X-RAY FLUORESCENCE AND DENSITY MEASUREMENTS ON SURFACE-TREATED ROMAN SILVER COINS*

Some 630 Roman silver coins excavated at Augusta Raurica (Switzerland) have been analysed by non-destructive X-ray fluorescence and X-ray diffraction. Furthermore, the density and the weight have been determined. The measured average density of plated and massive depletion-silvered coins is lower than the density calculated from the chemical composition, whereas massive coins display equal values for both of the densities. Based on experimental X-ray investigations of modern silver (tempering, acid treatment, production of corrosion layers), the nature of corrosion products on silver, their impact on surface analysis, and aspects of wearing-off by circulation are briefly discussed.     

Non‐Destructive Survey of Early Roman Copper‐Alloy Brooches using Portable X‐ray Fluorescence Spectrometry

This paper argues that portable X‐ray fluorescence spectrometry (pXRF) is a suitable elemental measurement technique to study the production of copper‐alloy artefacts. However, rather than try to imitate the accuracy and precision of laboratory techniques, it is more beneficial to deploy it in a survey role, one that attempts to model chronological and geographical changes within large quantities of artefacts. To achieve this, it was investigated to what extent corrosion and the issues surrounding surface measurements affect the potential of this type of research. Analyses on early Roman period brooches gathered in the Nijmegen region of the Netherlands were subsequently compared with published data.     

DAMAGE ASSESSMENT AND PRESERVATION OF AN EGYPTIAN SILVER VASE (300–200 BC)*

An Egyptian metal vase from the Ptolemaic period was investigated metallurgically and fractographically. The metal is nearly pure silver. Despite its high purity (in archaeological terms) it is severely embrittled and cracked, mainly along grain boundaries. Factors contributing to this damage are work-hardening and residual microstructural deformation (slip and twinning), corrosion along slip lines and twin boundaries, corrosion in bauds that are the remains of coring, large equiaxed grains, externally applied forces and internal residual stresses. The metal is now friable. This must be taken into account during any further restoration of the vase: a procedure is suggested.     

LEAD ISOTOPE ANALYSES OF BRONZE AGE COPPER‐BASE ARTEFACTS FROM AL‐MIDAMMAN,YEMEN: TOWARDS THE IDENTIFICATION OF AN INDIGENOUS METAL PRODUCTION AND EXCHANGE SYSTEM IN THE SOUTHERN RED SEA REGION*

The results of the lead isotope analysis (LIA) of 15 copper‐base artefacts from the Bronze Age site of al‐Midamman, Yemen, are reported. The LIA data suggest the existence of an indigenous Bronze Age metal production and exchange system centred on the southern Red Sea region, distinct from those in neighbouring regions of Arabia and the Levant. These preliminary results are highly significant for the archaeology of the region, suggesting that local prehistoric copper extraction sites have thus far gone unrecorded, and highlighting the need for systematic archaeometallurgical fieldwork programmes in the countries surrounding the southern Red Sea.     

ELECTROCHEMISTRY AND AUTHENTICATION OF ARCHAEOLOGICAL LEAD USING VOLTAMMETRY OF MICROPARTICLES: APPLICATION TO THE TOSSAL DE SANT MIQUEL IBERIAN PLATE

An essentially non‐invasive electrochemical methodology addressed to the authentication of archaeological lead is described. The method is based on the record of the voltammetric response of nanosamples from the archaeological artefact mechanically transferred to a graphite ‘pencil’ electrode in contact with aqueous buffers. Three diagnostic criteria for authentication are described based on the appearance of: (i) oxidative dissolution signals for trace metals like copper, arsenic, antimony and, often, tin and silver accompanying stripping peaks for lead, (ii) peak potential shifts for reduction peaks for patination products, and (iii) the presence of reduction peaks for PbO₂. The method is applied to the authentication of an Iberian lead plate from the Tossal de Sant Miquel (Llíria, Spain) site using a series of genuine and false pieces from different provenances in the Valencian region (Spain).     

AN ASSESSMENT OF COMPOSITIONAL AND MORPHOLOGICAL CHANGES IN MODEL ARCHAEOLOGICAL GLASSES IN AN ACID BURIAL MATRIX

The degradation mechanisms of glass in a buried context result in surfaces that have been depleted in various elements. The stability of the glass is primarily affected by the burial environment and the glass composition. However, in all archaeological glasses, the corroded layer that is formed on the surface tends to be low in alkalis, high in silica and lacking in cohesion. The extent to which the material has degraded, along with the physical nature of the corrosion, has a profound effect upon a wide range of factors affecting the stability of artefacts, as well as the choice of conservation techniques to be employed. This study has a number of objectives: determination of the morphology of the surface of the leached layer in glasses of two different compositions with different surface finishes; examination of the transition between the corroded material and the unaffected substrate; and investigation of concentration profile of different elements within the surface layers, as a function of depth. The study uses two glasses, fabricated under laboratory conditions, to replicate two common glass types found in the historical environment; a soda–lime–silica glass typical of those found in the Roman period throughout the Mediterranean and northwestern Europe, and high‐lime–potash glasses typical of those of Western Europe in the late medieval period. Three different surfaces have been prepared to mimic alternative manufacturing techniques such as blown, cast and ground surfaces for each composition. The glasses have been corroded under controlled laboratory conditions to replicate the buried environment. Imaging and chemical information is obtained using SEM–EDX and morphological information using IFM to produce 3‐D mapping from topographical surfaces.     

