Stones and Minerals
Sandstones, marbles, limestone, ...
It concerns all the objects constituted by minerals more or less organized, from metamorphism or other process of formation of rocks (sandstone cementation for example).
For the scientific expertise of stone sculptures, the study of a sample using scanning electron microscopy allows to determine the compatibility of the observed weathering processes close to the surface with the presumed antiquity. In particular, we can detect element indicating a surface treatment applied to simulate its ageing (such as strong acids).
The analysis of stone artifacts allows to determine their nature and participates, in particular, to trade reconstitutions purposes.
The particular case of Rock Crystal
Rock crystal, or hyaline quartz, is a silicate rock constituted of pure silica whose crystalline organization makes it a material strongly resistant to erosion. The study of a sample using scanning electronic microscopy does not allow the evaluation of the carving’s antiquity from the alteration processes, these being most often undetectable. Moreover, sampling an object made into this material could irreversibly damage it.
So, for the expertise of rock crystal objects, an other technique is used, ERDA (Elastic Recoil Detection Analysis). This non-invasive analysis allows the study of surface hydration. The analyzed phenomena correspond to processes occurring at molecular levels over long periods of time. Thus, we are able to determine if the hydration rate measured at the surface together with its penetration towards the inside part of the material are compatible or not, with the presumed antiquity of the rock crystal’s carving.
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