AAllmmaa MMaatteerr SSttuuddiioorruumm –– UUnniivveerrssiittàà ddii BBoollooggnnaa DOTTORATO DI RICERCA Science for Conservation Ciclo XXII Settore/i scientifico disciplinari di afferenza: CHIM/12 TITOLO TESI ARCHAEOLOGICAL BALTIC AMBER: DEGRADATION MECHANISMS AND CONSERVATION MEASURES. Presentata da: Gianluca Pastorelli Coordinatore Dottorato Relatore Prof. Rocco Mazzeo Prof. Jane Richter Esame finale anno 2009 AAllmmaa MMaatteerr SSttuuddiioorruumm –– UUnniivveerrssiittàà ddii BBoollooggnnaa DOTTORATO DI RICERCA Science for Conservation Ciclo XXII Settore/i scientifico disciplinari di afferenza: CHIM/12 TITOLO TESI ARCHAEOLOGICAL BALTIC AMBER: DEGRADATION MECHANISMS AND CONSERVATION MEASURES. Presentata da: Gianluca Pastorelli Coordinatore Dottorato Relatore ________________ ________________ Esame finale anno 2009 0 My work would be incomplete without a mention of the support given me by my cherished friend, Clara, to whom this thesis is dedicated. 1 Table of Contents. Abstract …………………………………………………………………… i List of abbreviations ……………………………………………………… iii List of tables & figures …………………………………………………… iv Acknowledgements ………………………………………………………. x 1 BACKGROUND OF THE RESEARCH PROJECT ………………………………... 1 1.1 Introduction – the amber …………………………………………………. 1 1.2 Amber in museums ……………………………………………………….. 10 1.3 Nature of the problem …………………………………………………….. 10 2 OVERVIEW OF THE RESEARCH PROJECT …………………………………… 13 2.1 Research project objectives ………………………………………………. 13 2.2 Degradation of Baltic amber – state of the art ……………………………. 15 2.3 Analytical techniques applied to Baltic amber – state of the art …………. 15 2.4 Research project strategy …………………………………………………. 20 3 PRELIMINARY INVESTIGATION – MATERIALS AND METHODS ……………… 21 3.1 Selection and characterization of the model material …………………….. 21 3.2 Preparation of amber samples ……………………………………………. 24 3.3 Ageing of amber samples ………………………………………………… 26 3.4 Examination of amber samples …………………………………………... 31 3.4.1 Visual examination by naked eye and photography …………………………... 32 3.4.2 Colour measurement by CIE L*a*b* spectrocolorimetry ……………….......... 33 3.4.3 ATR-FTIR spectroscopy ……………………………………………………. 34 3.4.4 FT-Raman spectroscopy ……………………………………………………. 36 3.4.5 Oxygen measurement by optical respirometry ……………………………….. 38 3.4.6 SPME-GC-MS headspace analysis ………………………………………….. 39 3.5 Overview of the experiment design ………………………………………. 42 3.6 Additional analyses ………………………………………………………. 43 3.6.1 C-H-N analysis …………………………………………………………….. 43 3.6.2 Micro-ATR-FTIR cross sections analysis ………………………………......... 44 3.6.3 Confocal profilometry ……………………………………………………… 45 3.7 Data analysis and documentation ………………………………………… 45 2 4 PRELIMINARY INVESTIGATION – RESULTS AND DISCUSSION ………………. 47 4.1 Analysis and interpretation of achieved data ……………………………... 47 4.1.1 Visual examination by naked eye and photography …………………………... 47 4.1.2 Colour measurement by CIE L*a*b* spectrocolorimetry ……………….......... 48 4.1.3 ATR-FTIR spectroscopy ……………………………………………………. 49 4.1.4 FT-Raman spectroscopy ……………………………………………………. 52 4.1.5 Oxygen measurement by optical respirometry ……………………………….. 55 4.1.6 SPME-GC-MS headspace analysis ………………………………………….. 55 4.1.7 C-H-N analysis …………………………………………………………….. 59 4.1.8 Micro-ATR-FTIR cross sections analysis ………………………………......... 60 4.1.9 Confocal profilometry ……………………………………………………… 61 4.2 Preliminary assessments ………………………………………………….. 62 5 ADVANCED INVESTIGATION – MATERIALS AND METHODS ………………… 64 5.1 Experiment design ………………………………………………………... 64 5.2 Preparation, ageing and examination of amber samples …………………. 68 5.3 Additional analyses ………………………………………………………. 74 5.3.1 Micro-ATR-FTIR cross sections analysis and Confocal profilometry …............ 74 5.3.2 ATR-FTIR analysis of archaeological material ………………………………. 74 6 ADVANCED INVESTIGATION – RESULTS AND DISCUSSION …………………. 75 6.1 Analysis and interpretation of achieved data ……………………………... 75 6.1.1 Visual examination by naked eye and photography …………………………... 75 6.1.2 Colour measurement by CIE L*a*b* spectrocolorimetry ……………….......... 75 6.1.3 ATR-FTIR spectroscopy ……………………………………………………. 76 6.1.4 FT-Raman spectroscopy ……………………………………………………. 81 6.1.5 Oxygen measurement by optical respirometry ……………………………….. 83 6.1.6 SPME-GC-MS headspace analysis ………………………………………….. 