Mold

Bob Muens <bob<-a t->flakeysol< . >com> writes

>I would like input from anyone out there, especially those of you in
>the tropics, who have any ideas about or experience in minimizing
>the risk of mold growth.

Until recently I was at the Australian Museum where we were doing
some investigation into this very subject.  Mould germinates and
grows when the Equilibrium Moisture Content (EMC) or "water
activity" of a surface stays above a minimum level for a long enough
period, assuming spores have settled on it.  Naturally enough, below
that minimum level (which varies according to species) no
germination or growth can occur.  Above that level but close to it,
germination takes a long time and growth rates are slow.  The higher
the EMC (up to a maximum point), the shorter the germination time
and the faster the growth rate.  Controlling the EMC in susceptible
materials is really the only reliable way of preventing mould
growth.

This can be done either by controlling the amount of moisture in the
material directly--through drying by some means--or indirectly, by
controlling atmospheric conditions--which is of course the usual
approach.  Ventilation appears to be helpful in preventing growth
but the exact mechanisms involved may be fairly complex--though
personally I cannot believe it can permanently prevent spores from
settling on surfaces or cause fatal damage.

Still-air conditions in the tropics--such as may be found in closed
cupboards or rooms, or even in sheltered corners--are quite likely to
result in mould growth if there is no humidity control of the
atmosphere.  Ventilation and air circulation could act by drying
surfaces directly or perhaps indirectly and temporarily by introducing
variable RH--and I ran some experiments in Sydney to investigate this.
The results showed that cycled atmospheric RH conditions produce
significantly longer germination times and lower growth rates than
steady conditions.

The ability of strong tropical sunshine to reduce daytime RH, to dry
walls and other surfaces are factors which may also help explain why
mould doesn't occur as often as may be expected in the tropics;
building defects combined with heavy rainfall, inoperable or
badly-maintained air-conditioning equipment and non-
"climate-sympathetic" or poorly-designed buildings are some factors
which may explain why mould actually does grow in particular
instances.  The problem with libraries is that books have a
relatively massive moisture storage capacity and as a result of this
and the usual method of storing them packed on shelves, variations
in atmospheric RH are unlikely to cause significant changes in their
EMC--this could of course be good or bad.

Dehumidifiers are likely to be the best of the mechanical options in
controlling EMC indirectly--provided visitor comfort isn't the
overriding priority and the equipment is reliable; air-conditioners
do have a similar effect in reducing RH but their main use is in
cooling air for human comfort and problems may occur when that
cooled air reaches warm surfaces, or vice versa once they've been
switched off after a while.  Ceiling- and oscillating-fans are
probably the most reliable and cost-effective option, though their
exact influence is still unclear.  The main problem with mechanical
solutions is that they have to run all, or most, of the time.
Continuity of power supplies and regular maintenance are not
guaranteed in a lot of tropical countries!

Vacuum-sealing of archives sounds like a good enough idea--provided
they are packed in the right conditions.  If they are not too
sensitive, they could be dried by heating in the sun before
packing--choosing the part of the day when RH levels are lowest
would be a less dramatic alternative.  Packing in a cool room could
cause problems if the air and the books are at different
temperatures and presumably a "front" of higher EMC would pass
through the book as heat dissipates out and it cools--though I don't
know whether this would persist long enough for germination or much
growth to take place.

Graeme Scott
Rijksmuseum voor Volkenkunde
Leiden, Netherlands

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                  Conservation DistList Instance 10:75
               Distributed: Wednesday, February 26, 1997
                       Message Id: cdl-10-75-005
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