Treatment of Mould
Mould is indicated by the presence of a ‘musty’ smell, stains and ‘fluffy’ or powdery growths on susceptible materials. Mould’s presence can be confirmed by either microscopic examination of the colony and/or culturing of the fungus under controlled conditions.
Initial steps in the treatment of mould (Guild and MacDonald 2004) include:
- protection of people;
- isolation of artefacts;
- deactivation of the mould by either air-drying or freezing; and
- cleaning of artefacts.
The first step in the treatment of mould is to protect all people in the vicinity of the outbreak from exposure to fungal spores. Moulds can cause health problems, especially for anyone with pre-existing respiratory ailments. Some moulds are toxins while others are primarily allergens. If the presence of mould is suspected it is important to take appropriate precautions when handling contaminated objects. Wear disposable gloves, a high quality mask or respirator, safety glasses and protective outer clothing (washable or disposable overalls).
Isolate mould-affected artefacts to prevent dispersal of spores. Seal small objects in polyethylene bags or boxes and wrap larger objects in plastic sheeting. If the objects are still damp or wet, minimise the time that objects are sealed in plastic so that further mould growth is not encouraged.
Mould must be deactivated before it is removed from an object. If living mould is physically removed from artefacts there is a danger of spreading spores to other objects, not to mention the associated health risks of such a practice.
Whereas chemical treatments such as paradichlorobenzene and thymol have often been recommended for mould treatment, avoid their use as they are harmful to both people and many materials in the collection. It is preferable to focus on deactivating the active mould using non-chemical methods (see below), cleaning up any affected storage areas and taking steps to eliminate the cause of the outbreak (water leak, air conditioning failure etc).
It is most important to do something about the conditions that led to the development of mould in the first place such as implementing passive environmental controls, effective housekeeping techniques and improved building maintenance. It is important to examine the area from which the object came, to treat this if necessary (see ‘Treatment of previous storage and display areas’ below) and to take steps to ensure that the risk of future outbreaks is minimised.
There are a variety of methods for treating mould-affected objects, including freezing, air drying between 30 and 40 °C and exposure to either ultraviolet radiation, sunlight or gamma radiation. As some of these methods can be damaging to objects, consult a conservator if you are unsure about possible treatment effects. Additional details of two of these methods are presented below.
Air Drying Treatment
Drying a mould-affected object will deactivate growing mould, minimise further mould growth and will also allow the object to be vacuum cleaned to remove fungal hyphae and spores from the surface.
If the object is still damp it should be supported, preferably on an absorbent, clean surface such as blotting paper and spread out as much as possible. If the object will not be damaged by elevated temperatures, raising the temperature to between 30 and 40 °C will deactivate growing mould. If this is not possible then dry the object using natural ventilation, fans or dehumidifiers. Fans used to increase the air circulation should not be aimed directly at the object to reduce the risk of damage to the object and the spread of spores.
Once the object is dry it may be cleaned by careful vacuuming using a HEPA-filtered vacuum cleaner. See the Textiles chapter for details on appropriate vacuum cleaning techniques.
Freezing artefacts is a further example of a non-chemical approach to mould treatment. Freezing has the potential to kill germinating spores and associated hyphae (fine fibrous strands), but may not be effective against dry spores (Florian 1994). Slow freezing to minus 20 °C and slow thawing enhance the effect of freezing on mould and spores (Florian 1997). Controlling environmental storage conditions following freezing treatment is critical to avoid activating dormant spores.
Whereas previously there were many objects for which freezing was considered unsafe (Berkouwer 1994), it is now considered safe to freeze most materials, including composite organic/inorganic items (Carlee 2003, Strang 2008). The low moisture contents of most museum objects, the inclusion of adsorbent, buffering materials with the object to be frozen and sealing in polyethylene bags all help to prevent damage to susceptible objects. Some objects that should not be frozen however include the following:
- desiccated or seriously degraded objects;
- some objects which contain a variety of materials, especially layers; and
- objects that have a high natural water content.
Such objects may not be able to withstand the fluctuations in relative humidity due to temperature changes and subsequent dimensional changes. Examples of such materials include:
- oil or acrylic paintings on canvas or panels, painted textiles and painted wood;
- glass plate negatives, glass colour transparencies, lantern slides and mounted glass slides, daguerreotypes, ambrotypes and tintypes;
- plant specimens or other objects that have high water contents (> 26 %); and
- computer, magnetic and grooved audio media such as tapes, disks, cassettes, discs and cylinders.
