Maria Filomena Guerra

Gold cultural heritage objects: a review of studies of provenance and manufacturing technologies

An overview of the use of the elemental composition of gold in the study of manufacturing technologies for objects as well as of the provenance of the metal is given. Depending on the objects, techniques based either on atomic physics or on nuclear physics and mass spectrometry are required to answer the questions surrounding the process of metalworking in the past. Several archaeological examples covering different periods of time and involving diverse analytical techniques are presented here. With those examples we illustrate the main research questions on precious objects and coins and show how far the elemental analysis of the objects can reveal the way they were made and provide information on the origin of the gold.

Analysis of archaeological metals. The place of XRF and PIXE in the determination of technology and provenance

Problems concerning the determination of technology and provenance of archaeological metals are presented using several historical examples and considering the heterogeneity of the alloys. PIXE and XRF are compared to other techniques (PIGE, nuclear activation, LA-ICP-MS); their limitations and complementary aspects in obtaining historical information are discussed.

SOUTH AMERICAN PRECIOUS METALS AND THE EUROPEAN ECONOMY : A SCIENTIFIC ADVENTURE IN THE DISCOVERIES TIME

Abstract Charged particles produced by a cyclotron were used to characterise and follow two important precious metals from the New World which changed the European economy at the time: Potosian silver in the 16th century and Brazilian gold in the 18th century. Coupling IBA techniques with LA-ICP-MS we made a first attempt to differentiate the American gold ores which took a major role in the economy of the Discoveries period.

Beyond the Great Wall: gold of the silk roads and the first empire of the steppes.

Fingerprinting ancient gold work requires the use of nondestructive techniques with high spatial resolution (down to 25 μm) and good detection limits (micrograms per gram level). In this work experimental setups and protocols for synchrotron radiation induced X-ray fluorescence (SRXRF) at the BAMline of the Berlin electron storage ring company for synchrotron radiation (BESSY) in Berlin for the measurement of characteristic trace elements of gold are compared considering the difficulties, shown in previous works, connected to the quantification of Pt. The best experimental conditions and calculation methods were achieved by using an excitation energy of 11.58 keV, a silicon drift chamber detector (SDD) detector, and pure element reference standards. A detection limit of 3 μg/g has been reached. This newly developed method was successfully applied to provenancing the Xiongnu gold from the Gol Mod necropolis, excavated under the aegis of the United Nations Educational, Scientific and Cultural Organization (UNESCO). The composition of the base alloys and the presence of Pt and Sn showed that, contrary to what is expected, the gold foils from the first powerful empire of the steppes along the Great Wall were produced with alluvial gold from local placer deposits located in Zaamar, Boroo, and in the Selenga River.

Practice-Based Comparison of Imaging Methods for Visualization of Toolmarks on an Egyptian Scarab

3D representations were made of a small Egyptian scarab with a gold band by a number of methods, based on photogrammetry and photometric stereo. They were evaluated for colour fidelity and spatial detail, in the context of a study of toolmarks and manufacturing techniques of jewellery in ancient Egypt. It was found that although a 3D laser scanner gave the best geometric accuracy, the camera-based methods of photogrammetry and photometric stereo gave better representation of fine detail and colour on the object surface.

Corrosion of silver alloys in sulphide environments: a multianalytical approach for surface characterisation

Sterling silver samples, prepared to simulate cultural heritage surfaces, were subjected to accelerated ageing tests through exposure to sulphide containing environments. Data obtained by X-ray diffraction, ultraviolet-visible spectrophotometry, contact angle goniometry, ellipsometry and scanning electron microscopy revealed, contrarily to what has been suggested, that the colour of corroded surfaces is related to the thickness of the corrosion layer and to the multi-layer structure of various corrosion products. At the early stages of corrosion, Cu prevails over the Ag compounds. In subsequent stages, AgCuS complexes were also detected. Ag2S is the prevailing corrosion product after longer periods of time.

The influence of the constituent elements on the corrosion mechanisms of silver alloys in sulphide environments: the case of sterling silver

The corrosion of copper and silver in sulphide environments was assessed by several analytical techniques to investigate the influence of each one on the corrosion of sterling silver. The surface colour changes with the immersion time due to the formation of corrosion products composed of particles with distinct sizes and shapes and the consequent layer thickening. Ag2S is the main corrosion product of silver. At early stages of corrosion, Cu develops Cu2O and Cu2S, and later Cu2O again. After the corrosion of sterling silver, pure Cu and Ag are compared, and the predominant influence of Cu at the first stages may be suggested whilst Ag mainly contributes to longer corrosion stages. The layer-by-layer corrosion structure observed for the sulphidation of sterling silver was not observed for its constituent elements.

An Inca silver figurine at National Museums Scotland: Technological study

The hollow silver male miniature figurine from National Museums Scotland is one of the tallest specimens made in precious metals attributed to the Incas. In spite of showing the expected characteristics of this type of Inca production for ritual offerings —regular proportions and standing pose, representation of its gender, bulging cheek associated to coca leaves chewing and elongated earlobes— this figurine differs from the regular figurines by the form of the top of its head. In addition, the skilled decoration includes details such as well portrayed feet. The technologies and the alloy employed in its manufacture were examined using a stereo microscope and by non-destructive X-ray fluorescence analysis. Comparison between this figurine with the hollow silver male figurines of equivalent stature and typology kept at Dumbarton Oaks Research Library and Collection and Museum of America shows that they are all made with the same technology and by employing silver alloys with copper contents below 0.5%. These alloys are richer in silver than the majority of the alloys employed to fabricate normal Inca hollow figurines of known provenance or from other museum collections.The hollow silver male miniature figurine from National Museums Scotland is one of the tallest specimens made in precious metals attributed to the Incas. In spite of showing the expected characteristics of this type of Inca production for ritual offerings —regular proportions and standing pose, representation of its gender, bulging cheek associated to coca leaves chewing and elongated earlobes— this figurine differs from the regular figurines by the form of the top of its head. In addition, the skilled decoration includes details such as well portrayed feet. The technologies and the alloy employed in its manufacture were examined using a stereo microscope and by non-destructive X-ray fluorescence analysis. Comparison between this figurine with the hollow silver male figurines of equivalent stature and typology kept at Dumbarton Oaks Research Library and Collection and Museum of America shows that they are all made with the same technology and by employing silver alloys with copper contents below 0.5%. These alloys are richer in silver than the majority of the alloys employed to fabricate normal Inca hollow figurines of known provenance or from other museum collections.