Analytical strategies for the identification and characterisation of thin corrosion layers on ancient gold artefacts

Tese de doutoramento, Engenharia Física, Universidade de Lisboa, Faculdade de Ciências, 2018The colour, morphology, thickness, elemental and structural composition of the thin atmospheric corrosion layer that develops on ancient gold objects exhibited or stored in museums were investigated using analytical techniques chosen after establishing the main requirements for those objects. An analytical protocol based on the advantages of each selected analytical technique and on the characteristics of the corrosion layer is presented and discussed. The study was carried out in two steps. Firstly, non-invasive portable and stationary techniques were tested directly on prehistoric goldwork in the collection of the National Museum of Archaeology in Lisbon, on jewellery by René Lalique in the collection of the Gulbenkian Foundation Museum, and on Egyptian gold foils in the collections of the Garstang Museum in Liverpool and the National Museums Scotland in Edinburgh. Secondly, and to overcome the restrictions related to moving gold objects from museums, the same techniques were applied to the study of samples made from binary and ternary gold alloys fabricated in this work to be subjected to natural and artificial corrosion in similar atmospheric conditions. The advantages and limitations of each technique are discussed. Based on the results obtained, it could be shown that portable equipment for in-situ analysis is not suitable for this type of investigation, but Scanning Electron Microscopy (SEM) could assess the morphology of the corroded layer, and the colour analysed by Ultraviolet-visible spectrophotometry (UV-Vis). The corrosion layer thickness could be estimated by ellipsometry. The elemental composition could be determined by micro-X-ray fluorescence spectrometry (μXRF) and by Energy Dispersive Spectrometry (EDS), and the structural composition obtained by X-ray Diffraction (XRD). The Treasure room of the National Museum of Archaeology exhibits gold objects with an accentuated corroded surface due to atmospheric corrosion. The main pollutants for indoor corrosion could be inferred and their sources identified. The environmental conditions were related to the corrosion products formed on the surface of the objects. Sulphur was identified as the principal pollutant of gold alloys in indoor environments; the main corrosion products are, thus, sulphur-based compounds. Data on René Lalique’s jewellery showed the influence of the fabrication techniques on the corrosion development and the study of the Egyptian foils allowed relating the colours of the corroded surfaces to the corrosion products formation. It could be shown that the atmospheric corrosion process leads to the development of a layer-by-layer structure consisting of corrosion products with different morphologies and distinct compositions. The characterisation of binary and ternary gold alloys samples corroded in sulphide-containing solutions was complemented with the study of corroded pure silver, pure copper and sterling silver samples. The role of the constitutive elements on the corrosion process of gold alloys was defined, the corrosion layer thickness was determined, and the relation between the colour of the corroded layer and the morphology of the corrosion products was discussed. The corrosion products formed on the silver and copper samples surfaces are mainly composed of silver sulphide and copper sulphides, and their different corroded colours depend on the corrosion layer thickness. The different colours of corroded sterling silver and gold alloy surfaces are related to the formation of distinct corrosion products with different morphologies, in a layer-by-layer structure. For sterling silver, it was revealed that at early corrosion stages, there is a prevalence of the formation of copper-based compounds, followed by the formation of silver-based compounds. For gold alloys, it was shown the formation of a two-layer film composed of a 60 nm thick inner-layer and a 20 nm thick outer-layer. The corrosion layer thickness of gold alloys and its composition limits the use of the selected techniques. UV-Vis analysis of the silver, copper and sterling silver samples allowed to differentiate corrosion products composed of particles with distinct sizes, but in the case of gold alloys it was only possible to show a reflectance decrease for the corroded areas. EDS analysis provided the quantitative elemental composition of the corroded layer of silver and copper samples as well as of each layer of the layer-by-layer structure that developed on sterling silver. However, in the case of gold alloys the identification of sulphur is difficult due the right tail intense peak corresponding to the Au-M lines close to the S-K line. XRD identified the compounds present on the Ag and Cu samples. The spatial resolution of the equipment is, however, inappropriate to identify the compounds that constitute each layer of the layer-by-layer structure that develops on corroded sterling silver and gold alloys