters, Die Bedeutung der Gema¨ldedurchleuchtung mit Ro¨ntgenstrahlen fu¨r die Kunstgeschichte dargestellt an Beispielen aus der niederla¨ndischen und deutschen Malerei des 15. und 16. Jahrhunderts. Vero¨ffentlichungen zur Kunstgeschichte 3, Frankfurt am Main 1938.

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M. Hours, A la Dećouverte de la Peinture par les Me´thodes Physiques. Analyse Scientifique et Conservation des Peintures, Office du Livres, Fribourg, 1976. R. Van Schoute and H. Verougstraete-Marcq (Eds.), Radiography, Chapter VII, in Scientific Examination of Easel Paintings, PACT 13 (1986), Conseil de L’Europe, Strasbourg, pp. 131 –154. K. Nicolaus, Gema¨lde-Untersucht-Entdeckt-Erforscht. Klinkhardt & Biermann, Braunschweig, 1979. J. Driehaus, Archaeologische Radiographie, 1968, Archaö-Physica Bd. 4. Du¨sseldorf, Rheinlandverlag, 112 pp. D. Robakowski, Ro¨ntgenfotographie zur Pru¨fung kulturhistorischer Objekte— Wege zum optimalen ro¨ntgenbild, Maltechnik-Restauro, 93(3) (1987) 32–48. G.N. Hounsfield, Computed medical imaging, J. Comput. Ass. Tomogr., 4 (1980) 665 –674. P. Reimers and J. Goebbels, New possibilities of non-destructive evaluation by X-ray computed tomography, Mat. Eval., 41 (1983) 732 –737. B. Illerhaus, J. Goebbels, P. Reimers and H. Riesemeierr, The principle of computerized tomography and its application in the reconstruction of hidden surfaces in objects of art, Proceedings of the Fourth International Conference on Non-destructive Testing of Works of Art, Berlin, Vol. 1. 1994, pp. 41 –49. M.M. Ter-Pogossian, M.E. Raichle and B.E. Sobel, Positron emission tomography, Sci. Am., 243 (1980) 140 –155. W.-D. Heilmeier, Technical investigation of Roman large bronzes, Fourth International Conference on Non-destructive Testing of Works of Art, Berlin, Vol. 1. 1994, pp. 11–20. W.B. Gilboy and J. Foster, Industrial application of computerized tomography with X- and gamma radiation. In: R.S. Sharp (Ed.), Research Techniques in Nondestructive Testing, Vol. 6. Academic Press, New York, 1981, pp. 255 –287. P. Keil, Fortschritte auf dem Gebiete der Ro¨ntgen-Computer Tomographie, Phys. Bl., 39 (1983) 2–8. H. Reiter, H. Moesta and W. Reinhard, X-ray methods as means of assessment of archeological findings as well as for the revealing of their production process (in German), Proceedings of the Fourth International Conference on NonDestructive Testing of Works of Art, Berlin, Vol. 1. Deutsche Gesellschaft zur Zersto¨rungsfreien Pru¨fung e.v., Berlin, 1994, pp. 75 –84. R.S. Sharpe and R.W. Parish, Engineering applications of microfocal radiography, Microfocal Radiography, Ely, R.V., London, 1980, pp. 43 –81. H.S. Tasker and S.W. Towers, Electron radiography using secondary betaradiation from lead intensifying screens, Nature, 156 (1945) 50–51. E. Mundry, D. Schnitger, J. Riederer, C. Schro¨der, Radiographie und Autoradiographie mit Elektronen. Fourth International Conference on NonDestructive Testing of Works of Art, Berichtsband 45 Teil 2. Deutsche Gesellschaft zur Zersto¨rungsfreien Pru¨fung e.v., Berlin, 1994, pp. 775–786. Ch.F. Bridgeman, S. Keck and H.F. Sherwood, The radiography of paintings by electron emission, Stud. Conserv., 3 (1958) 175 –182. E. Ziesche and D. Schnitger, Elektronenradiographische Untersuchungen der Wasserzeichen des Mainzer Catholicon von 1460, Archiv f. Gesch. d. Buchwesens (AGB)XXI, 5– 6 (1980) 1303– 1359.

