Depending on the type of metal (bronze, brass, silver, gold or iron) and the shape of the object, different methods are used for authenticity test.
Using electron microscopy (SEM), we define the composition of the metal, the nature and the degree of corrosion. These results form a body of evidence that can rule on the compatibility or otherwise with the presumed time of the object. A sample of a few mm is required for this analysis.
Moreover, if the object has a closed form and it contains a core melt residual, thermoluminescence test can be performed in addition to the analysis of metal.
Bronze, brass and silver
The first step is to determine the composition of the alloy and the nature of the inclusions to detect any formal indices of modernity, such as aluminum, manganese or phosphorus.
The second step determines the degree and source of corrosion of metal. If it penetrates deeply into the alloy, this is probably a natural process. If it remains superficial, it will surely come from a modern artificial treatment.
The study of the composition of the corrosion products (the "patina") also allows to verify the origin of the alteration and to identify any signs of chemical attack (recurrence chlorine, sulfur, phosphorus ).
The corrosion process of gold are almost nonexistent. Also, the study is more complex and requires the coupling of scanning electron microscopy (SEM) and extracted beam in PIXE (identical to the AGLAE C2RMF) or ICP / MS (Induced Coupled Plasma / Mass Spectrometer) .
The analysis of a gold object has three phases: study of surficial deposits, study of the composition of the metal major, minor and trace and surface scan to observe figures of micro-corrosion. Know more
Uranium-Thorium-Helium gold dating
We propose, in collaboration with university laboratories in Europe, dating gold (uranium, thorium - helium).
Principle: gold ore naturally contains uranium (U), thorium (Th) and samarium (Sm). These elements will decay over time and emit alpha particles (ie helium). Its concentration is proportional to the age of the last melting of the alloy. More the object is older and gold contain helium.
The measured amounts of helium are very low. This leads to considerable uncertainty in the measurements. But even if this method is not yet clear, it remains highly relevant to differentiate antiques copies of 19th or 20th centuries.
In contrast to gold, iron corrodes very quickly (rust). An object of a hundred years may have a patina similar to something 300 years. The analysis of the corrosion will not do in this case to decide on seniority.
Investigations by scanning electron microscopy will be based on the study of the composition of the alloy and mainly on the nature of the inclusions. These impurities are technology signs that also provide historical data. The presence of manganese is, for example, a formal index of modernity (metallurgy of 19th-20th centuries).
Important: Today, elaborated fake bronzes circulate on the market. They combine the old and the recent metal. The old metal is usually located at the base of the object where the samples are performed.
It is increasingly necessary to complete the study of the material with a display of the internal structure of the object, by X-ray radiography or CT Scan. Know more