Above all, it's important to know that works of art must be studied in terms of consistency between constituent materials and chronological attribution. The results of historical and stylistic aspects will have to be discussed by historians, art historians and art experts. In this context, CIRAM scientists are studying the physico-chemical properties of materials in order to search for chronological markers.
For the study of art objects, the first and most obvious type of marker is the search for traces of the passage of time. For organic materials (wood, textiles, ivory and paper, etc.), CIRAM laboratories prefer carbon-14 dating. However, for stone statues, there is no direct dating method that can characterize man's use of these materials.
Stages in the study of stone objects
To study stone sculptures, CIRAM's laboratories have set up an analytical protocol enabling them to search for relative chronological markers as well as significant technical markers. Discover the two main stages of analysis at CIRAM laboratories.
First step: identifying the rock
The first step is fundamental, as it determines the choice of analysis techniques to come. Techniques differ:
- For sandstone, limestone and marble: these materials are highly prone to weathering, and microsection studies will provide a binary indicator of their age. Logically, an early carved object will be heavily weathered, while a modern one will still be in good condition.
- Magmatic rocks (e.g. granites or diorites): these materials are only slightly altered, so there's no need to investigate the penetration of alteration with a sample. Our scientists then study the surface with minimally intrusive analyses. We recover only a very small amount of material from the surface using surface replicas.
It's important to remember, however, that while it is sometimes possible to determine a rock's geographical origin, this alone is not a definitive indication of authenticity.
Second step: investigating the surface of an object
The investigation begins with the examination of tool marks. These traces are technical markers of the shaping of the object and the polishing techniques used. In some cases, microscopic fragments of tool shavings can be found.
This initial approach enables us to verify the consistency between what scientists observe and our knowledge of ancient know-how.
Any traces of polychromy can be studied to compare the pigments found with those used at the object's presumed time. Residues of burial sediments are sometimes present on the surface of the object.
More specifically, our scientists look for traces of chemical treatments to detect an artificial recreation of the patina in order to simulate stone ageing. The presence of chemical traces produced by the reaction of strong acids with the stone allows the detection of modern surface treatments.
Surface replicas provide information on rock alteration. Dissolution, cracking or amorphization can be detected. We can also detect the recrystallization of iron and manganese oxides, as well as the development of micro-organisms.
To complete our investigation, we also study surface deposits and their interpenetration with the rock. This allows us to verify whether the deposits correspond to burial sediments, reflecting slow interaction with the object or, conversely, modern chemical treatment.
Interpretation of results
Surface investigations provide diagnostic elements and enable us to verify whether or not the indicators observed are compatible with the presumed age of the object.
It is important to note that the investigative methods mentioned above have their limitations:
- The presence of modern tool shavings or traces of mechanical polishing does not categorically determine falsification, as these may be traces of cleaning or restoration;
- The absence of rock weathering can be explained in several ways. Either the object is actually modern, or it is old, but has been preserved in a stable, non-aggressive environment (such as a sealed tomb). It should be noted, however, that the presence of fluorine is an indisputable indicator of falsification for the purpose of creating an artificial patina. Furthermore, hydrofluoric acid (HF) is a highly corrosive and dangerous product, and its possession is regulated.
Important: it is often easier to demonstrate that an object is fake, than to prove its authenticity, as the slightest inconsistency with the presumed context is enough to induce reasonable doubt. Conversely, failure to detect an inconsistency does not attest to the authenticity of the stone object.
Microanalysis for stone objects
Microanalysis remains little-known, as it does not provide direct chronological indicators, like carbon-14 dating, for example. However, this technique is the most suitable tool for discriminating between fakes and authentic items, particularly for rocks and metals that do not lend themselves to direct dating methods.
It's important to combine all the information in the quest for authenticity of a stone object.
