Authentication of stone sculptures

First and foremost, it is important to know that works of art must be studied with regard to a 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 physicochemical 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 favor carbon-14 dating. However, for stone statues, there is no direct dating method to characterize human use of these materials.

Steps in the study of stone objects

To study stone sculptures, CIRAM laboratories have set up an analytical protocol to search for relative chronological markers as well as significant technical markers. Discover the two main stages of analysis at CIRAM laboratories.

First stage: identification of the rock

The first stage is fundamental, as it conditions the choice of analytical techniques to come. The techniques differ:

• For sandstone, limestone and marble: these materials have a strong propensity to alteration, the study on microsection will provide a binary indicator in relation to their age. Logically, an object carved in the past will be strongly altered, while a modern object will still be in good condition.
• Magmatic rocks (granites or diorites for example): these materials do not alter much, so it is not necessary to investigate the penetration of alteration with a sample. Our scientists then study the surface with minimally intrusive analyses. We only recover a very small amount of material from the surface with replicas of the surfaces.

Let us remember, however, that while it is sometimes possible to determine the geographic origin of the rock, this alone does not constitute a formal indication of authenticity.

Second step: investigation at the surface level of an object

Investigation begins with the examination of tool marks. These traces are technical markers of the shaping of the object and the polishing techniques used. Sometimes we find microscopic fragments of shavings from the tools used.

This first approach allows us to verify the coherence between what the scientists observe and the knowledge we have of ancient skills.

It is possible to study possible traces of polychromy in order to compare the pigments spotted with those used at the presumed time of the object. Residues of burial sediments are sometimes present on the surface of the object.

More specifically, our scientists look for traces of chemical treatments in order to detect an artificial recreation of the patina in order to simulate an aging of the stone. The presence of chemical traces produced by the reaction of strong acids with the stone allows us to detect modern surface treatments.

Surface replicas give information about the alteration of the rock. Dissolution, cracking, or amorphization can be identified. We can also detect the presence of recrystallization of iron and manganese oxides, as well as the development of microorganisms.

To complete our investigation, we also study the surface deposits and their interpenetration with the rock. This allows us to verify that the deposits correspond to burial sediments, which testify to a slow interaction with the object or conversely to a modern chemical treatment.

Interpretation of the results

The surface investigations provide diagnostic elements and allow us to verify whether the observed indicators are compatible, or not, with the presumed period of the object.

It is important to note that the investigative methods cited above have their limitations:

• The presence of modern tool shavings or traces of mechanical polishing does not categorically determine falsification, as they may be traces of cleaning or restoration;
• The absence of rock weathering can be explained in several ways. The object is indeed modern or the object is old, but has been preserved in a stable and not very aggressive environment (such as a sealed tomb for example). It should be noted, however, that the presence of fluorine is an indisputable indicator of falsification in order to create an artificial patina. Moreover, hydrofluoric acid (HF) is a very corrosive and dangerous product whose possession is regulated.

Important: it is often easier to demonstrate that an object is fake, than to prove its authenticity, because the slightest inconsistency with the presumed context is enough to induce reasonable doubt. Conversely, not detecting an inconsistency does not attest to the authenticity of the stone object.

Microanalysis for stone objects

Microanalysis remains unrecognized, as it does not provide direct chronological indicators, such as carbon-14 dating for example. However, this technique is the most suitable tool for discriminating between fake and authentic, especially for rocks and metals that do not lend themselves to direct dating methods.

It is important to combine all information for the quest for authenticity of a stone object.