Production of reference “ancient” Cu-based alloys and their accelerated degradation methods

A large number of bronze artefacts found during archaeological excavations carried out in Italy in different contexts have been studied. Their chemical composition and metallurgical features have been determined by the combined use of different analytical surface and bulk techniques, such as optical microscopy (OM), scanning electron microscopy with energy dispersive X-ray micro-analysis (SEM-EDS), X-raydiffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The study of the corrosion products grown on the archaeological Cu-based artefacts and of their metallurgical features has revealed the quite ubiquitous and nearly constant presence of chlorine as the main corroding agent, and the different structures of the alloys. This information has been used to produce reference Cu-based alloys, whose chemical composition and micro-chemical structure is similar to that of the ancient alloys, and to propose the guidelines for carrying out the accelerated degradation tests to produce corroded samples for testing corrosion inhibiting products. The proposed tests were based on soil, chemical and (chemical+soil)-induced degradation, and the micro-chemical structure of the artificially produced corrosion layers has been compared to those grown on archaeological artefacts during burial. The comparison shows that the (chemical+soil)-induced degradation produces “patinas” that are similar to those grown on archaeological artefacts from a chemical, structural and micro-morphological point of view. PACS 68.55.Jk; 68.35.Dv; 68.37.Hk; 68.55.Nq; 81.05.Ba     

Micro-chemical and micro-structural investigation of archaeological bronze weapons from the Ayanis fortress (lake Van, Eastern Anatolia, Turkey)

Bronze weapons (VII cen BC) found during the archaeological excavation of the Ayanis fortress (lake Van, eastern Anatolia, Turkey) are investigated in order to determine their chemical composition and metallurgical features as well as to identify the micro-chemical and micro-structural nature of the corrosion products grown during long-term burial. Small fragments were sampled from the artefacts and analysed by means of the combined use of optical microscopy (OM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS). The results show that the bronze artefacts have been manufactured by using alloys with a controlled and refined chemical composition demonstrating the high level metallurgical competence and skill of the Urartian craftsmen and artists. Furthermore, the micro-structural and metallurgical investigations evidence the presence of equiaxed grains in the matrix, indicating that the artefact were produced by repeated cycles of mechanical shaping and thermal annealing treatments to restore the alloy ductility. From the degradation point of view, the results show the structures and the chemical composition of the stratified corrosion layers (i.e. the patina) where the copper or tin depletion phenomenon is commonly observed with the surface enrichment of some elements coming from the burial soil, mainly Cl, which is related to the high concentration of chlorides in the Ayanis soil. The results reveal also that another source of degradation is the inter-granular corrosion phenomenon likely increased by the metallurgical features of the alloys caused by the high temperature manufacturing process that induces crystallisation and segregation phenomena along the grain boundaries.     

Uncommon corrosion phenomena of archaeological bronze alloys

In the framework of the EFESTUS project (funded by the European Commission, contract No. ICA3-CT-2002-10030) the corrosion products of a large number of archaeological bronze artefacts are investigated by means of the combined use of scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), X-ray diffraction (XRD) and optical microscopy (OM) and tentative correlation of their nature with the chemical composition of the artefacts and the burial context is proposed.The results provide good insight into the corrosion layers and evidence in some bronze Roman coins and artefacts; the occurrence of uncommon corrosion phenomena that give rise to the formation of a yellowish-green complex chlorine-phosphate of lead (pyromorphite, (PbCl)Pb₄(PO₄)₃) and of a gold-like thick layer of an iron and copper sulphide (chalcopyrite, CuFeS₂). The micro-chemical and micro-structural results show that the coins were buried in a soil enriched in phosphorus for the accidental presence of a large amount of decomposing fragments of bones or in an anaerobic and humus rich soil where the chalcopyrite layer has been produced via the interaction between the iron of the soil, the copper of the coin and the sulphur produced by the decomposition of organic matter in an almost oxygen free environment. Finally, some unusual periodic corrosion phenomena occurring in high tin bronze mirrors found at Zama (Tunisia) are described. PACS 68.55Jk; 68.35 Dv; 68.37Hk; 68.55 Nq; 81.05 Bx     

Large scale investigation of chemical composition, structure and corrosion mechanism of bronze archeological artefacts from Mediterranean basin

