Stone analysis can determine the authenticity of objects made from stone by carrying out studies of the surface and cross section to ascertain its nature and degree of deterioration.
Each type of stone will have its own particular naturally occurring phenomena of deterioration, even if preserved from bad weather. This occurs in relation to its exposure to moisture, heat and the development of vegetable or animal organisms. Various mineral phases are likely to be differently weathered, especially the most fragile phases, when they are subjected to a more or less aggressive environment over a long period of time. The differentials between the carved surface and the inside of the stone of these mineral phases is to be expected, and it is the study of such which provides the basis of stone analysis.
With an extensive background in archaeomaterial physics and chemistry, Dr. Olivier Bobin, the scientific manager at Ciram Laboratory based in France performs the stone analysis. Since 2000, the focus of his work has been in investigating and analyzing works of art and objects of significant cultural heritage using coupled methodologies. His research continues to be in the use of new analytical methods for the study of ancient materials focusing on dating methods and characterization of corrosion processes.
In Asia, Antique Authentication is the sole agent for Ciram.
Most types of stone that are at least 300 years old are acceptable for analysis. Only rock crystal, granite and flint may be difficult to analyse as these stones are very hard and resistant to corrosion.
A Scanning Electron Microscope (SEM) with back scattered electron imaging (BSE) coupled with an energy dispersive x-ray spectrometer (EDS) which performs x-ray florescence is used. For jade analysis, Raman spectrometry is used. This process differentiates the stone between jadeite, nephrite, serpentinite and a jade simulant called antigorite, and will also show if any heat treatment may have been applied to simulate weathering.
A small sample of at least 5 mm from the carved surface is taken and processed as a cross section. Using cellulose acetate film, surface replicas which obtain impressions of the surface providing important additional information are also taken.
The type of stone and its composition must be determined as it is in direct relation to the nature of its weathering and corrosion processes. The weather gradient between the carved surface and the inside of the stone is examined, as well as the features of the surface porosity, grain decohesion, desquamation of the surface and reprecipitation of iron and manganese oxides.
The surface is studied for particulars on manufacturing techniques, such as evidence of tool marks from carving or polishing, as well as possible treatments such as cleaning or the use of acid. Ancient or modern polychromy, superficial deposits, burial sediments and modern applications can also be significant considerations in the analysis.
Within four to five weeks, a comprehensive report is provided explaining the experimental methods, analysis of the raw material with EDS spectra, analysis of the degree of weathering with SEM BSE images, a discussion of the findings and conclusions which are given as consistent or inconsistent with the presumed period.
If the findings indicate the object has been subjected to modern chemical treatment, this will also be stated.
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