CIRAM laboratories carry out carbon-14 (or radiocarbon) dating of your art objects in wood, ivory, bone, textiles... but also of organic materials in general. Discover CIRAM's protocols for dating ivory objects: dating method, calibration of results and interpretation. Our scientists interpret the results with the utmost precision, so you get the results you expect.

Carbon-14 dating techniques for ivory from CIRAM laboratories

Radiocarbon (carbon-14) dating quantifies the time that has elapsed since the death of an organism. The technique is based on measuring the amount of carbon 14 remaining in an organism after death. 14C is a radioactive carbon isotope that decays progressively according to a known exponential law. Its concentration is halved every 5,730 years. Beyond 60,000 years, the quantity of carbon 14 is too small to be measured. This is the dating limit.

Collagen extraction, the first step to precise dating

Ivory is composed of a mineral part, bioapatite, and an organic part, collagen. Collagen is the most suitable fraction and is normally used for radiocarbon dating. The first step in the dating process is therefore collagen extraction. To this end, samples are treated with hydrochloric acid (HCl, 1 M), then treated with sodium hydroxide (0.1 M) at room temperature and again treated with cold hydrochloric acid, to avoid absorption of atmospheric carbon dioxide. After washing with demineralized water, the samples are boiled to dissolve and recover the collagen.

Collagen analysis, a crucial step in ivory dating

The extracted collagen undergoes combustion at 920°C and is transformed into gas. During this stage, an initial check of the C/N ratio is carried out using an elemental analyzer (Elementar Vario ISOTOPE Select). This step is essential, as it constitutes a quality control. A C/N value between 2.9 and 3.6 indicates that the collagen is well preserved and will provide reliable dating. If the C/N ratio is outside this range, the collagen will not be C14 dated. In this case, the mineral part of the ivory can be used to date the bioapatite. Stable carbon and nitrogen isotope analysis will then be carried out using IRMS. These values will provide information on the terrestrial or marine origin of the ivory. At the same time, carbon dioxide from combustion is separated from other residues using a zeolite trap. This carbon dioxide is then catalytically converted into graphite using an automated system (AGE 3, Ion Plus).

Mass spectrometry for radiocarbon measurement

For radiocarbon measurement, we use mass spectrometry coupled to a particle gas pedal (AMS). This method requires very little material (1 mg of pure carbon compared with 1g previously), a minimum of time (around 1 hour of counting instead of several days or weeks in the past) and a more accurate result.

Gross age and calibrated dating: the importance of calibrating results

To obtain reliable and relevant data, it is important to calibrate the results based on :

• Gross age: expressed in BP years (before present, or before 1954), based on the assumption that carbon-14 concentration has been constant in time and space.
• To obtain calibrated dates, calibration curves are used, which take into account variations in carbon-14 content in space and time. These calibrated dates, combined with probability percentages, are used to verify the authenticity of an ivory object.

The problem of recent dating (after 1954)

It is possible to tell whether objects were made before or after 1954 by their carbon-14 content, but how?

This artificial "boundary" was man-made, as atomic bombs and atmospheric nuclear testing in the 50s and 60s doubled the amount of carbon-14 in the atmosphere.

In recent objects, our scientists have detected abnormally high levels of carbon 14. However, since the end of atmospheric nuclear testing in the mid-1960s, we have seen a steady decline in the quantity of C14. This phenomenon makes it possible to provide very precise dating of recent years (to within one or two years in the best case), used for dating wine, for example.

Carbon-14 dating in Tribal Art

Radiocarbon dating is relevant to Tribal Art, because carbon 14 can absolutely differentiate between objects made from materials that lived before and after 1954. Carbon 14 clearly identifies copies and fakes. The analysis is equally effective for older cultures.

But these scientific "tests" must be combined with a historical and stylistic study of a work. It is important to maintain a global and comprehensive approach to deliver accurate and reliable results. Carbon-14 dating must be used as an objective aid to decision-making.

Filing a CITES dossier with radiocarbon dating

Carbon-14 dating of ivory is also useful for filing a CITES dossier, to prove that the animal died before 1947. The 1975 UNESCO Convention restricts the sale and possession of elephant tusks.