A large number of Cu-based archaeological artefacts from the Mediterranean basin have been selected for investigation of their chemical composition, metallurgical features and corrosion products (i.e. the patina).The guidelines for the selection of the Cu-based artefacts have taken into account the representativeness of the Mediterranean archaeological context, the manufacturing technique, the degradation state and the expected chemical composition and structure of the objects.The results show wide variation of the chemical composition of the alloys that include all kinds of ancient Cu-based alloys such as low and high tin, and also leaded bronzes, copper and copper-iron alloys.The examination of the alloy matrix shows largely different metallurgical features thus indicating the use of different manufacturing techniques for producing the artefacts. The results of the micro-chemical investigation of the patina show the structures and the chemical composition of the stratified corrosion layers where copper or tin depletion phenomenon are commonly observed with a remarkably surface enrichment of some soil elements such as P, S, Ca, Si, Fe, Al and Cl. This information indicates the strict interaction between soil components and corrosion reactions and products. In particular, the ubiquitous and near constant presence of chlorine in the corrosion layers is observed in the patina of the archaeological Cu-based artefacts found in different contexts in Italy, Turkey, Jordan, Egypt, Spain and Tunisia. This latter occurrence is considered dangerous because it could induce a cyclic corrosion reaction of copper that could disfigure the artefact.The micro-chemical and micro-structural results also show that another source of degradation of the bronze archaeological artefacts, are their intrinsic metallurgical features whose formation is induced during the manufacturing of the objects, carried out in ancient times by repeated cycles of cold or hot mechanical work and thermal treatments. These combined treatments induce crystallisation and segregation phenomena of the impurities along the grain boundaries and could cause mechanical weakness and increase the extent of the inter-granular corrosion phenomena. PACS 68.55.Jk; 68.35.Dv; 68.37.Hk; 68.55.Nq; 81.05.Bx     

Combined use of SEM-EDS, OM and XRD for the characterization of corrosion products grown on silver roman coins

In the framework of the PROMET project (European Commission contract No. 509126) aimed to develop new analytical techniques and materials for monitoring and protecting metal artefacts and monuments from the Mediterranean region, the corrosion products grown on silver Roman coins during archaeological burial is studied by means of scanning electron microscopy combined with energy dispersive spectrometry (SEM-EDS), X-ray diffraction (XRD) and optical microscopy (OM) techniques. PACS 68.55.Jk; 68.35.Dv; 68.37.Hk; 68.55.Nq; 81.05.Bx     

Combined use of GDOES,SEM + EDS,XRD and OM for the microchemical study of the corrosion products on archaeological bronzes

By means of the combined use of glow discharge optical emission spectrometry (GDOES), scanning electron microscopy+energy dispersive spectrometry (SEM+EDS), X-ray diffraction (XRD) and optical microscopy (OM), corrosion products, i.e., the patina, grown on archaeological leaded bronze artefacts used by Punics and Romans, have been studied. This innovative approach has been utilised in order to gain further insight into the microchemical structure of the corrosion products as well as for selecting the cleaning and restoration methods. For all of the archaeological artefacts, the results show that via the innovative use of GDOES, it is possible to obtain reliable and reproducible quantitative elemental composition depth profiles for the outermost corroded layers, which are briefly discussed in combination with the microchemical features obtained via SEM+EDS, XRD and optical microscopy. Finally, the results show that GDOES, with its ability to provide routine and rapid analysis of layers of thickness up to 120 m, seems to be a powerful technique in studies of the corrosion products on archaeological bronzes, with the aim of restoring and conserving ancient high tin or low tin leaded bronzes. PACS 68.55Jk; 68.35 Dv; 68.37Hk; 68.55 Nq; 81.05 Bx     

Investigation of Cl corrosion products of iron archaeological artefacts using micro-focused synchrotron X-ray absorption spectroscopy

Synchrotron-based micro-X-ray absorption spectroscopy is used in the present study to obtain chemical information at the microscopic scale such as coordination and oxidation state of Fe atoms in phases constituting corrosion products within archaeological iron artefacts buried in soil. This technique is required in order to answer questions about the iron corrosion process related to the presence of chloride, particularly for restoration and conservation of metallic artefacts of the cultural heritage. The samples available for X-ray microprobe analyses are cross sections from corroded iron archaeological objects. Previously, complementary techniques have been used such as μXRD and μRaman. This specific study applies micro-X-ray absorption spectroscopy to determine the spatial variation of the predominant Fe oxidation state and to identify the corresponding crystallographic phase. The analyses performed at Fe and Cl K-edges (μXANES) reveal the correlation between the valence distribution in the corrosion products and the evolution of the chloride concentration. In addition to the presence of the well-known iron oxyhydroxide β-FeOOH: akaganeite, we highlight the presence of another important phase, the β-Fe₂(OH)₃Clhydroxychloride. These important findings help to gain new insights concerning the influence of such phases in the iron corrosion mechanism within their precise characterization. PACS 61.10.Ht; 61.10.-i; 68.49.Uv     

