Series editors' preface - Wood Tools

panying fungal attack. Animal fibres, mainly
wool and silk, provide foodstuffs for several
varieties of insects. The optimum RH for
clothes moths is 65–75% and for the brown
house moth about 90%, though even very low
levels of humidity may not deter them
completely. Some paint media, particularly
water colour used on ivory or vellum, may
contain a proportion of honey or glycerine
which can make them very sticky in damp
conditions. This provides food for microorganisms
and may cause the paint layer to
adhere to adjacent surfaces. The sapwood of
many timbers is also a good source of nutrients
for micro-organisms and fungi and may
also be attacked preferentially by several
wood-boring insects including powder post
beetle, death watch beetle, common furniture
beetle and longhorn beetle. Any type of object
which contains any of these materials in its
composition may be liable to attack.
Humidity is an important factor in the corrosion
of metals, in the fading and tendering of
textiles, in the movement of salts into and out
of ceramics and stone and in the phenomenon
known as glass disease. The consequences
and mechanisms of deterioration of these
materials are discussed in Chapter 8.
The mechanism of fading and tendering of
textiles is still not perfectly understood but the
photochemical degradation of textiles and
water colours requires oxygen and water
without which very little action takes place, as
demonstrated by the early work of Russell and
Abney on water colours (Brommelle, 1964).
The influence of humidity may possibly be
explained on the basis that most fading
reactions are bi-molecular and are diffusion
controlled. At higher levels of humidity, diffusion
of the reaction partners towards each
other becomes increasingly facilitated. The
excited species formed by absorption of light
then gets less time to dissipate its energy and
return to the ground state before reaction
occurs. In general, diffusion of the participants
in a chemical reaction will be increased at
higher RH and a film of moisture on surfaces
will provide an ideal medium for reaction
between objects and pollutant gases in the air.
Control of RH
Criteria for moisture control are discussed by
Brundrett (1990). Where water is an agent of
General review of environment and deterioration 257
deterioration, the humidity should be kept to
a minimum. Where the water content of an
object affects its physical properties, the
humidity should be kept stable. Based on a
compromise of the requirements of the different
types of materials encountered in a mixed
collection, a point in the range 50–60%,
usually 55%, with a variation of ±5% has been
chosen by many museums in the United
Kingdom. Specially sensitive items may have
to be isolated and treated individually if their
requirements cannot be met in this range. For
a collection of only one type of object a figure
can be chosen which best suits that type of
object. These set limits for variation do not
necessarily represent the best that could be
done for the sake of the objects. This is partly
because the complexity of possible interactions
mitigates against a single simple statement
of ‘ideal’ conditions for all objects and
partly because it is anything but easy to keep
relative humidity constant (even within the
above limits) in a building that is actually
being used.
Having decided on a desirable level of RH,
however, the question next arises as to how
this is to be maintained throughout the day
and throughout the year. There is no perfect
answer to this question but it is certainly influenced
by the resources available – especially
money. Probably the most complete answer to
humidity control is provided by air conditioning
(Chadderton, 1993). Air conditioning
should ensure that air in galleries is kept clean
and free from pollutant gases and microorganisms
and is maintained at a near constant
temperature and relative humidity. Before this
can be installed it is necessary to have at least
one and preferably two years readings for
temperature and relative humidity in those
buildings or parts of buildings which it is
proposed should be air conditioned. The
variables which most affect the system include
the requirement for filtration, the movement of
people in and out of the space, the lighting,
climate and daily and seasonal variations. The
heating and cooling requirements for the least,
mean and greatest situations must then be
worked out before air conditioning can be
installed. Once installed, an air conditioning
plant requires skilled attention to maintain it
in good working order and emergency backup
facilities in case any part of it breaks down.