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No. 112
Focus on conservation:
Guidelines for the prevention and treatment of mould-damaged archival and library material

by Catherine Craig-Bullen,
Senior Conservator, Conservation Treatment

Within archival collections, the presence of mould is not an uncommon phenomenon. Airborne mould spores (properly known as “conidia ”) are everywhere within our environment. When activated, these conidia begin to grow within the substrate (the support) or on the surface of most library and archival materials.1 The presence of mould or suspicion that a collection is contaminated with mould requires immediate attention as its presence places the collection and the people handling the collection at risk. Mould can be extremely destructive to archival material. Much of the damage, such as weakening of the support, staining or obliteration of the text or image is irreversible. Mould can also cause severe allergic reactions or aggravate existing health conditions in people exposed to or handling the collections. Our attention should be toward the prevention of growth (by reducing the number of and opportunity for conidia to grow) and the removal of the vegetative growth. In all circumstances, the issue of safety and health must be a consideration.

The first step in reducing the potential for the development of mould is to minimize the opportunity for growth.

Keep things clean

Conidia are microscopic2 and can easily enter a building. They are carried in on people, dust particles, in plants, food and through the HVAC (heating/ventilation/air-conditioning) systems. Air filters should be kept clean and should be small enough to trap the minute particles. Screens should be placed on air intake vents to eliminate particulate matter that may carry in microbes.3 Areas with water, such as trays holding a plant or drain pans in dehumidifiers should not be left to become stagnant. Areas should be kept clean at all times. Dust and dirt can contain spores and provide the nutrients required for fungal growth. Shelves, ledges and other surfaces such as the tops of boxes and books should be dusted and/or vacuumed. Floors should be kept clean and if they are mopped, the water should be changed regularly to avoid redistribution of the particles. Ideally, collections should be kept in cabinets or boxes or under dust covers to avoid contamination. Non-woven material (such as Remay) can also be used to cover artifacts and protect them from accumulating dust. Food,fruit and plants should not be allowed in storage areas and should be minimized and controlled in other areas throughout the building.

Maintain proper environmental controls

Germination and growth can be reduced if the temperature and relative humidity are controlled and maintained at specified levels.4 The possibility of mould activation increases substantially when the relative humidity is greater than 65 percent. Growth is also dependent on other variables such as pH, oxygen content, light and the osmotic characteristics of the substrate, however, these are not as easily controlled as temperature and relative humidity.5 Air should be circulated in exhibition and storage areas. The use of fans can help prevent a build up of "hot spots" or areas of high relative humidity. Dehumidifiers should be available in case they are needed. The creation of micro-environments should be avoided; for example, condensation can easily develop on metal shelves, windowsills and sinks. Collections should be stored away from outside walls which can be areas of high humidity and condensation. Shelves and cabinets should be at least 10 cm off the floor to protect collections from potential flood damage. Storage and exhibition areas should be monitored at all times. There are a variety of environmental monitors that measure and/or record temperature and relative humidity. These provide an accurate picture of a particular environment and can be used to identify any potential or existing problems. Items that do get wet should be attended to immediately as germination of conidia can occur within a very short time (24 hours).

Ensure safe and healthy working environments

Close up of the bottom right corner of a mouldy and damaged book

Collections which are identified as being contaminated should be isolated from other collections and confined to an enclosed space as much as possible.

People handling contaminated material should be advised of the attendant risks which increase with frequency and duration of handling. Individuals with specific allergies and/or respiratory ailments should be particularly cautious. In most cases, people handling such documents infrequently have little risk of developing a sensitivity to the fungi; however, as frequency and duration of handling increases, so does the probability of developing a sensitivity to the antigens (toxins) produced by the mould growth. Sensitivity usually occurs through inhalation, though contact with the skin and other tissues can cause a reaction. Allergic reactions could include symptoms of bronchial asthma, allergic alveolitis and dermatitis. Individuals handling contaminated material should wear latex gloves, surgical face masks, lab coats and possibly goggles.6 The masks should be impermeable to small particulate matter. Ideally, this protective clothing should be disposable so that it can be bagged and sealed immediately after treatment. Handling the material in a fume hood would also help reduce some risks. Cleanliness is extremely important, and hands, face and protective clothing should be thoroughly washed and work surfaces wiped down after contaminated material has been handled.

When dealing with mould, err on the side of caution. If the concerns over safety and health (of the staff, collections and clients) cannot be adequately addressed by the institution, local or provincial health authorities should be called.7

Perform effective treatments

In many cases, it is very difficult to determine exactly what species of mould is present, and it is a job which should be left to a trained mycologist. Regardless of the species, the concern for curators and conservators is that any mould infestation should not be present and the focus should be on its eradication and possible removal. A collection exhibiting mould may or may not have active mould. If the material is dry and there is no odour, it is likely that the mould is inactive; however, the conidia could be dormant but still viable. Mould spores or conidia in the dormant state have a hard cell wall, low metabolism and are dehydrated. Once they are activated by one or many of any number of factors, they become activated and germination begins.

Freezing the material to below -20 degrees Celsius for 48 hours and thawing it slowly is one way to kill the active mould. Care should be taken to ensure that the material can indeed be frozen and that any possibility of distortion is reduced, particularly if the material is wet or damp.8

If items become wet, they should be dried as soon as possible to avoid mould developing. This can be accomplished through the use of blotters or spreading the items out to air dry. Fans should be used to circulate the air. If it is not possible to dry the material immediately, it should at least be refrigerated to inhibit any mould growth, and if possible, it should be frozen.

