The Prevention and Treatment of Mould Outbreaks in Collections

Transcription

1 National Preservation Office The Prevention and Treatment of Mould Outbreaks in Collections NPO Preservation Guidance Preservation in Practice Series

2 Author R.E. Child Head of Conservation National Museums & Galleries of Wales ISBN Design The British Library Design Office October 2004

3 The Prevention and Treatment of Mould Outbreaks in Collections Fungi: moulds and rots Biological deterioration is one of the major causes of deterioration of organic materials. Attack by mould, bacteria, insects and even mammals is inevitable for material that is neglected, exposed to a poor environment or stored in unsuitable conditions. A better understanding of how biological organisms function will lead to better long-term control measures rather than short-term palliative treatments. Fungi are a complex group of organisms that are neither plant nor animal, but which obtain their food from living or dead material. Those living on dead material, such as paper or leather, secrete enzymes deep into the foodstuff and absorb the resulting solution, thus breaking it down both physically and biochemically. Different fungi will grow on different materials but a number of similar types will often be found on an affected area. Mould is the common name given to the visible growth of various fungi growing on dead material and is usually seen as fluffy spots on the surface of materials such as textiles and cardboard boxes which have been stored in warm, damp conditions. Rots affecting historic material are usually fungi growing in wood, producing brown and white rots, wet and dry rots. Foxing is the term used for the brown spots and stains seen on affected paper which may have a fungal origin but may also have other causes such as iron staining. The biology of fungi is complicated, but if much simplified it can be explained as follows: The life-cycle of a mould starts from the spore. Spores are loosely comparable to seeds in the vegetable kingdom. They are minute (typically 1µm upwards in diameter) and are ubiquitous in the air. 1

4 The more humid the air, the greater the concentration of spores. Because of their small size, spores are carried long distances by air currents and only settle on surfaces in very still air. The spores can stay dormant for long periods of time, but when the conditions are right they will germinate. The necessary conditions for germination are generally: Temperatures of 10-35ºC with optima of 20ºC and above. Relative humidities greater than 70% to give a high enough moisture content in the material for fungal growth. Wood becomes vulnerable to fungal attack when it has a moisture content above 20%, which equates to a relative humidity of above 80%. Suitable organic foodstuffs include paper, leather, wood, organic dirt, soiling and treatments such as leather dressings. In general, mould growth is encouraged by dark conditions and a lack of air movement, but other factors such as surface ph can influence development. If conditions are suitable, the spores will germinate and produce hair-like hyphae which both penetrate into the material and grow on the surface of the object. Hyphae are filaments that are the growing stage of the mould. They secrete enzymes at their tips which dissolve organic material. The resulting solution is absorbed back by the hyphae and used as food for growth. When a mass of hyphae are present, the fungal colony is known as a mycelium. Mycelia exist in two basic types: vegetative mycelia which penetrate deep into the material to grow, and reproductive mycelia which grow on the surface as visible mould. When the colony is mature, the reproductive mycelium can change to produce sporophores which are specialised hyphae, containing spores which are dispersed into the air to create new colonies. Monitoring for fungi is normally carried out by monitoring the ambient relative humidity of the environment and measuring the moisture contents of objects. In general, organic materials in an environment with relative humidities over 70% are vulnerable to mould growth. This is because organic materials absorb moisture at high humidities, to a point when they can support fungal growth. 2

5 Where mould is already present, lower humidities and moisture content may still continue to support mould growth for some time. Control measures Long-term prevention of mould and control of outbreaks is only possible by effective environmental control to ensure that conditions are unsuitable for growth. The use of chemical fungicides and fumigants, while often temporarily successful, is not normally acceptable as they have limited efficiency, may be toxic, and have the potential for damaging objects. The use of chemical fungicides and fumigants also does not address the underlying problem of poor environmental control or building defects. Good preventative measures include: Low temperatures Below 10ºC inhibits spore germination and slows growth. Very low temperatures (-40ºC) will eventually kill fungi. Low relative humidity Below 70% to reduce the equilibrium moisture content of materials to a level where fungi cannot grow. Anoxic environment Oxygen-free environments will slowly kill most moulds. N.B. high temperatures above 50ºC will dehydrate and kill most fungi within a few hours. This can, where appropriate, be used as a control measure. Storage guidelines Where full environmental control is not possible, the following guidelines should be followed for the storage of vulnerable material: Ensure the building is watertight and do not allow damp to penetrate through the fabric from blocked drains, faulty roofs or other building defects. 3

6 Keep material off floors and away from outside walls, especially cold, northfacing walls, as these may be liable to high humidities and condensation. Store and stack books, boxes etc., in such a way as to allow a circulation of air around them. Increase ventilation where possible with fans, etc. Opening doors and windows periodically on dry days may be useful in uncontrolled, damp environments. Install dehumidifiers in all rooms where necessary. Keep material clean and protect from dust and water with loose covers of polythene or Tyvek, where appropriate. Regularly inspect material, especially leather bindings and collections on lower shelves. Treatment of mould outbreaks Active mould attack should be addressed with caution, as the mycelium and spores are respiratory sensitisers and can cause allergic reactions in some people. Respiratory sensitisers are subject to the Control of Substances Hazardous to Health (COSHH) Regulations 2002 In areas where mouldy material is present, and when handling mouldy material, protective equipment should be worn, such as gloves and masks. Dust masks, if used, should conform to European Standard EN 149 category FFP2S. Affected material should be isolated in sealed bags or areas until it can be dealt with properly. Remove visible mould by careful brushing in a well-ventilated area, in a fume cupboard, or by using a suitable extract ventilation, for example a fume extraction system. Lightly vacuum with a vacuum cleaner fitted with High Efficiency, Particulate Air (HEPA) filter to retain the fungal spores and mycelium fragments. A conventional vacuum cleaner will not do this. Respiratory protective equipment in the form of dust masks should only be used as a last protective measure and should conform to EN 149 category FFP2S. Do not use warm air to dry out wet organic material ( e.g. after flood damage) as this can encourage rapid mould growth. Use fans, dehumidifiers and cool air. If necessary, surfaces can be sterilised with 70% ethyl alcohol, but all use of fungicides and sterilants must be carried out with professional advice 4

7 and conform to the Control of Pesticides Regulations In an emergency, objects can be bagged and placed in a freezer to avoid further damage until remedial action can be taken. Additional sources of information Florian, Mary-Lou, Heritage Eaters: insects and fungi in heritage collections, London:James and James, Florian, Mary-Lou, Fungal Facts: solving fungal problems in heritage collections, London:Archetype Publications, Guild, Sherry, and Macdonald, Maureen, Mould prevention and collection recovery, Ottawa:Canadian Conservation Institute, 2004 (CCI Technical Bulletin 26). Websites: (URLs checked 1 September 2004)

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