Nuclear techniques have been used for the preservation of cultural heritage since the seventies. One of the first aspects exploited was the biocidal effect of radiation. This actually helped to preserve the mummy of Ramses II! He was one of the ancient Egypt's greatest rulers more than three thousand years ago. The mummy (Fig. 1) discovered in 1881 was placed in a non-sealed glass cover at the Egyptian Museum in Cairo but after several years Egyptologists realized that it was deteriorating at an alarming rate. In the frame of an art exhibition in Paris the pharaoh’s mummy was moved to France for examination. Researchers discovered that the deterioration was due to a fungal infection. The disinfection of the mummy was effectively performed thanks to ionizing radiation.

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Figure 1 - Mummy of 19th dynasty King Rameses II, the original uploader was ThutmoseIII at English Wikipedia - CC BY-SA

In time other abilities of ionizing radiation proved to be useful for cultural heritage preservation. For example, the ability to initiate polymerization and crosslinking allows the consolidation of deconstructed porous materials.
Preservation involves the treatments applied to stop or reduce the rate of deterioration of an artifact. This step follows the characterization, which provides information that could help to preserve the object. As for any other action in cultural heritage, preservation has to follow the ethical principle of keeping the object “as close to its original condition as possible and for as long as possible”. The requirement of effectiveness with minimal intervention makes nuclear techniques very attractive.

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Figure 2 - Deteriorated old Ottoman documents.

A lot of cultural heritage objects are made of natural polymers, such as wood, leather, textiles or paper. These materials are vulnerable to environmental factors. Deterioration can be due to exposure to external agents such as temperature, humidity, light or water, and this degradation is usually slow.
In order to protect the object preventive conservation is applied. This refers to the control of environmental conditions and the protection of the object against accidents. Of course, this step is always applied but the artifact could be also damaged by biological attack such as from fungi, bacteria or insects. The object becomes their nutrient and the degradation proceeds at high speed. The aggressors are present inside the object and action must be undertaken to stop degradation.
Interventive conservation refers to any act that involves a direct interaction with the cultural material. It includes simply the cleaning, treatment or repair of the object, or even more strong actions such as the removal of covering varnish or the replacement of parts of the object that might be too damaged. In this case it is essential to fully justify any intervention, following the ethical principle, keeping them to a minimum.

Classical techniques used for disinfestation have been borrowed from medicine and agriculture, such as the use of ethylene oxide or methyl bromide gases. However, this approach has the disadvantage of hard penetration inside the object in addition to the safety issues related to the use of dangerous gases. Irradiation sterilization is widely used in industry and both the treatment itself and side-effects are known. It provides several advantages including limited manipulation of the object and the absence of any residues after treatment.
Sterilization by radiation started to be applied to cultural heritage in the 70s at the ARC-Nucléart laboratory of the CEA in Grenoble. It provides very high effectiveness and reliability, and the death rate is effectively controlled. As for the industrial treatment, doses of less than 1 kGy to 10 kGy are employed. Irradiation can be applied to many materials, including organic ones without damaging them.
Limits on the material are imposed depending on the dose. Doses up to 1 kGy have insecticide effects. At such doses the treatment is so harmless that it can be applied to almost every material

except for transparent ones in which color may be altered, such as gems. At higher doses more side effects start to appear and it is important to evaluate the benefits of the treatment against the drawbacks of doing nothing. At fungicidal doses, more chemical effects are induced. In order to understand the final effect on the artifact, such chemical effects have to be translated into the amount of degradation.
The use of radiation to consolidate porous materials is called densification. It is less commonly employed than disinfestation and is mostly used on wood and stone porous artefacts. The process is simple: the material is impregnated by a resin that fills all of the micropores and is then cured by radiation. It can also act inside the bulk of the artifact. The resulting mechanical properties of the treated artefact are better than the ones obtained with other impregnation techniques and so is the appearance of the object. However, the physico-chemical properties are changed so the method is applied only when necessary.
The most used resin is the acrylic monomer methyl-methacrylate: it has a very low viscosity

and so it can penetrate into porous materials, but its high volatility impairs the effect of the treatment on the final surface. Alternatively, standard unsaturated polyester resins which are much more viscous result in a very hard insoluble material even on the surface.
The polymerization initiated under gamma irradiation does not require any chemical additives and is performed at room temperature. The crosslinking proceeds under irradiation until the formation of the solid polymer and all of the free radicals have reacted. The polymerization reaction rate is proportional to the irradiation dose rate: the treatment can be controlled by varying the dose rate. The highest dose rates applied are around 1 or 2 kGy/h, up to total doses of 20 to 30 kGy.
Nuclear techniques can be fruitfully exploited in cultural heritage preservation thanks to the expertise of several professionals. Besides competences in history, archaeology, material science, biology and chemistry, experts in radiation chemistry and radiochemistry are required to properly choose and setup the irradiation treatments.