MAJOR CONSTITUENTS OF ARTEFACTS AND THEIR DEGRADATION BEHAVIOURTHEIR DEGRADATION BEHAVIOUR

Our ancestors could choose raw materials for their artefacts from the three kingdoms: regnum lapideum (the mineral kingdom), regnum animale (the animal kingdom) and regnum vegetabile (the vegetal kingdom), which all make up the imperium naturae described by Linnaeus.

2.2.1. The degradation of inorganic artefacts — telluric transformation Heritage artefacts made of inorganic substances can contain all known chemical elements. Some materials are taken from nature and used in their natural state, after only minor physical modifications. This happens with some pigments and stones such as flint and obsidian. The chemical compounds in these materials are very stable. They appeared over time periods that far exceed the human time horizon, in the process called ‘telluric transformation’. Those events took place hundreds of millions of years ago. The materials we are talking about now result from a process in which the original raw materials, whatever they were, were subjected to extreme temperatures and pressures.

The degradation of inorganic objects of this kind usually requires physical/

mechanical damage, caused by water, temperature variations (fire, frost), wind or mechanical stress. All these factors may have destroyed the shape of the object but not its chemical composition. The oldest artefacts of this type date from the palaeolithic era — a historical period which began 2.6 million years ago. For their conservation, any normal indoor environment is sufficient.

Most metals, ceramics and glass, and some mineral pigments, are inorganic materials obtained through human technological intervention upon natural raw minerals. Ceramic and glass are very stable materials. We can find them in artefacts tens of thousands of years old. On the other hand, human-made metals and alloys such as bronze, brass and iron are less stable. In the presence of water, oxygen, sulphur and nitrogen oxides from the air or humic acids from the soil, these metals suffer chemical changes.

Artefacts made of inorganic materials do certainly interact with the living world. Microorganisms have been identified that are able to grow on purely inorganic substrate (lithophilous). They are the pioneers in the establishment of complex colonies comprising organisms ranging from bacteria, algae, lichens and fungi up to macroflora and even animals [2.1]. Biodegradation occurring in such conditions affects buildings and monuments situated in outdoor environments, buried archaeological artefacts and immersed objects. The biodegradation in such circumstances is usually slow. Occasionally, biological degradation has

been incorrectly blamed for damage produced by a physical/mechanical rather than biological action, as in the case of damage produced by plant roots.

As a rule, inorganic artefacts kept in controlled environments in museums, subjected to periodical maintenance operations, are not in danger of biological attack.

Generally, remedial conservation techniques like irradiation are not applied for decontamination and/or consolidation of inorganic artefacts. However, there have nevertheless been some notable scientific experiments. One was a study in Lisbon, Portugal, at the Nuclear and Technological Institute, involving the irradiation of tiles as a biocide against infiltrating microorganisms whose metabolism products induced a pigment colour change [2.2]. In Grenoble, France, Nucléart Regional Conservation Workshop (ARC-Nucléart) consolidated porous structures of some types of gypsum and stone by radiation induced polymerization [2.3].

2.2.2. The degradation of organic artefacts — the biological cycle

Another large category of artefacts consists of those made from organic materials. Carbon prevails in their composition. Organic materials come into the world in a process of transformation that one can call the ‘biological cycle’, in which the main players are plants, animals and microorganisms. The biological cycle refers to the production of organic matter during life, followed by its disintegration as a consequence of death. The remains of this process serve as nutrients for the recommencement of the cycle. The duration of a biological cycle is much shorter than that of a telluric transformation. In most cases, life itself does not last more than 100 years. The duration of disintegration is at least an order of magnitude shorter in natural conditions. There are exceptions, of course, but statistically, this is the approximate average timing.

The artefacts in this category are made of wood, leather, parchment, paper and textiles. From the moment the trunk of a tree becomes a piece of furniture, a musical instrument or a structural element in a building, and from the moment an animal skin becomes parchment, a coat or part of a piece of furniture, the organic matter involved will be kept in conditions more favourable to its preservation than those in nature. As long as it is in use, the degradation of the object is slow. After the period of use has passed, many artefacts go through a period of abandonment (in the worst scenario, being buried in the ground or immersed in water). And then they are rediscovered (in the worst scenario, by an amateur) and often kept in bad conditions. During this stage, degradation advances quickly.

From the moment they arrive in a museum, the artefacts become cultural heritage and will evade the natural degradation pattern and its timing. The life cycle in which they are now involved will be purposefully lengthened.

The most important chemical compound present in wood, paper, or textile fibres derived from cotton, flax, hemp or jute is cellulose — a polysaccharide.

In materials of animal origin, such as parchment, leather or textile fibres of wool and silk, proteins are dominant (collagen, keratin, sericin and fibroin). Both cellulose and proteins are biopolymers.

The degradation of natural organic materials consists primarily of the breaking of their structural biopolymers. The phenomenon is governed by vulnerabilities, opportunities and preservation conditions.

The vulnerabilities result from the structural characteristics of the biopolymers. The monomer units in cellulose are kept together by glycosidic bonds. These bonds are the vulnerable points in the polymeric chain. In the structure of a protein molecule, peptide bonds dominate, and they are the weak link of the polymeric chain in this case. The degradation products of organic matter are involved in trophic chains. They are the necessary nutrients for the beginning of a new life, and provide the opportunity for resumption of the biological cycle. In this way, the degradation of organic matter plays an essential role in life.

Vulnerabilities and opportunities are the factors leading to the degradation of natural organic materials. However, extreme dryness or perfect isolation from oxygen and water promote conservation even in a natural environment.

For instance, coal strata perfectly insulated wooden hunting spears found in good condition at Schöningen, Germany [2.4], and dated to the palaeolithic era. Mummies from the third millennium B.C. and manuscripts on leather and papyrus dated from the 3rd century B.C. were discovered in good condition in similar environments. However, these examples have to be regarded as exceptions from the general rule of quick degradation under natural conditions.

In conclusion, all organic substances participating in the life cycle have low thermodynamic stability in natural conditions. Exceptions are products with a high degree of mineralization: bone, horn and ivory. Proper environmental conditions in museums and frequent hygienic treatments control the development of biodeteriogens (the organisms that degrade cultural artefacts), preserving the artefacts.

Irradiation techniques are applied successfully to preserve artefacts in this category. Irradiation can be used as a method of physical biocide. Polymers obtained through irradiation can strengthen porous, degraded wooden structures or waterlogged wood.