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- Looking into historical cracks analysing repaired silver objects by electron microscopyPublication . Tissot, Isabel; Barreiros, M. Alexandra; Lemos, Manuel; Tissot, MatthiasABSTRACT: The identification of structural alterations of silver alloy objects of cultural interest is essential to assess their state of conservation and define maintenance programmes. Alterations induced by previous repair and cleaning actions strongly affect the conservation condition, influencing the type and extension of conservation procedures. The detailed analysis of ancient repairs is a subject to be investigated. In this work, we show the potentialities of scanning electron microscopy in the characterisation of structural repairs and their influence on the conservation state of silver objects. The study of distinct silver objects dated to the 16th and 18th centuries allowed us to show, among others, the non-intentional dealloying of copper due to the use of acid solutions in surface cleanings and cracks resulting from incorrect handling. The data collected by microscopic examination contributed to the definition of preservation and maintenance methodologies applied to those silver objects.
- When gold stops glittering: corrosion mechanisms of René Lalique's Art Nouveau jewelleryPublication . Tissot, Isabel; Correia, Jorge; Monteiro, Olinda C.; Barreiros, M. Alexandra; Guerra, M. F.ABSTRACT: Art Nouveau jewellery created by René Lalique is presently corroded. To identify the corrosion processes, Au-Ag-Cu alloys with compositions comparable to those used in the René Lalique’s jewellery were fabricated to be exposed to sulphide-containing environments. Using SEM-EDS, XRD, UV-Vis spectroscopy and ellipsometry, it was for the first time demon-strated that at the surface of tarnished Au alloys forms a corrosion film with a layer-by-layer structure. Considering the complex refractive indices of bulk Cu and Ag oxides and sulphides, a two-step corrosion mechanism was proposed. The formation of Cu-based compounds during the early corrosion stages is followed by the formation of Ag-based compounds. The thinness of the formed film, shown for one of the gold alloys to be of 80 nm, is due to a corrosion kinetics controlled by the presence of Au and by the formation of an Au-S self-assembled monolayer. The corrosion mechanism of gold alloys raises a new conservation challenge concerning the removal of nanometric layers.
- The earrings of Pancas treasure: Analytical study by X-ray based techniques – A first approachPublication . Tissot, Isabel; Tissot, M.; Manso, M.; Alves, L. C.; Barreiros, M. Alexandra; Marcelo, Teresa; Carvalho, M. L.; Corregidor, V.; Guerra, M. F.The development of new metallurgical technologies in the Iberian Peninsula during the Iron Age is well represented by the 10 gold earrings from the treasure of Pancas. This work presents a first approach to the analytical study of these earrings and contributes to the construction of a typological evolution of the Iberian earrings. The manufacture techniques and the alloys composition were studied with three complementary X-ray spectroscopy techniques: portable EDXRF, μ-PIXE and SEM–EDS. The results were compared with earrings from the same and previous periods.
- A multi-analytical approach to gold in Ancient Egypt: studies on provenance and corrosionPublication . Tissot, Isabel; Troalen, L. G.; Manso, M.; Ponting, M.; Radtke, U.; Barreiros, M. Alexandra; Shaw, I.; Carvalho, M. L.; Guerra, M. F.Recent results from a three-year multi-disciplinary project on Ancient Egyptian gold jewellery revealed that items of jewellery from the Middle Kingdom to the New Kingdom were manufactured using a variety of alluvial gold alloys. These alloys cover a wide range of colours and the majority contain Platinum Group Elements inclusions. However, in all the gold foils analysed, these inclusions were found to be absent. In this work a selection of gilded wood and leather items and gold foil fragments, all from the excavations by John Garstang at Abydos (primarily from Middle Kingdom graves), were examined using Scanning Electron Microscopy-Energy Disperse Spectroscopy (SEM-EDS), X-Ray Fluorescence (µXRF), Particle Induced X-Ray Emission (µPIXE) and Double Dispersive X-Ray Fluorescence (D2XRF). The work allowed us to characterise the composition of the base-alloys and also to reveal the presence of Pt at trace levels, confirming the use of alluvial gold deposits. Corrosion products were also investigated in the foils where surface tarnish was visually observed. Results showed that the differences in the colour of corrosion observed for the foils are related not only to the thickness of the corrosion layer but also to a multi-layer structure containing the various corrosion products.