How an isotope can help combat fraud in works of art

Since the discovery of nuclear energy, many applications have been developed, such as producing electricity, diagnosing and treating diseases, caring for the environment, etc. However, one is little known: the study, conservation, restoration, and disinfection of cultural, artistic, or historical heritage.

In this article, we focus on how an isotope can help combat art fraud, but if you want to know more about what else can be done, look at our article: What can nuclear technology do for artistic, cultural, and historical heritage?

Forgery can be a very lucrative criminal activity. Therefore, the authenticity of a work of art is crucial to determine its historical and economic value. One of the first steps is to know its history, identify the artist, and place it in a specific period.

To do this, some more advanced and precise techniques can be applied to verify its authenticity by using radioactive isotopes, especially carbon-14, whose technique is known as radiocarbon dating. As a result, it is possible to:

• Measure the proportion of carbon-14 in organic materials, making it possible to determine their age with high precision.
• Identify and quantify specific isotopes in small samples, providing detailed information on the composition and origin of the materials.
• Provide information on original materials and those used in previous restorations, helping conservators to plan interventions.
• Identify the presence of contaminants or harmful elements that could affect the conservation of the objects.
• Reveal inconsistencies in the isotopic composition of objects that could indicate that they are counterfeits.
• Compare the composition of objects with known reference materials, providing additional evidence of authenticity.

What is the carbon-14 nuclear technique?

The carbon-14 or radiocarbon dating technique is used to determine the age of organic materials, such as objects made of wood, cotton, paper, leather, wool, silk, or bone. This technique often allows us to discover whether what is being analyzed is genuine or fake.

All living organisms, such as animals and plants, absorb carbon. When they die, carbon-14, an unstable isotope of carbon, begins to decay at a known and constant rate. Using Accelerator Mass Spectrometry (AMS), specialists measure the proportion of carbon-14 to determine how much time has elapsed since its creation.

This technique makes it possible to determine the age of heritage objects up to 50,000 years old. The radioactivity due to the presence of carbon-14 is halved every 5,730 years. For this reason, when its activity (and quantity) is accurately measured, it is possible to calculate the age of the sample.

The amount of carbon-14 in the atmosphere has fluctuated in recent years, especially since the mid-1940s and 1950s, as a result of nuclear weapons testing. The concentration of carbon-14 in the atmosphere peaked around 1964 and has decreased since then.

The process of how this is done is:

• Collection of samples of organic materials, such as wood, bones, tissues, or paper.
• Sample preparation. Samples are cleaned and prepared to remove any contaminants that may affect the results. This may include the removal of carbonates, acids, and other unwanted matter.
• Carbon-14 measurement. The prepared sample is placed in a particle accelerator or mass spectrometer to measure the amount of carbon-14 present.
• Age calculation. The proportion of carbon-14 in the sample is compared to the proportion of carbon-14 in the atmosphere. Since it decays at a constant rate, the amount remaining in the sample can be used to calculate the time since the death of the organism.
• Calibration. The results obtained are calibrated using calibration curves that take into account variations in the concentration of carbon-14 in the atmosphere over time. This improves the accuracy of the dates obtained.

If you want to know more about this technique, click on the following link: Monograph: What can be done with the carbon-14 nuclear technique?

The following are some examples:

Carbon-14 dating of lead white

Through non-destructive analysis, we can obtain information on the ‘fingerprints’ of the piece, i.e. the micro-constituent elements of raw materials that can vary according to the author and the time frame.

Notably, late medieval wall paintings from the Château de Germolles in French Burgundy and plaster fragments from the wall of a missing choir loft in the church of Cordeliers in Fribourg (Switzerland) were dated with certainty in 2020 by carbon-14 measurement as they contained a pigment that was widely used in painting, also known as lead white, to define skin tones or as a preparatory layer in most paintings.

The paints are complex and contain carbon of organic origin from lead white, which can be radiocarbon dated, and also contain inorganic carbon that could come from another white pigment made from calcium carbonate or building materials.

To make this discovery, the researchers isolated the carbon from the lead white by thermal separation: heating the paint samples at a low temperature. Under these conditions, only the carbon atoms are released from the lead white in the form of carbon dioxide (an easily recoverable gas), and the other atoms remain attached to the calcium carbonate, which is stable at temperatures up to 600ºC.

The isotopic fraction of carbon-14 in the carbon dioxide samples is then measured and, after a statistical process, the carbon-14 age associated with these levels was determined using the carbon-14 calibration curve. These ‘ages’ are composed of several time intervals that can sometimes be reduced with historical information.

For the Château de Germolles, the date of acquisition of the château helped the researchers to date the painting to 1380-1400 and, in the case of the Church of Cordeliers, the results made it possible to differentiate between the two paintings studied, the older one dating from 1426-1460.