UV-, IR- and X-ray imaging 82 83

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R.S. Holt, et al., Gamma-ray scattering techniques for non-destructive testing and imaging, Nucl. Inst. Meth., 221 (1985) 98. W. Niemann and W. Roy Modern X-ray imaging techniques: radioscopy and backscatter imaging. Fourth International Conference on Non-Destructive Testing of Works of Art, Berlin Berichtsband 45 Teil 1. Deutsche Gesellschaft zur Zersto¨rungsfreien Pru¨fung e.v., Berlin, 1994, pp. 21 –30. F. Mairinger, Strahlenuntersuchung an Kunstwerken, E.A. Seemann, Leipzig, 2003, pp. 186–187. K.H. Weber, Die sixtinische Madonna, Maltechnik-Restauro, 90(4) (1984) 9–28. C.F. Bridgeman, Radiography of paper, Stud. Conserv., 10 (1965) 8–17. F. Drilhon, L’Examen radiographique de sculptures en cire du XVIe au XIXe sie`cle. ICOM Comm. Conserv. Seventh Triennial Meeting Copenhagen 1984, 84.1.58 –84.1.61 L. Hatziandreov and G. Ladopoulos, Radiographic examination of the marble statue of Hermes at Olympia, Stud. Conserv., 26 (1981) 24 –28. G.F. Alfrey and K. James, The gamma-ray radiography of decorative plasterwork, Stud. Conserv., 31(2) (1986) 70 –76. V. Vitali, J. Darcovich and W. Williams, Construction of a fudo-myoo sculpture: an X-radio-graphic study, Stud. Conserv., 31(4) (1986) 185 –189. J. Goebbels, J. Haid, D. Hainisch, B. Illerhaus, H.-J. Maltte and D. Meinel, Ancient bronze—a challenge to radiographic technique, Fourth International Conference on NDT Testing of Works of Art, Berlin, Vol. 2. 1994, pp. 733 –742.

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Chapter 3

Electron microscopy and its role in cultural heritage studies A. Adriaens and M.G. Dowsett

3.1 3.1.1

INTRODUCTION Why use electron microscopy?

In the sense of straightforward imaging, the electron microscope allows a sample to be examined at far higher magnification and lateral resolution than is possible with light microscopy. A further advantage is a very large depth of field which, for example, allows rough samples to be studied with the whole surface remaining in focus at one time. However, where cultural heritage materials are concerned, perhaps it is in the huge range of contrast mechanisms, and the possibility of simultaneous imaging and localized chemical analysis where the main advantage of the electron microscope lies. Cultural heritage related studies present a very heterogeneous challenge to the microscopist, and involve materials and problems similar to those encountered in many other areas of materials science—e.g., ceramics, metal alloys, biological materials, manufacturing assessment and corrosion studies. Because of this we have included a fairly complete overview of electron microscope techniques applied to such materials. However, a chapter such as this can only give a brief description of electron microscopy and its application in this area. For more basic information on the technique itself the reader is strongly urged to consult Goodhew et al. [1] and Watt [2]. Applications have been limited to the study of inorganic materials. Itemization is done from the perspective of museum-specific problems. 3.1.2

Imaging with electrons

The human eye provides the ultimate limitation in microscopy. It has a resolution reye of ,0.1 mm—about the diameter of a human hair. This means Comprehensive Analytical Chemistry XLII Janssens and Van Grieken (Eds.) q 2004 Elsevier B.V. All rights reserved

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A. Adriaens and M.G. Dowsett

that two small objects placed about 20 cm from the eye can just be seen as distinct when they are ,0.1 mm apart. The limitation arises from the intrinsic magnification of the eye, and the separation of the sensing elements on the retina. An optical microscope has a resolution rmic and improves that of the unaided eye in a way which is limited by the wavelength of the light used to illuminate the object. For visible light, this corresponds to rmic ,0.2 mm. There is no point in building a microscope with a significantly higher magnification than reye /rmic—the result would be widely separated but fuzzy images with no improvement in detail. If one allows that 0.1 mm is straining the eye somewhat, a magnification ,1000 is the useful limit for conventional light microscopy [3]. Louis de Broglie [4] established that a wave is associated with any moving particle, and that the corresponding wavelength l is given by l¼

h p

ð3:1Þ

where h is Planck’s constant, 6.626 £ 10234 J s, and p is the particle’s (relativistic) momentum. For an electron with energy E (eV) [1], the associated wavelength is 1:225 l ðnm21 Þ ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi E þ 1026 E2

ð3:2Þ

For the energy range 103 – 106 eV used in modern electron microscopes, l is comfortably smaller than the atomic diameter. The resolution of the microscope is then determined by other factors, such as imperfections in the imaging (aberrations) due to defects in the optical components. Magnifications of (depending on the microscope) 105 – 106 can be achieved allowing single atoms (or even orbitals within them) to be imaged directly at the highest end of the range. In the first place, therefore, electron microscopy allows far smaller detail in an object to be examined than does light microscopy. This benefit comes with some inherent features of the electron microscope, which may be disadvantageous in some circumstances, especially where delicate materials are concerned. Electrons will not travel far through air, and electron microscopes are (usually) vacuum-based instruments—therefore, the specimen must be vacuum tolerant, and not a significant gas source in its own right (but see section 3.1.4). In addition, intense irradiation by electrons can damage or destroy a sample through heating, or other effects. Even in quite good vacua, samples can become coated in the microscope obscuring true surface detail and distorting