Effect of laser treatment on the surface of copper alloys

The paper presents the results of laser cleaning of the archaeological metal objects using two time widths of pulsed laser radiation, which are around 150 μs and around 120 ns. Two archaeological objects made of copper alloys were studied: a bow and a ring. Both objects came from a cemetery which is located in the garden complex of Wilanow Palace in Warsaw and are dated from XII to XIII century. The bow and bronze ring had ornamental longitudinal grooving and were part of burial jewellery. The materials of which these artefacts were made of, as well as corrosion products on these objects, were studied by using a variety of analytical techniques. The phase composition of the corrosion layers was determined by using Raman spectroscopy. The surface topography as well as the chemical composition of the deposits and cleaned surfaces were investigated. The samples were examined using scanning electron microscopes equipped with EDS. The investigations included observations in SE and BSE modes and point analyses of the chemical composition by EDS.     

Formation processes of CuCl and regenerated Cu crystals on bronze surfaces in neutral and acidic media

The paper is devoted to investigating the formation of CuCl and regenerated Cu crystals on bronze. Electrochemical behaviour of bronze in simulated anoxic edaphic media and occluded cell (O.C.) solutions was studied with cycle voltammetry (CV) and X-ray diffraction (XRD). Within potential range of −800 to +800 mV, oxidation occurred was largely a process in which Cu is oxidized to CuCl and the reduction process was a reverse of it. An atomic force microscopy (AFM) was used to observe the morphology of CuCl crystals, regenerated Cu crystals and corrosion interface at nm level. The deposition of regenerated Cu on simulated archaeological bronzes was simulated under experimental conditions for the first time. CuCl could be thoroughly reduced to pure Cu if reduction time interval were sufficiently prolonged. This provided a theoretical and experimental basis for getting rid of harmful CuCl patina from archaeological bronzes with electrochemical means.     

Complementary use of X‐ray methods to study ancient production remains and metals from Northern Portugal

Archaeological artefacts recovered at Castanheiro do Vento (Northern Portugal) were characterised by integrating macro and micro‐energy dispersive X‐ray fluorescence spectrometry (EDXRF) and scanning electron microscopy with X‐ray microanalysis. The collection includes metallurgical remains (ceramic crucibles, a metallic nodule and a vitrified fragment) and metals (tools and ornaments) whose chronology spans from the Chalcolithic to the Roman Age. The study of production remains was able to identify distinct copper‐based metallurgical operations including the smelting of copper ores, the melting of copper and tin and/or the melting of bronze scrap. Micro‐EDXRF identified copper and arsenical copper tools as well as bronze and leaded bronze ornaments. The composition of tools (Cu with varying As contents: 0.46–3.6%) reveals an incipient technology, typical of the Chalcolithic till the Middle Bronze Age. On the contrary, ornaments are composed by different alloys – low tin bronze (4.8% Sn), high tin bronze (14.9% Sn) and high tin‐leaded bronze (16.5% Sn and 2.4% Pb) ? evidencing technological and economic choices that clearly indicate a late period such as the Roman Age. In conclusion, this multiproxy approach was able to study those ancient artefacts with a minimum impact on their archaeological and museological significance while providing important answers to the interpretation of the archaeological settlement and to better understand the metallurgical evolution in the Portuguese territory. Copyright © 2014 John Wiley & Sons, Ltd.     

Bronze roman mirrors: the secret of brightness

The surface microchemical structure of high tin leaded bronze Roman mirrors has been studied by means of scanning electron microscopy combined with energy dispersive spectrometry (SEM-EDS), X-ray diffraction (XRD) and optical microscopy (OM) techniques. The results allowed understanding of the origin of their high chemical stability and silvery-lustrous appearance. Indeed, some areas of the selected Roman mirrors are still characterised by a highly reflective and silver coloured surface even though they have been buried in the soil for about 2000 years. The micro-chemical results obtained from these areas have revealed that the mirror surface was tin enriched via inverse-segregation phenomenon by tailoring the cooling parameters. Furthermore, the presence of tin could be likely enhanced via cycles of oxidation and selective copper corrosion processes, thus resulting in a tin surface enrichment as a semi-transparent amorphous-like tin oxide (SnO₂) film, as well as a copper depletion at the outer surfaces.     