84 6.1.7 Micro-ATR-FTIR cross sections analysis ………………………………......... 85 6.1.8 Confocal profilometry ……………………………………………………… 85 6.1.9 ATR-FTIR analysis of archaeological material ………………………………. 86 6.2 Final assessments …………………………………………………………. 87 Conclusions ………………………………………………………………. 93 Reference list ……………………………………………………………... 96 Websites list ………………………………………………………………. 103 3 Abstract. The National Museum of Denmark has around 17,000 archaeological objects of Baltic amber in its collections, which are among the largest in Europe. Approximately 45% of the amber objects has degraded and requires urgent conservation treatment. Degradation has resulted in restricted access to the collection by curators, students, archaeologists and visitors. The National Museum of Denmark’s collection is of interest to archaeologists and curators because amber jewellery reflects the economic, social, religious and other cultural beliefs of the peoples who made and wore it. It is important that the collections are preserved for study and cultural enrichment for future generations. Baltic amber is sensitive to physico-chemical environmental factors, consequently it is extremely predisposed to degradation. The aim of this project was to achieve a deeper understanding of the mechanisms by which amber degrades, in order to develop techniques for preventive conservation with the purpose to slow down the rate of degradation of archaeological amber objects. Early active conservation treatments comprised impregnating crumbling amber with natural and synthetic polymers; with time, these conservation materials have deteriorated and damaged the amber surfaces they were supposed to protect. For that reason it is more appropriate to create a preventive conservation strategy based on the control of factors, such as temperature, illumination and atmosphere, in the environment where amber objects are placed during storage, transport and display. A clear understanding about interaction between amber and environmental factors is essential to comprehend degradation processes. Current state of knowledge presents few degradation features regarding Baltic amber and new experiments were necessary to provide evidence. In order to study deterioration of Baltic amber, a starting point was to identify and monitor surface and bulk properties which are affected during degradation. The way to operate consisted of the use of accelerated ageing to initiate degradation of raw Baltic amber samples and, successively, in the use of non/micro-destructive techniques to identify and quantify changes in visual, chemical and structural properties. For the preliminary experimental phase, a large piece of raw Baltic amber was selected for its visible homogeneity and prepared in two sample sizes: prisms and powder. Samples were placed inside Pyrex glass flasks and subjected to two different kinds of accelerated ageing: thermal-ageing and photo-ageing. For both the accelerated ageing procedures, samples were exposed to six different microclimatic conditions, in order to study the role of different environmental parameters (relative humidity, presence of oxygen, pH) on the degradation of amber. Samples were checked regularly by several analytical techniques, including spectrocolorimetry, infrared and Raman spectroscopies, optical respirometry and gas chromatography – mass spectrometry. i From the interpretation of the obtained results it was possible to identify some relations between amber and environmental factors during the degradation process and to select the conditions to apply for the second ageing experiment. In the advanced experimental phase, pellet-shaped samples obtained from pressed amber powder (which appeared more appropriate than prisms and free powder in terms of analytical repeatability and homogeneity of results) were exposed to eleven different microclimatic conditions, subjected only to thermal accelerated ageing (since photo-ageing already resulted in considerable degradation) and analysed regularly according to the same techniques and purposes that were employed in the preliminary investigation. Initial results from the preliminary investigation showed colour change, oxidation of the molecular structure and off-gassing of formic and acetic acids vapours, that is a new important finding for Baltic amber. Final