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Moulds: a primer

Mould is important as:

  • a direct risk to collections
  • an indicator of poor environments and
  • a potentially serious health risk.

The life cycle of moulds generally proceed as follows; spores germinate to form a germ tube which samples the environment and, if conditions are still favorable, it grows branched filaments called hyphae. These collectively form the mycelium which is the visible, vegetative part of the organism. Given the right age (or appropriate environmental conditions) the mycelium produces a fruiting body e.g. a mushroom. The mould growth on paper, leather etc generally fruits as a coloured, powdery fuzz.

Spores are microscopic, from 3 to 30 micrometers in diameter and extremely abundant in air. Everything is covered in them, however they remain dormant till conditions are favourable. Their size enables them to colonise microclimates such as damp crevices or condensation behind frames, areas which may not otherwise be noticed during routine inspections and environmental monitoring.

Some fungi can grow at 0°C, some at 55°C but most range from 15° to 40°C and, most optimally, at 25° to 30°C. With relative humidities (RH), 65% is usually considered the minimum level but the higher the better. Some fungi, such as those which cause foxing, can grow at an RH level as low as 60%. Some dry rot fungi will transport their own water to dry sites allowing colonisation. Almost invariably however, mould growth will be present as a result of wetting of the object. Mould damaged or stained items may enter the collection dry but with considerable spore accumulation. This is usually visible as a very fine, black, green, yellow or brown dust. In this case the mycelium is most likely dead but the spores are still viable.

Fungi have adapted to survive on an enormous and unlikely range of foods. Their growth in aviation fuel (actually in the water at the bottom of fuel storage tanks) was a serious problem in the Pacific theatre of WW2. Films of dirt or organic materials absorbed from the air can also prompt and support growth.

The damage caused by moulds can be due to the secretion of extracellular enzymes into the material for pre-digestion of food before absorption. Staining may be caused by metabolites excreted by the organism (the striking pink, green and yellow staining sometime seen on paper items). Damage can even be as the result of a physical rupture of the surface of an item by fungal growth.

Moldy collections pose a serious human risk due to the exposure of an individual to a high concentration of common but normally thinly spread organisms. There is a risk of inhalation of fungal spores when, for example, inspecting a new collection of documents. Turning the pages of a moldy book fans spores straight into the face of the handler. This can cause allergic reactions, sensitisation, and, most seriously, a variety of pulmonary diseases including aspergillosis and aspergilloma.

Some advice with mould:

  • when handling suspect collections, wear a dust mask and wash your hands after handling them
  • be vigilant for its presence and anticipate its growth when leaks or condensation occurs
  • treat any mould you discover as a potential disaster
  • seek professional advice if investigations reveal that you do indeed have a disaster.

If an item with active mould is referred to you, your immediate response should be to arrest its growth. This can best be done by drying it as this kills the mycelium. Sunlight is very effective at killing mould (a combination of drying and high UV) but unless done with great care can cause serious damage to the object through rapid dehydration. If it is impossible to deal immediately with a wet moldy item (in a disaster for example) one option may be to bag it and freeze it.

Spores are usually removed from a dry item by brushing in a fume hood or a well ventilated space; the use of a suitable dust mask for this process is essential. There are a variety of fumigation methods to kill mould but they can be very damaging to the item and there is no residual effect to discourage re-infection. Some of the substances used to treat mould are also powerful solvents for some materials so these procedures should not be used by other than conservators or similarly qualified professionals. The best cure is prevention. Environments must be stabilised as much as possible and monitored for the continued safety of the item.

The control of mould therefore, as with other pests, requires good housekeeping practices. This involves the maintenance of stable temperatures and relative humidity values and a clean, well ventilated environment. If your building is air conditioned it is crucial that those systems functions efficiently, are regularly maintained and within design parameters.



Updated 12 August, 2005 , webmaster, CPBR (