Production of reference alloys for the conservation of archaeological silver-based artifacts

In the framework of the EC PROMET project, the chemical composition and metallurgical features of a large number of archaeological artifacts were investigated by different analytical surface and bulk techniques, such as Optical Microscopy (OM), Scanning Electron Microscopy coupled with energy dispersive X-ray micro-analysis (SEM-EDS), X-ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS). The study of the corrosion products grown on the archaeological Ag-based artifacts revealed a quite ubiquitous and nearly constant presence of chlorine from the soil as the main corroding agent, mainly producing chlorargyrite (AgCl). Results of this extensive chemical, physical and metallurgical characterization were used to produce modern reference Ag-based alloys with a chemical composition and micro-chemical structure similar to that of ancient alloys. Furthermore, these reference Ag-based alloys were submitted to an accelerated degradation method in order to produce corroded samples to be used as sacrificial materials for testing corrosion inhibiting products. The production of artificial “patinas” and corrosion layers was made by a chemical and soil-induced degradation procedure. The comparison of the micro-chemical structures of natural and artificial corrosion layers shows that the selected degradation method produces “patinas” resembling those grown on archaeological artifacts from a chemical, structural and micro-morphological point of view.     

Combining chemical data with GIS and PCA to investigate Phoenician–Punic Cu-metallurgy

This study focusses on the development and application of an innovative protocol which combines chemical data, GIS (geographical information system) and PCA, involving numeric (chemical composition) and categorical (typology of object, archaeological context, chronology and geographical areas) variables, as a simple tool to help in the visualisation and interpretation of large multidisciplinary datasets on Cu-based alloy archaeological artefacts influenced by Phoenician–Punic contacts. The protocol is a useful tool for highlighting existing connections between specific alloy chemical compositions, the location of the original settlement where the artefact had been produced and the proximity to mining resources, waterways, and allochthonous presence such as, in the specific case of this study, the Phoenician and Punic influence in the Iberian bronze production during the Late Bronze Age–Iron Age. The protocol was tested successfully in a case study concerning the precise dating and provenance of bronze statuettes of unknown age and provenance from the Evora Museum collection in Southern Portugal where it confirmed and further refined earlier hypotheses based solely on archaeological and/or chemical studies. The results were interpreted with a unique perspective, to validate the GIS system in combination with experimental chemical-physical data to yield the identification of metallurgical sites of bronze production.     

拉曼光谱对古青铜矛腐蚀情形的无损研究

应用显微拉曼光谱对云南楚雄万家坝出土的古青铜矛进行无损研究 ,可确定其腐蚀产物的成分及在样品上的位置 ,还可帮助分析青铜矛所经历的腐蚀环境。腐蚀产物的主要成分有 Cu2 O和 Cu CO3.Cu( OH) 2 。本文还辅以电子探针测试 ,对矛尖的抗腐蚀性作了分析研究。与传统的鉴别方法如电镜、X光衍射等分析法相比较 ,拉曼光谱被证实是对金属器物作无损检测的一种非常有用的方法     

Diamond-like carbon coatings for the protection of metallic artefacts: effect on the aesthetic appearance

Plasma-enhanced chemical vapour deposition (PECVD) is an environmentally friendly process used to deposit a variety of nano-structured coatings for the protection or the surface modification of metallic artefacts like the SiO₂-like films that have been successfully tested on ancient silver, bronze and iron artefacts as barriers against aggressive agents. This paper deals with the preliminary results of a wider investigation aimed to the development of eco-sustainable coatings for the protection of Cu and Ag-based artefacts of archaeological and historic interest. Diamond-like carbon (DLC) coatings have been deposited by PECVD in different experimental conditions, in a capacitively coupled asymmetric plasma reactor, placing the substrates either on electrically powered electrode (cathodic mode) or grounded electrode (anodic mode) with and without hydrogen addition in the gas mixture. The final goal is to develop a coating with good protective effectiveness against aggressive atmospheres and contemporarily with negligible effects on the aesthetic appearance of the artefacts. The evaluation of possible colour changes of the surface patinas, due to coating process, was performed by optical microscopy and colorimetric measurements. Furthermore, to evaluate the reversibility of the thin DLC layer, an etching treatment in oxygen plasma has been successfully carried out and optimized. The chemical-physical characterization of the deposited DLC coatings was performed by means of the combined use of micro-Raman and XPS spectroscopies. The results show that the DLC films obtained in the anodic mode, may be proposed as a viable alternative to polymeric coatings for the protection of metallic ancient objects.     