Many textiles and wooden artefacts, most paper-based objects including modern photographic prints, acetate film, books, herbarium specimens and artefacts made from materials of plant origin (leaves, seeds etc), leather artefacts, natural history specimens and objects made from materials of animal origin (feathers, fur, horn, bone etc) may be frozen to eradicate mould and insects.
If mould attack has occurred already, then the affected object, its storage area and any materials in contact with the object should be treated following the steps described below (Dodd 1991):
- isolate all affected material to stop further contamination;
- treat affected materials and storage areas;
- determine the cause of the mould outbreak; and
- rectify any problems which may have contributed to mould formation.
Isolation and Preparation for Freezing
The steps involved in the isolation of mould-affected objects are summarised below:
- place affected objects in plastic bags and remove them from the collection;
- if the mould outbreak is due to wetting of an object then allow it to dry out partly (see above), acclimatise to a relative humidity of 50 - 60 % and then wrap it in clean tissue paper, clean white cotton or linen;
- place the object in a polyethylene bag and remove most of the air from the bag. Carry out this procedure in a well-ventilated area and wear appropriate personal protection to reduce the risk of inhaling fungal spores; and
- seal the bag with tape or by heat sealing.
Treatment of Affected Materials
To ensure treatment of mould-affected materials is effective and that the risk to the artefacts is minimised, follow these steps:
- either wrap the objects in absorbent material like clean white cotton or linen or clean tissue paper or add these materials as packing in the bag to provide additional protection against relative humidity changes during freezing;
- seal the objects in polyethylene bags and place them in a freezer for at least four days at minus 20 °C or below ;
- allow objects to thaw slowly (in a refrigerator) before removing them from the refrigerator. Allow them to warm to ambient conditions before unsealing; and
- use a HEPA-filtered vacuum cleaner in a well-ventilated area to remove dead hyphae and spores from objects. Correct vacuum cleaning techniques are described elsewhere (see the chapter Textiles).
Vacuum clean objects in a fume cupboard or similar containment enclosure. If a suitable enclosure is not available then clean the objects outdoors in a dry, well ventilated area away from air-conditioning intakes and other people.
Unless access to industrial freezers is possible, there are limitations on the size of objects that can be treated in this fashion.
Treatment of Previous Storage and Display Areas
If possible, dry storage and display equipment at elevated temperatures. Spores cannot survive three to four hours of temperatures greater than 36 °C. While this is desirable, it may not be possible for large storage racks or indeed may damage the equipment. If the climatic conditions are warm enough, seal contaminated shelving units in black plastic and evacuate as much air as possible. Place the sealed shelving in the sun for the required period. As long as the plastic is well sealed the shelving will not be desiccated as there will be insufficient exchange of moisture from the shelving to the surrounding air.
Unless the storage equipment is likely to be damaged by contact with alcohol, swab the equipment with a 70 % alcohol/water solution as a further precaution against re-contamination. Methylated spirits can be used to make up the alcohol solution. Carry out this treatment in a well-ventilated area. Alternatively use a 0.5 % solution of sodium hypochlorite (household bleach) to wash the equipment. If bleach solution is used, wet the surfaces for 15 to 20 minutes with disinfecting solution to ensure that complete disinfection occurs (Guild and MacDonald 2004). There are also commercially available products containing ethanol, water and ortho-phenylphenol that are suitable for use on non-painted surfaces.
Determining the Cause of the Outbreak
All possible causes of mould growths should be examined and maintenance organised if a leaking roof, overflowing gutter or similar building problem contributed to increased relative humidity levels in the collection area. Implement relative humidity control strategies if the mould outbreak was the result of adverse weather conditions (see the chapter Preventive Conservation: Agents of Decay).
For some mould-affected objects, non-chemical techniques may be neither effective nor appropriate. In such cases it may be necessary to consider chemical fumigation. If there is no alternative to chemical treatment, consult a conservator to obtain the most recent information on chemical techniques and their effects on specific material types. This is important because many chemical treatments are damaging to objects and great care is needed. It is preferable to avoid chemical intervention if possible.
Use commercial pest control agencies to deal with heavy infestations of either fungi or insects. Again, consult a conservator prior to such intervention to minimise risks to artefacts.
When removing dead fungal matter, wear suitable safety gear such as a high quality dust mask or respirator, disposable gloves, protective outer clothing and glasses or goggles. Wear protective gloves and goggles when handling methylated spirits and other potentially dangerous chemicals.