The scientific analyses generally used to study gold are particularly suited to issues of material provenance and research into manufacturing techniques. But the art market needs to go beyond these results. We need to use these analytical tools to decide whether gold alloys are ancient or modern.
CIRAM's analytical approach to authenticating gold objects.
CIRAM has developed a methodology dedicated to the authentication of gold objects using optical and electron microscopy. This analytical protocol is based on a multi-criteria approach that combines surface examination to observe tool marks and corrosion patterns, to define the nature of surface deposits and to detect possible ancient or modern chemical treatments. At the same time, the chemical composition of the gold is analyzed by ICP-AES (Inductively coupled plasma atomic emission spectroscopy), quantifying concentrations of major and minor elements (up to 0.1%) and trace elements or impurities (concentrations below 0.1% or 1000 ppm - parts per million). It's important to remember that 1 ppm = 1 mg/kg.
Examination of tool marks and polishing techniques
Examination of tool marks is one of the technical markers of object manufacture and polishing techniques employed. Heterogeneous, multidirectional striations are the result of hand tools, while modern, mechanical tools produce clusters of fine, parallel striations.
The importance of surface deposit analysis
The analysis of surface deposits of sedimentary origin or corresponding to corrosion products will be an important step in establishing the age diagnosis. Indeed, the presence of silver chloride microparticles will indicate deliberate chemical attack, in order to simulate the natural weathering of the material. Alternatively, an amorphous surface deposit of iron and chlorine indicates residues of an acidic solution, such as ferric chloride.
Natural weathering of gold alloys
If electron microscopy of the gold surface reveals micro-pitting, microporosities and alterations at the grain boundaries, this is consistent with natural corrosion processes.
The markers of age and modernity in golds
The bibliography indicates that ancient golds "contain a series of trace elements in significant concentrations, such as PGMs, elements of the platinum group (platinum, ruthenium, rhodium, palladium and iridium), but also lead, tin, bismuth, antimony, samarium, arsenic... These impurities are the result of ancient gold extraction and refining techniques, which were not totally free of them. This is why the presence of trace elements is a marker of age for gold alloys. On the other hand, modern extraction and refining techniques, such as the electrolytic process, ensure very pure gold. The purity of gold, in the sense of the absence of impurities and not its fineness (number of carats), will be an indication of modernity, as will the presence of cadmium. Like aluminum and phosphorus in bronzes, cadmium is considered a formal indicator of modernity. After all, its use in metallurgy only began in the 19th century.
