Most people have seen mould growths on old bread, cheese, jam, damp wood and leather. This mould growth, usually appearing as a fine fluffy mass is referred to as the ‘fungal colony’. Mould growth is favoured under warm, dark and moist conditions when air circulation is limited. The presence of dirt, particularly organic detritus also increases the risk of mould growth.
The appearance of the colony changes when conditions are favourable to mould growth, usually when the relative humidity is high. The older parts take on a different colour and fruiting structures appear. These fruiting structures, which contain the individual reproductive bodies called spores, are usually only visible with a microscope. The fruiting structures stand up above the mass of the colony so that the spores may be discharged into the atmosphere and carried by air currents to suitable growth surfaces.
Under such conditions spores absorb water, enlarge rapidly and branch repeatedly, forming a new colony. When the colony is established and large enough, fruiting structures appear, spores are produced and the cycle is repeated.
As long as the environment is suitable mould will form on organic materials (dead plant and animal matter). Provided there is sufficient moisture available, either in the object itself or in the environment, fungi will feed on leather, cotton, wool, bone, paper, wood, bark, fur and rawhide (Figure 1). Fungi are also capable of growing on metal and stone surfaces, particularly those with a coat of dust or organic debris. Some species even grow on creosote-treated wood while others can incorporate poisonous, chlorine-containing compounds into their diets.
Examples of undesirable effects of fungal infestation include the complete destruction of wooden and paper materials, staining of textile and paper-based objects and attack on photographic gelatin.
Some fungi can tolerate temperatures as low as minus 10 °C while others can survive in temperatures as high as 110 °C. Fungi are affected more by relative humidity levels than by temperature. A few species can survive below 60 % relative humidity but the majority require a relative humidity of at least 70 % to survive and reproduce. Fungal bodies normally die if the relative humidity drops below 60 %. The spores released at such adverse times will lie dormant however until suitable conditions for growth are available again.
Preventing Mould Growth
Preventing contamination by fungal spores within any building is practically impossible as spores are everywhere in the air and will be present on all surfaces. It is possible however, to control conditions so that it is very difficult for spores to germinate. This can be done by controlling the relative humidity and air circulation. Methods used to control relative humidity are described elsewhere (see the chapter Preventive Conservation: Agents of Decay).
If the relative humidity is maintained at a sufficiently low level spores cannot germinate. A relative humidity range of 40 to 65 % is considered safe. Too low a value (below 40 %), while stopping mould formation, can damage susceptible materials such as paper, leather and silk.
As mould growth is encouraged under stagnant conditions, it is also important to maintain adequate ventilation in storage and display areas. The use of something as simple as a fan will help to stop the build up of high relative humidity microclimates in susceptible areas such as against exterior walls and in basements.
Although elimination of the sources of fungal spores is not possible, surface contamination of indoor objects can be reduced by:
- sealing windows and doors and using central air-conditioning;
- storing objects in dustproof containers;
- covering objects that have been temporarily removed from containers; and
- vacuum cleaning objects carefully prior to storage or display.
It is more efficient to control collection conditions and prevent mould growth rather than have to treat objects contaminated by active colonies.
Material on Display
Provided the temperature does not vary greatly individual display cases can act as a buffer zone within a collection, helping to maintain relative humidity at a reasonably constant level. High relative humidity periods may occur sporadically (average relative humidity below 65 % but with occasional spells above this level) or endemically (average relative humidity above 65 %). Different methods are needed to control display case humidity under these conditions.
Sporadically High Relative Humidity
The strategy adopted to deal with occasional periods of high relative humidity will be largely determined by the nature of the storage or display equipment and the condition of objects in the collection. High relative humidity can usually be controlled by using a well-sealed display case or cabinet as a buffer against the ambient conditions. If this is not successful it may be necessary to use a dehydrating agent, such as self-indicating silica gel, to absorb the excess moisture. A change in colour from orange-yellow to either dark green, pale yellow or colourless (the colour change is determined by the type of silica gel used) indicates that the silica gel is saturated with moisture and needs to be changed. Note that the formerly recommended blue self-indicating silica gel is now classified as a toxic substance and should not be used. Other alternatives for controlling relative humidity levels are described elsewhere (see the chapter Preventive Conservation: Agents of Decay).
A disadvantage of using silica gel is the large quantity required to achieve and maintain a reduced relative humidity environment. For transporting objects or for short periods of high relative humidity, one kilogram of silica gel is needed for each cubic metre of space. If the problem is endemic then 20 kg/m3 is necessary (Thomson 1986).
Endemically High Relative Humidity
The options for controlling prolonged periods of high relative humidity are to use silica gel (20 kg/m3) in a well-sealed display case or to improve the ventilation so that mould growth is not possible. Although the use of air curtains and fans greatly improve conditions in tropical regions these may have to be combined with the use of silica gel or other desiccants if mould growth continues to be a problem.