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Electron microscopy and its role in cultural heritage studies

chemical analyses (at 1026 mbar, approximately one mono-atomic layer of gas hits the sample surface per second). Many samples, not least those relating to cultural heritage, are insulating, and charge up under the electron beam. This degrades the resolution, and usually requires some countermeasure such as coating the sample with a conducting layer. However, because of the way that electrons interact with matter, the electron microscope has the ability to do far more than just produce magnified images of the sample. Through secondary electron emission it allows the imaging of topography and major compositional changes (changes of density or atomic number). Via electron diffraction it can provide information on the crystallography and other features of a sample microvolume (e.g., strain). Because electrons can lose characteristic amounts of energy to individual atoms, causing internal excitation and X-ray emission, for example, the chemistry of the microvolume can be determined by using techniques such as energy dispersive spectroscopy (EDS) to measure the emitted X-ray spectrum or electron energy loss spectroscopy (EELS) to determine the energy lost by the primary electron in the interaction. What is more, the chemistry and crystal structure can be correlated with the other electron-imageable properties, depending on the type of microscope used. 3.1.3

Varieties of electron microscopy

There are two basic types of electron microscope (Fig. 3.1). If the detectors are mounted on the same side of the sample as the impinging beam, particles emitted from the front of the sample are detected, and one has a means of characterizing the surface and near-surface regions of a sample of any thickness. This is the basis of the scanning electron microscope (SEM) (Fig. 3.1(a)) and its close cousin the electron probe microanalyser (EPMA). Whereas the design of the SEM usually emphasizes resolution and multi-technique imaging, which results in (typically) a fine electron probe with a small current, EPMA is optimized for the analysis of X-rays characteristic of the elemental composition of the material. The effect on the instrument design is to allow for the use of a higher current broader probe, and more accurate wavelength dispersive spectroscopy (WDS) for the X-ray analysis, in addition to EDS. If the detectors are mounted behind the sample, then electrons and other particles transmitted through a thin section of material can be detected, and the internal details of such a sample can be examined. This is the basis of

75


Fig. 3.1. General types of electron microscope. (a) Scanning electron microscope (SEM); (b) transmission electron microscope (TEM).

transmission electron microscopy (TEM) (Fig. 3.1(b)), and scanning transmission electron microscopy (STEM). A further division in the microscopy can be discerned in the way the magnification is achieved. In the scanning microscopies (SEM, STEM), the optics of the microscope are used to form a sharply focussed electron beam which may be from ,1 mm – ,0.1 nm in diameter. This is scanned across a small square area of the sample, dwelling on a regular array of points known as pixels. More than 106 pixels may cover the scanned area. Typically, the most efficient spacing between pixels is determined by the area around the point of impact of the beam from which secondary particles are emitted (the diameter of the interaction volume – see section 3.2). The signals from each pixel are recorded and stored individually. The magnification is

76


determined by the ratio of the size of the displayed image to the scanned area. The resolution is determined ultimately by the probe diameter, but more typically, by the size of the interaction volume. In the TEM, magnification is achieved by using lenses underneath the sample to project the image formed by the transmitted electrons onto a recording device. In this case, the magnification is determined by the optical system and the resolution by the aberrations (imperfections) in the lens performance. The interaction volume plays a diminishing role as the sample becomes thinner, and as the energies of the detected electrons approach that of the primary beam. Most electron microscopy of cultural heritage materials uses SEM, usually with a magnification at the low end of the possible range. The microscopist usually requires complementary information to that available from optical microscopy, perhaps combined with chemical analysis on the micron scale. However, an outline of other modes of operation is included here because of their potential use in specialized applications. 3.1.4

Recent developments in commercial SEM

The increasingly widespread application of the Schottky field emission gun (FEG), originally developed ca. 1969, (e.g., Swanson and Crouser [5]) combined with greatly improved pumping technology has given rise to rapid development of the commercial SEM. Two related areas, highly relevant to cultural heritage materials are described here. As we explain later, the Schottky emitter allows SEMs (and TEMs) to achieve far smaller, brighter foci at the sample than with the older tungsten filament technology, and at much lower beam energies (SEM). Beam diameters quoted at 1.5 nm at 15 kV are not uncommon. First, electron beam energies as low as 1 keV can be used at high resolution in the low-voltage SEM (LVSEM). Even with a conventional electron gun, low-voltage SEM may have some attractions for the examination of cultural heritage materials. The resolution will still be adequate for many purposes, and selection of a (sample-dependent) electron energy somewhere in the 1 –15 keV range such that the secondary electron emission coefficient is $1 can minimize charging problems, thus simplifying sample preparation. Aspects of electron beam damage, especially those due to localized heating, can also be reduced because of the lower power density input. Note, however, that low-energy electrons cannot excite some X-ray transitions which may be required for chemical analysis.