Dating study of two rock crystal carvings by surface microtopography and by ion beam analyses of hydrogen

Two artefacts made of rock crystal (quartz) from the collection of the Musée du quai Branly in Paris, France, a skull approximately half of the size of a real cranium and a smaller anthropomorphic head, purportedly attributed to pre-Columbian Mesoamerican cultures, were studied to assess their authenticity. The surface of the artefacts were examined by means of optical microscopy and scanning electron microscopy (SEM) and were analyzed nondestructively by ERDA (Elastic Recoil Detection Analysis), an ion beam analytical method that can measure hydrogen concentration profiles in depth. Optical and SEM imaging of tool marks indicates that the skull has been cut from a rock crystal block using machine lapidary techniques unavailable to pre-Columbian artisans, whereas the anthropomorphic head has more likely been carved and polished with manual techniques comparable to ancient ones. Hydrogen depth profiles in the first micron below the surface of the artefacts have been measured by ERDA with a 3-MeV He beam in a controlled helium atmosphere. Recently the progressive penetration of water at the surface of a quartz sample exposed to the natural environment has been proposed as a dating method (labeled quartz hydration dating or QHD) applicable to archaeological artefacts made of this material. The shallower penetration of H clearly indicates that the rock crystal skull was manufactured more recently than the reference quartz sample cut in 1740. As for the anthropomorphic head, the deep penetration profiles indicate an older artefact. Thus the converging micro-topographical examinations and hydrogen profiles of the samples surfaces indicate that the skull is probably not a pre-Columbian artefact but has been carved in the 18th or 19th century. The anthropomorphic head, on the other hand, could have been carved in the pre-Columbian period. In addition, the ERDA method carried out with an external beam presented here provides a new and simple approach for the nondestructive authentication of quartz-based archaeological artefacts by QHD.     

Analytical Methods for the Characterization of Organic Dyes in Artworks and in Historical Textiles

Abstract

Dyes are among the most significant components in works of art and archaeological findings. In the scientific examination of historical artefacts, the identification of natural dyestuffs is a challenging task, due to the complexity of their chemical composition and the possible presence of mixtures of chromophores and degradation products. For this reason, in the last few decades, new analytical procedures and techniques have been developed and improved for the characterization of organic dyes and their identification in microsamples. This review looks at the chemical composition of natural organic dyeing materials used in the field of the cultural heritage and focuses on several analytical methods based on spectrometric and chromatographic techniques that have contributed to the study of organic dyes in works of art and archaeological findings.     

Cultural heritage and archaeology materials studied by synchrotron spectroscopy and imaging

The use of synchrotron radiation techniques to study cultural heritage and archaeological materials has undergone a steep increase over the past 10–15 years. The range of materials studied is very broad and encompasses painting materials, stone, glass, ceramics, metals, cellulosic and wooden materials, and a cluster of organic-based materials, in phase with the diversity observed at archaeological sites, museums, historical buildings, etc. Main areas of investigation are: (1) the study of the alteration and corrosion processes, for which the unique non-destructive speciation capabilities of X-ray absorption have proved very beneficial, (2) the understanding of the technologies and identification of the raw materials used to produce archaeological artefacts and art objects and, to a lesser extent, (3) the investigation of current or novel stabilisation, conservation and restoration practices. In terms of the synchrotron methods used, the main focus so far has been on X-ray techniques, primarily X-ray fluorescence, absorption and diffraction, and Fourier-transform infrared spectroscopy. We review here the use of these techniques from recent works published in the field demonstrating the breadth of applications and future potential offered by third generation synchrotron techniques. New developments in imaging and advanced spectroscopy, included in the UV/visible and IR ranges, could even broaden the variety of materials studied, in particular by fostering more studies on organic and complex organic–inorganic mixtures, while new support activities at synchrotron facilities might facilitate transfer of knowledge between synchrotron specialists and users from archaeology and cultural heritage sciences.