Protecting wooden artefacts for future generations through ionizing radiation

Let’s accept it, over time all objects start to perish. This will happen for varied reasons such as environmental conditions, physical harm or purely following the inevitable process of natural decay. The cultural heritage of legacy comprising of physical artefacts that have been passed on by the previous generations need to be preserved for posterity sake.

The tangible cultural heritage of humankind involves various works of art, artefacts in museum collections, making museums important institutions of national identity and a booster for economy through tourism.

In order to maintain the objects in conditions enabling them to survive for the enjoyment and education of future generations, it is vital that museums follow proper preservation methods. This is where Irradiation techniques, widely practiced by international organisations like United Nations Educational, Scientific and Cultural Organisation (UNESCO), the International Council of Museums – Committee for Conservation (ICOM-CC).

Organic and inorganic

All museums artefacts can broadly be classified under two categories – organic and inorganic. Organic artefacts include those made from animal products such as fur, leather, wool, silk, bone, ivory, or feathers and also those made from plant products such as wood, paper, cotton and other natural fibres. Inorganic artefacts are those made from non-living materials such as metal, ceramics and glass. Even stone artefacts belong to the category of inorganic.

Material of organic origin: textile, wood, paper, leather etc are readily attacked by bio-deteriorants: insects, fungi, yeasts, moulds, bacteria etc. which thrive and feed on these materials.

This piece looks at the restoration of wooden artefacts through the use of ionizing radiation.

Preserving wooden artefacts

Wood has perhaps been the most important natural organic material for humans since time immemorial. It has been used for several crucial purposes such as making shelters, weapons, boats and coaches, religious or symbolical artworks not to talk about fire, which made life possible on earth. It is simply because of this reason that wood and wooden artefacts have emerged as objects of exceptional importance for heritage conservation. As ancient wood and wooden artefacts are invaluable, their appropriate restoration is of particular importance.

Unfortunately, wood is also among the most vulnerable organic materials. From termite attack to water retention, wooden products are susceptible to harm in so many ways. Not just waterlogged wood, even wood dried in the forest or in uncontrolled conditions may be attacked by fungi; and become more vulnerable to xylophagous insects.

Archaeological wood is often found in a poor state of conservation since it has been subjected to physical, chemical and biological degradation. Bringing in such decayed wood into the museum could contaminate the indoor environment with fungi and insects. Irradiation decontamination is a fast and reliable solution that can save them.

Saving art from vagaries of war in Croatia

The long-spanning war in Croatia from 1991 to 1995 put many objects of cultural significance into great danger of destruction. As part of the saving efforts about 5000 objects, comprising about 3000 altars, with polychrome sculptures, alter parts and other wonder objects were moved from its museums and galleries to previously determined storage spaces.

A large part of the evacuated cultural heritage – mostly wooden objects – were saved from direct damage from war , they, however, were facing vagaries of nature in the form of biodeterioration. The large number of artefacts made it an onerous task for the conservationists to arrest the massive deterioration at the earliest. This was made possible by the panoramic gamma irradiation facility in Croatia that made quick treatment of a large quantity of cultural heritage objects at risk of bio-degradation feasible.

Irradiation decontamination

The irradiation method is based on the ability of high energy photos, electromagnetic radiation to induce chemical damage of DNA of all biological contaminants. Some studies have been performed to evaluate the use of ionizing radiation in the recovery and preservation of wooden artefacts. The discussion and research on the aspect actually took off around 1970s. This was a time when gamma rays were applied to destroy the living organisms present in woods.

Since wood exposed to environment conditions is fungi and insects infected, it needs to be disinfected. The first step in restoration is the detection and quantification of wood pests and decay.

Besides disinfestation prior to restoration, sterilisation of wood is done to test its resistance and its products against wood-destroying organisms. For both purposes, restoration and resistance testing, gamma radiation is considered a suitable decontamination method.

Radiation dose is the most important parameter for the treatment. Depending on the degree of degradation, the artefacts can undergo conservation treatments to stabilise the wooden structure and avoid shrinkage and collapse. Irradiation decontamination is the preferred solution because it is the only method that can effectively penetrate the wooden pieces with large dimensions and weight. The irradiation has shown itself to be a fast and efficient process to eliminate infestations by both insects and microorganisms; and no quarantine is required because of the process does not generate toxic residues.

Museum objects attacked by insects can also be treated with low temperatures, microwaves, or gamma radiation, while other electromagnetic waves are used comparatively seldom.

Gamma radiation

Gamma radiation as a high energy, ionising electromagnetic radiation, easily penetrates wooden objects. It is known to be very effective in the not only for dis-infestation of wooden artefacts but also for wood sterilisation. Contrary to alpha and beta rays, which penetrate only very thin layers, gamma radiation fully penetrates wooden objects. The energy-rich gamma rays modify molecular structures and lead to unexpected function of living cells or to their death.

Gamma rays, electromagnetic waves with high penetrating power, pass through materials without leaving any residue. Fungi have been successfully inactivated from different materials, such as paper, wood and soil with radiation doses ranging from 6 to 15 kGy (kilogray is a radiation absorbed dose measurement unit).

The scientific community is divided over how the duration of the exposure impacts the wooden artefacts. While some say that the treatment time depends on the power of the irradiation source, and there is no significant difference if the wood was irradiated with a weaker source for a longer time or with a stronger source for a shorter time. Some maintain that dose rate and total dose of gamma radiation differently affect both the bending strength and some chemical components in tested wood.

Sterilisation by gamma radiation is very easy, fast and effective, but at doses higher than disinfestation doses it changes the molecular structures not only in wood decaying organisms but also in wood cell walls. Gamma radiation at a level of 30 to 150 kGy causes irreversible and permanent changes in chemical and mechanical properties of wood. Also gamma radiation leads to significant colour changes of wood. With increasing radiation dose the darkening of the specimen increases.

However, overall use of gamma radiation is effective for the restoration and conservation of wooden artefacts and has been helping the humankind preserve articles of importance for future generations to watch, appreciate and learn from.