While refrigeration allows time for some follow-up treatment, there are some disadvantages. Minimal growth can still occur, resulting in the development of metabolic products. Also, due to the adverse conditions, pigmentation may increase causing staining and discolouration. In addition, soluble inks may continue to run. When freezing the artifact, care should be taken to ensure that it can withstand the process, and measures should be taken to ensure that any distortion is minimal. (Some photographic processes such as wet collodian on glass or metal should never be frozen.)

Immersion in a bath of 70 percent alcohol and 30 percent water will kill active mould and dormant conidia. (A small percentage of water is necessary to hydrate the conidia which breaks the cell wall and allows the alcohol to penetrate.) Again, care must be taken to ensure that the medium is stable in this solution, and treatment should be performed by a qualified conservator.

The use of fungicidal treatments is not recommended for a variety of reasons. Many treatment techniques present a considerable health hazard and must be done in accordance with very strict rules and regulations. Although hydrated conidia are vulnerable to many fumigants, dehydrated inactive conidia are resistant to treatment. Some fumigants can work as an activator by triggering the development of dormant conidia.

Mould can be found on the surface of the object and within the substrate below. It is the mould on the surface which is most easily removed. The elements within the substrate are more difficult to remove without damaging the artifact.

Surface mould can be removed by lightly vacuuming or brushing the surface. The media and/or support should be carefully examined to ensure that it can tolerate such a treatment. When the cleaning is performed, it should be done with great care to ensure that any friable or loose elements are not dislocated or lost. The use of a vacuum aspirator provides a safe and controlled method for the removal of surface mould.9 As a safety precaution, it is advisable to work within a fume hood. Any dry cleaning should be done in such a way as to prevent the spores from becoming airborne and further contaminating the surrounding environment.

Mould produces pigments, particularly under adverse conditions. These pigments may be found in various locations - in the conidia, the mycelium (a thread-like mass of growth) or within the substrate itself. If the pigmentation is in the conidia, it can be removed through vacuuming or brushing the artifact. Pigments found within the other locations are more difficult to remove. Pigments found within the mycelium can only be removed by removing the mycelium or by breaking down its cell wall using enzymes. Some pigments are soluble in certain solvents, and if they are found in the substrate, can be removed through the application of a specific solvent(s).10 Some permanent discolouration may also occur from the weak acids produced by the mould.

Mould affects the chemical and physical make-up of the artifact. With the presence of moisture, the chemical process of hydrolysis occurs, enzymes and acids react with the artifact and there can be a change in the pH (level of acidity). The mould rarely attacks the structure of the paper, that is, the fibres, but prefers the amorphous organic materials such as sizes, proteins and amino acids.11 As a result, there can be a considerable loss of strength. The mat of mycelium can “felt” the paper causing it to become soft, fibrous and without substance, thereby providing an area for considerable moisture regain and retention which can lead to further deterioration.

A qualified conservator should be consulted before treating paper which has been damaged by mould. It is essential that the physical and chemical limitations of the artifact be recognized prior to treatment. A conservator will also be able to recommend or perform treatment required to repair and strengthen damaged areas.


Institutions should be constantly on the alert for the development of mould within their collections and have a plan of action should mould be detected. This plan should include procedures for reducing exposure, handling, cleaning and eradication.12 Individuals handling the collections should be informed of these procedures and be aware of the potential risks when dealing with contaminated material.

Controlling the outbreak of mould within a collection must begin with prevention. Mould requires specific physical and/or chemical influences to activate germination. By ensuring that these elements are not available in necessary quantities, germination can be controlled. If the potential for a problem exists, that is, if a collection is contaminated but not necessarily active or is wet, then prompt action is a priority. By killing and/or removing the conidia or drying the wet or damp object(s), the possibility of growth and contamination can be reduced if not eliminated. Isolating those items that are contaminated from the rest of the collections also helps to reduce the potential for widespread outbreak. Through monitoring, prevention and prompt emergency actions, it should be possible to free the collections from the threat of, or actual infestation of, mould.


  1. M.L. Florian, “Conidial Fungi (Mould) Activity on Artifact Materials - A New Look at Prevention, Control and Eradication,” International Council of Museums, Committee for Conservation, Vol. II, pp. 868-874, 1993.
  1. Ibid.
  1. W. Glenn, “Getting Rid of Unwanted Guests,” Occupational Health and Safety Canada, pp. 40-45, Nov./Dec. 1993.
  1. National Archives of Canada, Headquarters Accommodation Project, Architectural Program: Gatineau Building, Vol. 1, pp. 85-103, 1989.
  1. M.L. Florian, “Conidial Fungi (Mould) Activity on Artifact Materials - A New Look at Prevention, Control and Eradication,” International Council of Museums, Committee for Conservation, Vol. II, pp. 868-874, 1993.
  1. J. Bissett, “Recommended Procedures for Examining and Conserving Mould-Contaminated Documents,” Correspondence, Biosystematics Research Centre, Agriculture Canada, l989.
  1. Strang, J.K., and J. Dawson, “ Controlling Museum Fungal Problems,” Technical Bulletin 12, Ottawa: Canadian Conservation Institute, 1991.
  1. Library and Archives Canada, Guide on Emergency and Disaster Control, pg. 1.H.1., 1993.
  1. Price, Lois, “Managing a Mold Invasion: Guidelines for Disaster Response,” Technical Series  No. 1 - Mould, Conservation Centre for Art and Historic Artifacts, 1994.
  1. H. Szczepanowska and C. Lovett Jr., “A Study of the Removal and Prevention of Fungal Stains on Paper,” Journal of the American Institute for Conservation, Vol. 31, pp. 147-159, 1992.
  1. N.I. Hendey, “How Fungi Attack Materials,” Science Journal, pp. 43-49, 1966.
  1. National Archives of Canada, “Precautionary Measures for Dealing with Mould Contamination of Archival and Library Collections and Locally Contaminated Buildings,” 1996.