77


The fact that the electron microscope is a vacuum-based instrument has already been mentioned. This restricts the types of sample which can be examined to those which are both vacuum tolerant, and which will preserve the vacuum (which do not, for example contain significant amounts of water, other volatile species, or gas as part of their structure, or through porosity). Conversely, it implies that samples can rarely be examined in their natural state, and that, at best, the chemical composition of the surface may have changed through loss of adsorbed gas. The second development, the environmental SEM (ESEM) is a solution to this problem. First introduced in around 1981 by Danilatos and co-workers [6,7], and commercially available from 1988, ESEM microscopy is a rapid growth area. It has long been known that the primary electron beam is not seriously defocused in SEM by transit through a millimetre or so of gas at pressures approaching 1 atm. Instead, electrons which interact with gas molecules are strongly scattered and form a more or less uniform halo around the beam. The resolution of the microscope is therefore preserved, although there may be an increase in background noise from the samplerelated signals excited by the halo. In the ESEM, therefore, the sample is placed in a separate chamber with its own pumping system. This is linked to a second independently pumped chamber containing the electron column by a pressure-limiting aperture. Detectors may be placed in either chamber, depending on type and the signal detected. The sample is placed close to the aperture (typically 0.5 mm from an aperture diameter 0.5 mm) through which the primary and secondary electrons pass. The microscope column, and the electron gun provide further stages of pumping, which is important because the Schottky field emission source, requires an operating vacuum ,1029 mbar. High vacuum conditions can then be maintained in the microscope column and, to a lesser extent around the detectors, and the electrons need only to travel a few mm in poor vacuum over the sample. A resolution down to 10 nm can be maintained, and analytical techniques such as EDS can be carried out on, for example, wet or hydrated specimens in their natural state [8,9]. Phenomena such as crystallization and dissolution can be observed directly as they happen. Moreover, samples may be analysed in controlled atmospheres. A key advantage is that ionization of the gas over the sample minimizes charging effects, and insulators can be analysed without coating. However, as always, there exists the possibility of electron beam damage which may be increased in some experiments by the presence of species such as water which generate highly reactive radicals under electron irradiation [10,11].

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3.2

THE INTERACTION OF ELECTRONS WITH A SOLID—CONTRAST MECHANISMS

The power and versatility of electron microscopy derives from the variety of ways in which the primary electron beam interacts with the sample, and the fact that the strength of the various interactions is very dependent on the sample’s physical structure, topography, crystallography and chemistry, thus giving rise to contrast between different regions in an image. Many types of secondary or modified primary particle are emitted, and most of them find a use in topographic, structural or chemical characterization. Some of the emission is summarized in Fig. 3.2(a,b). Electrons are very strongly scattered by matter, even at the level of a single atom. The signals that contribute to contrast in an image are, therefore, themselves strong, and this is another reason why the electron microscope can image such small structures. One important effect of the interaction is that the achievable resolution is dependent on the emitted species examined, and is usually worse (often much worse) than the probe diameter or the limitations imposed by aberrations. This is because most of the incident electrons scatter strongly and repeatedly, loosing their energy in a tear-drop shaped volume (Fig. 3.2(a)), typically 1 mm across, exciting secondary electrons and photons in the process. If the sample is thinned (as it must be in TEM), more electrons pass through relatively unscattered, and the effect is reduced (Fig. 3.2(b)). The lateral resolution of EDS and SEM can be strongly dependent on the size of the interaction volume, but techniques such as electron diffraction, and EELS which employ electrons which have elastic, or near elastic scattering

Fig. 3.2. Interaction of electrons with a solid showing effects of interaction volume. (a) SEM sample; (b) sample thinned for TEM.

79


have resolution similar to the probe size (or the aberration limited performance in TEM). Some of the possible interactions and associated contrast mechanisms are summarized below, together with their microscopical or analytical application. 3.2.1

Scattering

An electron is scattered when its direction of travel is changed through interaction with atoms or electrons in the sample (Fig. 3.3). Most individual scattering events are peaked in the forward direction—i.e., the probability of the electron being scattered through a large angle is small. Thus if the sample’s thickness is similar to the mean distance between scattering events (

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