Module 4: Deterioration Mechanisms; Methods of Survey & Analysis
Today Ornella Salvadori, a biologist and the director of the Scientific Laboratory at the Ministry for Cultural Heritage (Soprintendenza per i Beni Artistici e Storici di Venezia), spoke to us on the topic 'Microbiological Deterioration', following on from the lectures from Giulia Caneva last week (see Friday 20th May 2011; Biodeterioration & Visit to Instituto Superiore per la Conservazione ed il Restauro). It was particularly interesting to have Ornella with us because she attended one of the first ICCROM Stone Conservation Courses in 1979.
Ornella described biodeterioration as almost a new science because in the past biodeterioration was only considered to be a discolouration of the surface. Even starting at this point it is not as simple as discolouration simply due to the visual observation of the organisms. There are other reasons for the visual change which include the alteration of minerals by the organisms present and the production of pigments which are then deposited within the stone. For this reason when analyses are carried out in relation to biological activity it is always necessary to combine chemical and biological analyses to fully understand the alteration.
We discussed the 'bioreceptivity' of stone, that is the ability of a stone to be colonised. Ornella showed us the three environments in which organisms live within stone; epilithic (on the surface), chasmolithic (within cavities) and endolithic (organisms that have bored from the surface into the stone through mineral grains). Where growth is present on the stone's surface it will not always be visible and even where it is we will not necessarily be able to identify if there is also growth below the surface. It is essential to understand this when carrying out stone cleaning because although a stone may be superficially clean, not all organic material will necessarily have been removed.
It can be difficult in the field to identify if a dark patina on stone is organic or inorganic, but the general rule that can be followed is that where water is frequently feeding the area, the patina is likely to be organic and where the area is likely to remain fairly constantly dry then this is likely to be inorganic. If you wet the darkened area an inorganic layer (e.g. pollution crust) would show very little change, whereas an organic layer would take on a slimy texture and you may see a slight alteration in colour to brown-green which can give you an additional indication of its origin.
Environmental conditions such as humidity are very important in dictating the type of biological growth likely to be present on an organism; bacteria generally need >90%RH to develop whereas it is much less for fungi. Ornella's specialism is in lichens and she explained to us how lichens are formed by the symbiosis between a cyanobacteria and a fungi. Lichens come in several different epilithic forms and also endolithic forms. Ornella summarised the three main groups of organisms found on masonry for us, these are cyanobacteria, fungi and lichens.
The main message of the lecture was that there can be such a wide variety of biological growth on stone that we should not generalise and simply say 'biological growth' in our observations; different species can and will have very different implications.
This afternoon we went to the Non-Catholic Cemetery to visit out work sites and inspect the biological growth present on each of the six tombs to be conserved and we identified a wide range of lichens, algae and fungi.
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