Phd Thesis Salvemini.Pdf
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DOTTORATO DI RICERCA IN Scienza per la Conservazione dei Beni Culturali CICLO XXVI COORDINATORE Prof. Piero Baglioni Development of innovative methodologies for non-invasive characterization of metal artefacts of archaeological, historical, and industrial interest, through neutron diffraction and neutron imaging techniques Settore Scientifico Disciplinare FIS/07 Dottorando Tutore Dott. Salvemini Filomena, Floriana Prof. Zoppi Marco Co-Tutore Prof. Grazzi Francesco Coordinatore Prof. Baglioni Piero Anni 2011/2013 Alla mia famiglia Abstract Metal artefacts of archaeological and historical interest represent a large fraction of the rich cultural heritage which is conserved in national and private museums worldwide. As such, these objects, when properly studied, can reveal secrets of the past human history and we are only allowed to treat them using the most delicate care and avoiding, as much as possible, to accelerate the natural and unavoidable aging process. Moreover, as the technological knowledge evolves, we are not allowed to investigate these objects using invasive tools that might prevent further more involved techniques to operate properly on the same objects by future generations. Until recently, the research activity in this field was mainly based on standard point-based analytical techniques like, for example, metallography. Traditional analysis, however, though extremely effective, is not always suitable for rare and unique objects of high scientific, cultural, and economic value. Lots of interesting metal items fall in this category: sculptures and artistic artefacts, archaeological finds, rare coins, and even meteorites. It has been shown that this technique, which is mainly based on a careful surface analysis following a cleaning and preparation process, can be effectively complemented by a well-recognized, non-invasive experimental approach, based on the use of thermal neutrons. Thanks to their high penetration power, neutrons represent an ideal tool to probe the microscopic properties of bulk dense materials and can be used to characterize the microscopic structure (at the atomic level) of metal artefacts. As a matter of fact, neutron techniques are used to determine the qualitative and quantitative presence of different phases, as well as the presence and distribution of texture and residual strain at the atomic level. From this wealth of data it is possible to obtain information on the conservation status of the artefact, as well as on the smelting and smithing procedures, through identification of some peculiar signatures related to these processes. In fact, the study of forging techniques, and their time-evolution, represents one of the most interesting topics in the investigation of the manufacturing methods, which are considerably different among various cultures. To this aim, neutron diffraction can be used to quantitatively determine the relative amount of the various crystal phases composing the artefact, its conservation status, the presence of texture, measurements of residual strains, and the determination of the grain size of the phases in the sample. In addition, neutron imaging techniques allows obtaining bulk detailed information on the micro-structural properties of the samples, through the tomographic reconstruction of the object’s macroscopic cross section. However, this technique can be further improved, to achieve materials discrimination, by a proper selection of the neutron energy. In fact, monochromatic neutron beams give the possibility of modifying the image contrast for different phases taking advantage of the abrupt change of the attenuation coefficients in the proximity of the so-called Bragg cut-off. In the present work, we have developed a comprehensive study of the metallurgy of museum metal artefacts of historical and archaeological interest aiming to investigate composition, assembly methods, and structural variations pertaining to different cultures and historical environments. The study has been developed in cooperation with several Museum Institutions (like, for example: Museo Stibbert – Firenze (IT) and The Wallace Collection - London (UK)) and some private collectors. i Beyond time-of-flight neutron diffraction, which represents the workhorse of the neutron techniques, white beam and energy selective radiography, tomography, and laminography have been successfully applied to determine the material composition and microscopic structures of the analysed samples. From these results, we could obtain information on the smithing techniques and the working process of the studied of ancient artefacts. For this investigation several experimental European facilities have been used, like the ICON, NEUTRA and POLDI beam-lines at the Paul Scherrer Institut (CH), the CONRAD II beam-line at the Helmholtz-Zentrum Berlin (DE), and the INES and ENGIN-X beam-lines at ISIS (UK). Japanese sword (katana) blades pertaining to the Koto (987–1596) and the Shinto (1596– 1781) periods in Japan have been characterized using neutron diffraction and neutron- imaging techniques (in this case both white