How radiation can restore an object of historical, artistic or cultural importance

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 of them is little known: the study, conservation, restoration, and disinfection of cultural, artistic, or historical heritage.

In this article, we focus on nuclear science and technology applied to the restoration of historical works and objects, however, if you want to know more about what else can be done, have a look at our article: What can nuclear technology do for artistic, cultural and historical heritage?

Sometimes an object is not only studied and/or disinfected, but also needs to be restored. This is done by irradiation to restore the chemical bonds of the damaged object, thus helping to strengthen and solidify them. This method involves soaking the object or material in a liquid resin and then exposing it to radiation to harden and strengthen it.

Thanks to the consolidation method, the object appears virtually intact, however, the material and its physical properties have been transformed in such a way that they have become stronger and denser.

One of the most commonly restored materials is wood as it is organic composite, consisting of cellulose fibres in a lignin matrix, hygroscopic and anisotropic, which makes it highly biodegradable and, without conservation precautions, it can be quickly destroyed due to its low chemical stability.

Therefore, environmental conditions as well as human activities can significantly promote its degradation. In fact, it is very sensitive to climate, light (especially ultraviolet rays) and variations in air temperature and humidity. During climatic variations in the environment, the internal humidity level of the wood varies, causing swelling or shrinkage, which can lead to warping and cracking, splitting or peeling.

Another important cause of dry wood degradation is biological attacks by xylophagous insects. The most common in furniture and statues are the lyctus, the anobium punctatum or small woodworm, and the reticuliternus or termite. The larvae of the first two feed by digging galleries in the wood, away from light, and at the end of their life cycle, the larva transforms into a flying insect, leaving a ‘flight hole’ on the surface.

Fungi are also a problem for wood because they cause wood to decay, cause painted wood to swell and crack.

Below are some examples of objects that have been restored using nuclear techniques:

Barge Arles Rhône 3

Waterlogged archaeological materials have sometimes remained for centuries in a humid or aqueous environment and appear to be intact, but their internal structure has suffered irreversible damage. In other cases, such as at the bottom of lakes, rivers, seas and soils where the water level has remained constant over time, the absence of oxygen (anaerobic environment) can preserve wood, leather, and fibers from rapid degradation although they continue to dissolve under the effect of water.

In 2011, irradiation was applied to a 1st-century Roman wooden boat found in the Rhône River in Arles (France), called ‘Arles-Rhône 3’ measuring 31 meters in length. After its discovery, as the boat dried out, the wood began to fall apart. To save this ancient relic, ARC-Nucléart specialists used the technique of freeze-drying consolidation with PEG (polyethylene glycol), i.e. triggering a chemical polymerization reaction using gamma radiation:

• Impregnate the object with a resin of a mixture of styrene and polyester in an autoclave using a vacuum-pressure process.
• Once the object is impregnated and cleaned, exposure to gamma radiation will act as a catalyst and induce cross-linking of the two components, which will polymerize and thus create a solid three-dimensional network on the object.

In this way the object has become a wood-plastic composite, resistant to moisture (the styrene-polyester mixture is hydrophobic), very dense and much less sensitive to the climatic conditions of the storage or exhibition place where it will be kept.

The Madonna and Child of Noiron-sur-Seine

Another example is the restoration of the Virgin with baldachin from Noiron-sur-Seine by ARC-Nucléart. It had a precious polychrome underneath and the wooden structure was completely dusty. After vacuum impregnation with radiopolymerisable resin, followed by exposure to gamma radiation, the piece was restored to its original beauty.

Preservation of books

The ‘Gantt’ case occurred in 1980 in the United States when Johns Hopkins University inherited a collection of valuable documents owned by Mr. Gantt. The collection was in a house in very poor condition, infested by insects, rodents, cats, and dogs. The condition of the collection was truly deplorable. After studying the possible options to treat the collection and finally ‘clean it up’, it was concluded that the only way to recover such a valuable collection was to ‘clean it up’ by irradiation. 4.5 kGy of gamma rays (60Co) were administered. Those in charge of the collection have stated that in the years since that operation, there have been no problems with the irradiated documents.

In 1997, the library of the University of Colorado was flooded due to heavy rains that fell incessantly for three days. As a result, a large number of documents in the library were damaged by water. Attempts were made to recover the documents by different methods (various chemical methods or freezing). Finally, they decided that the only way to recover these valuable documents (about 500,000 copies) was to irradiate them. They used gamma rays (60Co), administering doses of 15 kGy.

So far, irradiation for the preservation of books and archival documents has been very successful in emergency circumstances.

Combination of nuclear techniques to recover a bronze statue

Sometimes, in order to recover an object and learn everything about it, it is necessary to apply several nuclear techniques together, as was the case with the study and restoration of a bronze statue that was discovered in the waters of the Adriatic Sea at a depth of more than 45 meters in 1996. For a decade, researchers used nuclear techniques to determine its age, origin and even the methods of manufacture.

After its removal from the sea it was badly damaged by corrosion, so it underwent a long process of desalination and restoration until 2005. Upon completion, they realised that the theme of this statue of Apoxymenus depicting an athlete removing sweat and dust from his body is recurrent, although it was not known whether it was Roman or Greek. Therefore, utilizing carbon-14 dating, it was possible to determine that the statue was made between 100 BC and 250 BC.

In 2009, using the accelerator-based technique called microparticle-induced X-ray emission (PIXE), they determined the original composition of the alloy, and together with inductively coupled plasma multi-collector mass spectrometry they gained a better understanding of the isotopic composition of the statue's lead.

Isotopes are specific forms of chemical elements that have different atomic mass and physical properties. By starting from the ratio of the different lead isotopes present in a sample and comparing that ratio with the known properties of the geographical areas, scientists can determine the provenance of the sample. Thanks to this accelerator-based analytical technique, the origin of the lead in the statue could be located in the Eastern Alps or Sardinia, and they concluded that the statue was a Roman copy of a Greek original.

In 2014, the researchers re-examined it using the high-resolution lateral PIXE technique and discovered that the lips embedded in the statue were made of very pure, unalloyed copper. With the help of X-ray radiographs, they were able to learn how the inlays were inserted and attached and to determine the sophisticated casting and joining techniques used for the limbs. The conclusion is that Apoxymenus was undoubtedly a copy of a much older statue from the mid-4th century BC, made by an indirect lost-wax casting technique using an alloy with a low lead composition.

If you want to know more about nuclear techniques applied to preserve cultural heritage with examples, you can consult this publication of the International Atomic Energy Agency (IAEA) entitled: "Uses of Ionizing Radiation for Tangible Cultural Heritage Conservation" or the website of Arc-Nucleart