Whether for the art market or archaeology, carbon-14 dating is a major advance. Over the years, this analysis has become a precious and indispensable tool for CIRAM laboratories.

Discover CIRAM's thermoluminescence test protocols, measurements and results interpretation.

The thermoluminescence laboratory test

While dating materials is the most effective way of obtaining a chronological marker to authenticate an art object, it is important to distinguish between the event being dated and the event being dated. Thus, dating a material is not the same as dating its use by man.

Our scientists use carbon-14 dating for organic materials, but we also use other dating methods for inorganic materials. For heated objects, CIRAM uses thermoluminescence, which works on ceramics, porcelain and terracotta in general, with a chronological range of around 300,000 years.

Please note that materials such as stone, metal, glass and pigments cannot be dated. CIRAM laboratories will use analytical alternatives (electron microscope, chemical analysis...) to authenticate them.

The phenomenon of luminescence

In 1664, Boyle reported the phenomenon of heat-stimulated luminescence or thermoluminescence (TL). But its physical properties were not studied until the 1930s, thanks to the development of photomultiplier tubes. In the 1950s, the potential of thermoluminescence was explored for the dating of archaeological materials, particularly ceramics. But the application of TL to dating did not come to fruition until the mid-60s in England. It was not until 1967 that the first TL dating studies were carried out in France.

The principle of thermoluminescence

TL is based on the study of natural radioactivity and the ability of the crystals contained in a terracotta to accumulate the effects of this irradiation. This energy (also called irradiation dose) is proportional to the time the crystals are subjected to this irradiation. In other words, TL corresponds to the emission of light (luminescence) caused by a thermal (thermo) input of energy. Note that the intensity of this luminescence is proportional to the energy absorbed by the crystal, and therefore proportional to time.

TL tests are used to determine the natural radiation dose (QNat) received since time zero (i.e. the last time the material was heated). For true TL dating, two values need to be measured:

• QNat: actual irradiation dose ;
• I: annual radiation dose.

The TL study of objects outside their burial context does not allow for real dating. However, the QNat measurement will assess the age and provide objective information on the authenticity or otherwise of the object studied. For this purpose, we assume an annual radiation dose of between 3 and 6 mGy/year, which corresponds to the vast majority of cases encountered. This lack of annual dose measurement means that the relative accuracy of thermoluminescence age tests is of the order of 20-30% of the estimated age.

Distinction between forgeries and authentic objects

By examining thermoluminescence curves, it is possible to distinguish between modern fakes and authentic objects.

Unlike carbon-14, it is impossible to detect a significant gap between the dated event and the event being dated. Since the firing of terracotta is the last anthropogenic act recorded by the material. The relevance of the result to the problem posed leaves no room for doubt.

The thermoluminescence test

TL testing is relatively simple to perform, but the diversity of materials makes it difficult to adapt measurement protocols and interpret results. Porcelain, for example, is problematic for TL because its manufacturing process involves high-temperature firing, which results in total or partial amorphization of the material. As a result, the physical properties that condition TL operation are altered or even destroyed. In this case, the results of conventional TL will be unusable, and we will use "predose", an alternative technique.

The evolution of counterfeiters' methods, combined with their growing understanding of the analytical techniques we use, makes it increasingly necessary to use TL to detect fakes.

We'll see in a later article that thermoluminescence tests need to be combined with other investigative techniques to detect the so-called "new fakes", which are more successful.

The quest for authenticity in terracotta objects 

Factual data derived from physico-chemical analysis, correlated with accumulated knowledge of the evolution of shapes and techniques, fuel the quest for authenticity.

To make a precise diagnosis, our scientists always propose a combination of clues, while remaining aware of the limits of each approach. If certainty is the absolute goal of authentication, it is rarely attainable. However, the detection of a fake will always be more obvious than the certainty of an authentic object. But thanks to our expertise and know-how, the result will always be reliable, objective and relevant.

CIRAM is proud to participate, in our own modest way, in the restoration of Notre Dame Cathedral in Paris. A few weeks ago, our teams were on site to take samples.

See our stay in pictures :