beam and energy selective imaging have been used). However not only the blades were the object of the present investigation, but also the composite structures of the metal components at the handle’s extremities of Japanese swords have been investigated: the hand-guard tsuba, the hilt collar fuchi, the pommel kashira, the small knife-handle fitting into the scabbard kozuka. In addition, we extended the study to the inner metal structure and manufacturing techniques of kabuto (helmets), which represent beautiful examples of past technology made in Japan in the 17th Century. The present investigation, however, was not limited to the study of the Japanese historical objects but was extended to other cultural environments. Thus, we have done experiments on a set of five kris, the distinctive weapon of Malaysia and Indonesia, and on a kanjar, the Indian dagger. Morphological information and material distribution in the whole volume of these objects allowed us to identify the working procedures and, when possible, to define the authenticity of the investigated samples. The study has been enriched by a methodological test, aiming to demonstrate the equivalence of the information content between the classical (invasive) technique of metallography and neutron diffraction and imaging techniques (non-invasive). An enlightening comparison could be obtained studying the cross sections of a Toledo-like sword, which has been studied thought neutron imaging. Fragments, taken at different height of two sacrificed samples, were investigated through energy selective images and the results have been compared to those obtained from standard metallography. Once the neutron techniques were validated, on a qualitative and quantitative basis, a massive investigation was conducted on a large number (91) of styli, the small bar made of either iron, bronze or bone, used by the Romans for writing onto wax-coated surface of wooden flat tablets. All these samples, coming from the same archeological site of Julia Concordia - Venice (Italy), were carefully analyzed by neutron techniques and have disclosed peculiar morphological features related to the working techniques of ancient Romans. Finally, it is worthwhile to note that, beyond the standard neutron diffraction and imaging experiments, a relevant number of samples (two selected Japanese swords and 11 Roman styli) that have been analyzed to obtain the residual strain mapping aiming to obtain information on the method and direction of working. ii Contents Abstract i 1. Introduction 1 2. Ancient metallurgy 2 3. Materials 8 3.1. Metals 9 3.2. Crystal structure 10 3.2.1. Cristallographic planes and directions 10 3.3. Alloys 11 3.3.1. Phases and phase diagrams 11 3.3.2. Continuous cooling transformation 13 3.4. The microstructure of metals 14 3.4.1. Casting 14 3.5. Working 15 3.5.1. Plastic deformation in polycrystalline materials 15 3.5.1.1. Slip system 15 3.5.1.2. Mechanisms of strengthening in metals 16 3.6. Stress and strain 17 3.7. Texture 18 4. Methodology 20 4.1. The Interaction of Neutrons with Matter 21 4.2. Neutron Imaging Methods 22 4.2.1. Working principles of radiography and tomography 23 4.2.2. Energy selective neutron radiography and tomography 24 4.2.2.1. Why cold neutrons 25 4.2.2.2. Beam monochromatization 26 4.2.3. White beam and energy selective neutron laminography 27 4.2.4. Experimental Facilities 28 4.2.4.1. ICON 28 4.2.4.2. NUETRA 28 4.2.4.3. CONRAD II 28 4.3. Neutron Diffraction Methods 29 4.3.1. Experimental Facilities 30 4.3.1.1. POLDI 31 5. Measurements and Results 32 5.1. The Samurai sword 33 5.1.1. Test measurements on fragments of ancient Japanese swords 35 5.1.2. Quantitative characterization of two ancient katana 43 5.2. Revealing the secrets of kabuto 52 5.3. European Blades 61 5.4. Kris, weapon of the Malay world 68 5.5. Kris and kanjar: an authentication study 87 5.6. Neutron laminography: test measurements on ancient metal artifacts 100 5.7. Koshirae, components of Japanese swords 110 iii 5.8. The Roman settlement of Iulia Concordia 127 5.8.1. Morphological characterization of Roman styli 128 5.8.2. Residual strain mapping of Roman styli 136 6. Conclusion 141 Appendix A 143 Appendix B 146 Appendix C 149 Appendix D 154 References 160 Acknowledgments 169 iv 1 Introduction An accurate knowledge of the composition (phase distribution) and the microstructure (size, shape, and orientation of grains) of materials represents the equivalent of the Rosetta stone in material science. Until recently, especially concerning metal objects, this task was mainly fulfilled basing on standard analytical techniques like, for example, metallography. Traditional analysis, however, though very accurate, is not always suitable for rare and unique objects of high scientific and economic value. Lots of interesting items related to cultural heritage fall in this category: sculptures and artistic artefacts, archaeological finds, rare coins, and even meteorites. It has been shown that this well established technique, which is mainly based on surface analysis, can be effectively complemented by an emerging non-invasive experimental approach based on thermal neutron analysis.