Deterioration by Microorganisms

In relation to the microbial requirements for humidity, the influence of the climate becomes especially obvious for objects that at some stage have been inaccessible, creating a stable climate. Recently, newspapers reported on microbial problems in a cave in France that had been recently opened to the public. Even if only a limited number of visitors are allowed, it is necessary to consider the amount of humidity that is introduced by people. Therefore, in most cases, wall paintings opened to the public are subject to enhanced humidity, due to both human influences and the effects of the external climate after opening. Only very rarely can investigations of the climate and air contamination be carried out before documentation, investigation and consolidation are started. One of the exceptions to be described is the tomb of Queen Nefetari, in the Queens Valley, Thebes. Caves like Altamira or Lascoux have now been completely closed to tourists and are now only accessible to selected visitors with specific interests. 

The most important deterioration problems on wall paintings, induced or influenced by microorganisms, can be summarised as follows (i) deterioration due to biofilms, (ii) metabolism of organic bindings as substrates, (iii) alterations of mineral pigments, (iv) excretion of mineral or organic acids, and (v) interaction of microbes and faunal elements. 

Metabolism of organic bindings as substrate. It is well known that natural organic bindings, like animal or plant glue, lose their binding effect when 

Although some species of the genus Verticillium have been described that grow on chitinous substrates (important components of the insect exoskeleton), 

Grazing might also provide a kind of stimulatory effect on microbial growth and deterioration after mechanical disturbances of the biofilm. However, we have to consider that there might be a direct detrimental input of arthropods on mural paintings as well, since grazing is not restricted to the biofilm but may destroy the painting or plaster directly. 

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Purity control limits contamination with pathogenic bacteria Staphylococcus aureus, Escherichia coli. Salmonella-species, Pseudomonas aeruginosa, Clostridium-species and others), yeasts, moulds, microbial toxins (aflatoxins, endotoxins), toxic heavy metals (lead, cadmium, mercury, arsenic e.g. from industrial emission), pesticide and herbicide residues, fumigants (ethylene oxide, methyl bromide, phosphine) and radionuclides. Furthermore, impurities with other plants parts ( foreign organic matter ) are limited. Moist levels must be below a certain maximum to avoid deterioration by microorganisms. Excreta of animals and dead insects must be absent. The ash value and acid-insoluble ash limits the amount of inorganic impurities (soil, sand). 

PP is now used in indoor or outdoor carpets. However, the first PP products deteriorated rapidly when subjected to sunlight because of the presence of tertiary hydrogen atoms present. Explain. Which is more resistant to attack by microorganisms (a) PVC or (b) plasticized PVC ... 

Conditions which affect deterioration after manufacture include temperature, relative humidity, radiation, atmospheric contamination, and attack by microorganisms, insects, and rodents. These are the responsibility of those purchasing and using or storing the paper. Although they may profoundly influence the permanence of paper in use, they are beyond the scope of the present discussion. 

As mentioned in the introduction, fermentation has been used since ancient times to conserve and alter food. Also today, it is still applied on a very large scale for this purpose. A few typical examples are described in this chapter. The principle is similar in most cases. Lactic acid produced by bacteria protects the food from deterioration by inhibiting the growth of mold and other microorganisms. Most vitamins and nutrients of the food are preserved during fermentation. Three examples are discussed in more detail below The production of sauerkraut, soy sauce, and milk products (Table 9.3). 

Stability is improved by protective colloids, which are polymeric materials with highly polar and non-polar features in their molecular structures. They provide additional steric stabilization. If the protective colloids are not truly compatible with the film-former, gloss will be reduced and the film weakened. Frequently the colloids in water paints (e.g. water-soluble cellulose derivatives) are affected by microorganisms, so fungicides and bactericides may be included to prevent deterioration. 

Paints are susceptible to deterioration brought about by microorganisms. These include particularly bacteria, yeasts and fungi. 

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