National Preservation Office
The Prevention and Treatment
of Mould Outbreaks in Collections
NPO Preservation Guidance
Preservation in Practice Series
Author R.E. Child
Head of Conservation
National Museums & Galleries of Wales
ISBN 0 7123 4895 6
Design The British Library Design Office
October 2004
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.
I 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.
I 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.
I Rots affecting historic material are usually fungi growing in wood, producing
‘brown’ and ‘white’ rots, ‘wet’ and ‘dry’ rots.
I 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
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:
I Temperatures of 10-35ºC with optima of 20ºC and above.
I 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%.
I 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.
I 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.
I 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
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:
I Low temperatures
Below 10ºC inhibits spore germination and slows growth. Very low
temperatures (-40ºC) will eventually kill fungi.
I Low relative humidity
Below 70% to reduce the equilibrium moisture content of materials
to a level where fungi cannot grow.
I 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:
I 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
I Keep material off floors and away from outside walls, especially cold, north-
facing walls, as these may be liable to high humidities and condensation.
I Store and stack books, boxes etc., in such a way as to allow a circulation
of air around them.
I Increase ventilation where possible with fans, etc. Opening doors and
windows periodically on dry days may be useful in uncontrolled, damp
environments.
I Install dehumidifiers in all rooms where necessary.
I Keep material clean and protect from dust and water with loose covers
of polythene or Tyvek®, where appropriate.
I Regularly inspect material, especially leather bindings and collections
on lower shelves.
Treatment of mould outbreaks
I Active mould attack should be addressed with caution, as the mycelium and
spores are respiratory sensitisers and can cause allergic reactions in some people.
I Respiratory sensitisers are subject to the Control of Substances Hazardous
to Health (COSHH) Regulations 2002
I 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.
I 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.
I 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.
I 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
and conform to the Control of Pesticides Regulations 1986.
I 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, 1997.
Florian, Mary-Lou , Fungal Facts: solving fungal problems in heritage
collections, London:Archetype Publications, 2002.
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)
www.nla.gov.au/pres/conver/100196.html
http://amol.org.au/recollections/3/3/11.htm
http://palimpsest.stanford.edu/byorg/abbey/
www.solinet.net/emplibfile/moldnew.pdf
www.ccaha.org/mold_eng.html
www.acaeum.com/Library/Mold3.html
www.bodley.ox.ac.uk/dept/preservation/training/mould/intro.htm
www.mold.ph/mold_species.htm
www.botany.utoronto.ca/ResearchLabs/MallochLab/Malloch/Moulds/Moulds.html
5
National Preservation Office
The British Library
96 Euston Road
London NW1 2DB
Tel: 020 7412 7612
Fax: 020 7412 7796
Email: npo@bl.uk
www.bl.uk/npo
National Preservation Office
is supported by
The British Library, with
additional support provided by
The National Archives
The National Library of Scotland
Trinity College Library Dublin
Cambridge University Library
The National Library of Wales
Oxford University Library
Services
The National Library of Ireland
The Public Record Office
of Northern Ireland
The National Archives
of Scotland
The Heritage Council of Ireland
The National Archives of Ireland
The Consortium of National and
University Libraries (CONUL)
The Society of College, National
and University Libraries
(SCONUL)