results from the advanced investigation confirmed some of the previous findings and gave new important outcomes, especially concerning the degrading roles of oxygen, relative humidity and pH. After the interpretation of the final achieved data, combined with initial information from the preliminary investigation, the main relations between amber and environmental factors during the degradation process became clearer and it was possible to identify the major pathways by which amber degrades, such as hydrolysis of esters into carboxylic acids, thermal-oxidation and photo-oxidation of terpenoid components, depolymerisation and decomposition of the chemical structure. At the end it was possible to suggest a preventive conservation strategy based on the control of climatic, atmospheric and lighting parameters in the environment where Baltic amber objects are stored and displayed. ii List of Abbreviations. This list contains all the abbreviations used in the thesis. Units of measurement and chemical formulas were used according to the standards in the scientific field (International System of units –SI–; International Union of Pure and Applied Chemistry –IUPAC–) and they are not included in this list, as well as commercial acronyms related to reagents, consumable experiment supplies and technical equipments. ANOVA ANalysis Of VAriance ATR Attenuated Total Reflectance BCE Before Christian Era C-H-N Carbon, Hydrogen, Nitrogen (content) CIE Commission Internationale de l'Eclairage DRIFT Diffuse-Reflectance Infrared Fourier Transform DTGS Deuterated TriGlycine Sulphate ESR Electron Spin Resonance FT Fourier Transform FTIR Fourier Transform InfraRed GC Gas Chromatography ISO International Organization for Standardization LED Light Emitting Diode MCT Mercury Cadmium Telluride MS Mass Spectrometry NIR Near InfraRed NMR Nuclear Magnetic Resonance PC Personal Computer PDMS PolyDiMethylSiloxane PTFE PolyTetraFluoroEthylene Py Pyrolysis Ref. Reference RH Relative Humidity SCI Specular Component Included SPME Solid Phase Micro Extraction UV UltraViolet YAG Yttrium Aluminium Garnet iii List of Tables & Figures. Tables. Table 1.1. Main physical properties of amber Table 1.2. Mean elemental composition of amber Table 1.3. Most important tree resin-producing plant families which are considered potential sources of amber Table 1.4. Historical-literary classification for some European amber deposits Table 1.5. Structural classification system for fossil resins (modified from Anderson et al., 1992) Table 1.6. Main chemical composition of succinite Table 1.7. Early conservation treatments for deteriorated Baltic amber Table 2.1. Analytical techniques commonly used for amber studies Table 2.2. Main infrared bands characteristic for Baltic amber Table 3.1. Characteristics of the Perkin Elmer Spectrum One FTIR spectrometer used for the analysis of amber fragments Table 3.2. Operative settings for ATR-FTIR analysis of amber fragments Table 3.3. Operative conditions for thermal-ageing of amber samples Table 3.4. Operative conditions for photo-ageing of amber samples Table 3.5. Microclimatic conditions used for the accelerated ageing of amber samples Table 3.6. Analytical techniques used for the examination of amber samples Table 3.7. Operative settings for photographic recording of amber samples Table 3.8. Operative settings for spectrocolorimetric analysis of amber samples Table 3.9. Operative settings for ATR-FTIR analysis of amber samples Table 3.10. Characteristics of the Bruker RFS 100 FT-Raman spectrometer used for the analysis of amber samples Table 3.11. Operative settings for FT-Raman analysis of amber samples Table 3.12. Operative settings for respirometric analysis of amber samples Table 3.13. Characteristics of the SPME-GC-MS equipment used for the analysis of amber samples Table 3.14. Operative settings for SPME-GC-MS headspace analysis of amber samples Table 3.15. Ageing experiment design. Unless it is indicated otherwise, each container held one prism or one gram of powder. The different analytical groups are defined according to table 3.6 in paragraph 3.4 Table 3.16. Characteristics of the Perkin Elmer AutoImage FTIR microscope used for the analysis of amber cross sections Table 3.17. Operative settings for micro-ATR-FTIR cross sections analysis of amber samples iv