IDENTIFICATION OF FORGERIES BY MEASURING TIN ISOTOPES IN CORRODED BRONZE OBJECTS*

The reliable identification of fakes consisting of bronze often presents problems, because traditional methods such as stylistic studies, optical microscopy, chemical analysis or X‐ray diffraction of the corrosion may not be conclusive. We present a method that is based on the comparison of the tin isotope ratios ¹²²Sn/¹¹⁶Sn and ¹¹⁷Sn/¹¹⁹Sn in the metal and in the adherent corrosion layer. An artificial patina is usually generated within a short time period. It has been observed that such a procedure leads to a depletion of the light tin isotopes in the corrosion layer, while in naturally corroded authentic archaeological objects no isotopic fractionation has been detected. The method has also been applied to archaeological objects and it could be confirmed, among other examples, that the famous ‘Sky Disc of Nebra’ is authentic.     

GILDINGS AND FALSE GILDINGS OF THE BAROQUE AGE: CHARACTERIZATION AND CONSERVATION PROBLEMS

This work investigates the decayed decoration of the ‘Vergine del Carmelo’ chapel, with a particular interest in the residual gildings. Samples of the finishing layers were examined by VIS‐UV optical microscopy and scanning electron microscopy coupled with EDX spectrometry, in order to evaluate the unusual degradation phenomena. Mineralogical composition of the stucco substrate was characterized by X‐ray diffraction (XRD) and FTIR spectroscopy. The results showed the unexpected presence of a thick layer of brass flakes, applied without any binder, used as a substitute for the damaged original gold leaf, and highlighting a very uncommon selective corrosion process of the brass.     

A CASE STUDY OF ARTHROPODS PRESERVED ON ARCHAEOLOGICAL BRONZES—MICRO‐ARCHAEOLOGICAL INVESTIGATION HELPS RECONSTRUCTING PAST ENVIRONMENTS

This paper offers new perspectives for improving our knowledge of arthropods in wet and aerobic archaeological contexts. The very‐near environment of uncleaned buried bronze artefacts, two ronde‐bosse ornaments of a Celtic grave from the Iron Age (275 to 250 bc , La Fosse‐Cotheret, Roissy‐en‐France), was studied by means of optical and scanning electron microscopies. Taking into account recent knowledge on bronze corrosion (decuprification), this micro‐investigation (micro‐archaeology) allowed identification of two fossilized arthropods—Cryptophagus sp. (Insecta, Coleoptera, Cryptophagidae) and Coccotydaeolus sp. (Acari, Tydaeoidea, Iolinidae)—which sheds some light upon the complex taphonomic process that occurred after the burial of a Celtic chariot. These first results support the idea that the local corroded metal/soil system has to be investigated for a better understanding of archaeological microenvironments. Efficient minute investigation of soiled bronzes could be performed prior to systematic cleaning of artefacts.     

MICROSTRUCTURAL CHARACTERIZATION OF EARLY WESTERN GREEK INCUSE COINS*

In this research, we studied the compositional, crystallographic and microstructural properties of a series of incuse silver didrachmae stemming from the Achaean colonies of Metapontum and Caulonia. In this paper, we address the following points: (i) the metal sources, (ii) the fabrication process and (iii) degradation phenomena, such as incrustation and embrittlement. In this investigation, we employed energy‐dispersive X‐ray fluorescence spectroscopy, X‐ray diffractometry, and scanning electron and optical microscopies. The patina is mainly composed of chlorargirite. The coins consist of a silver‐rich alloy containing ～ 1% of Au and Cu. Metallographic and local compositional analyses revealed a complex scenario of inclusions. In one instance, unalloyed copper grains, two‐phase copper/bismuth globuli and high‐bismuth filaments were observed. In other cases, globular Cu ₂ S (chalcocite) inclusions were noticed. The presence of SiO ₂ and iron oxide inclusions is ubiquitous in these samples. Distorted twin lines and strain lines can be detected, denoting work‐hardening of recrystallized flans. Grain polygonalization can occasionally be noticed, hinting at secondary recrystallization processes. The irregularly shaped iron oxide particles often act as crack initiation sites. Fracture facets are generally intergranular. On some areas, intergranular decohesion is also observed. Open cracks sometimes contain AgCl. The strain lines that can be noticed on the fracture surfaces indicate work‐hardening and residual microstructural deformation. Information regarding inclusions and the presence of significant amounts of gold can be tentatively used to address provenancing and fabrication issues.