In both cases the dates of the paintings are documented and are consistent with the results of the carbon-14 analysis.

Carbon-14 dating of lead white began after the discovery of carbon-14 in lead carbonates that were used as cosmetics in Egypt and Greece in antiquity.

The Capitoline Wolf (Italy)

According to legend, Rome was founded by Romulus and Remus, twin brothers who, after being abandoned as children, survived thanks to a she-wolf. The image of the young men at the she-wolf's teat has been recreated countless times, but few are as famous as the ‘Capitoline Magnifying Glass’ which has been in the Capitoline Museums in Rome since 1471.

This 75-centimetre bronze statue was discovered in the 15th century and was believed to be over 1,000 years old. It was not until the 2000s that a radiocarbon dating analysis surprisingly indicated that it was made in the 12th century.

Determining the age of an old aquaculture system

The Budj Bim Cultural Landscape, located on the traditional land of the Gunditjmara people in south-eastern Australia, consists of three serial components that contain one of the world's oldest and most extensive aquaculture systems and is inscribed on the UNESCO World Heritage List.

The Budj Bim lava flows provide the basis for the complex system of canals, dams, and dykes developed by the Gunditjmara to trap, store and harvest kooyang (shortfin eel - Anguilla australis ) that was highly productive and provided an economic and social base for Gunditjmara society for 6,000 years.

Through the application of ultra-sensitive isotopic analysis for radiocarbon dating, its age was determined and evidence of the extensive history of Gunditjmara use and management was obtained.

Are they genuine or fake these pictures?

During an investigation into possible forgeries carried out in 2019 by France's Central Office for Combating Illicit Trafficking in Cultural Property, two paintings from a collection believed to date from the late 19th and early 20th centuries were tested.

Investigators collected fiber samples from the canvases and reduced them to approximately one milligram of carbon, which was then measured using the AMS technique and proved to be forgeries as the excess carbon-14 detected in the fibers indicated that they had been manufactured around the 1950s and 2000s and that the paintings could not have been painted in those centuries by the supposed artists who had died in the 1940s.

Accelerator Mass Spectrometry (AMS) is an ultra-sensitive technique that combines conventional mass spectrometry techniques with a particle accelerator to give the particles much higher energies than usual.

Other examples of objects that were either proven to be authentic or counterfeit include:

• Riace bronzes. This technique was used to determine their composition and origin and to confirm their authenticity.
• Gold and precious stone objects from the Kingdom of Ayutthaya existed in present-day Thailand between the 14th and 18th centuries.
• The authenticity of the 500-year-old gold statue ‘The Saltcellar’ by the sculptor Benvenuto Cellini, which was stolen and later recovered in the Vienna Woods (Austria), was proven.
• The composition, age, and origin of a century-old portrait of St. George in Albania was proven.

The Holy Shroud of Turin

The Shroud of Turin is the alleged shroud that wrapped the body of Jesus Christ after his death, and is preserved in Turin (Italy).

In the image of the shroud, the figure of a man with the marks of crucifixion can be seen more clearly in the negative, properly contrasted, than in the original sepia colour.

In 1988, radiocarbon analysis was carried out on a small fragment of the sheet in three independent laboratories.

The results showed that the cloth dated to the Middle Ages, specifically between 1260 and 1390, suggesting that it could not be authentic.

However, it is now thought that the sample taken is not representative of the whole cloth and that, in addition, that part contains an “invisible” patch added during a repair in the Middle Ages. This would have "rejuvenated" the sample.

In addition to the radiocarbon technique, other complementary methods are used for the study, conservation, restoration, disinfection, and authentication of cultural heritage. Some of the most common are:

• X-ray fluorescence (XRF). Used to analyze the elemental composition of objects, particularly useful for identifying pigments in paintings and the composition of metals.
• Raman spectroscopy. Used to identify molecules and crystalline structures in cultural materials. Particularly for analyzing pigments and organic materials.
• Infrared spectroscopy. Used to identify organic and inorganic materials in cultural objects, such as the analysis of paint layers and filler materials.
• X-ray diffraction (XRD). Analyzes the crystalline structure of materials, providing information about their composition and state of preservation.
• Scanning Electron Microscopy (SEM). Provides high-resolution images of the surface of objects, revealing microscopic details about their composition and structure.
• Ultrasound. Used to detect cracks and other internal defects in solid materials such as stone and metal.

If you'd like to learn more about nuclear techniques applied to preserving cultural heritage, including examples, you can consult this publication from the International Atomic Energy Agency (IAEA): "Uses of Ionizing Radiation for Tangible Cultural Heritage Conservation" or take a look at the following monograph: What can be done with the carbon-14 nuclear technique?