Series editors' preface - Wood Tools

about by localized conditions such as those
found in display cases, packing cases or
stores. Hydrogen sulphide is oxidized by
ozone to sulphuric acid.
Other pollutant gases and vapours All timbers
may give off organic acid vapours (particularly
acetic acid) that can be damaging to certain
metals, particularly lead and its alloys, zinc
and some copper alloys. This effect is more
pronounced with some species such as oak
and Douglas fir. Formic acid, used in the
tanning of some leathers, in the coagulation of
rubber latex and as a catalyst for certain
adhesives, also gives rise to organic acid
vapours and hence to damage. Marine animal
shells containing carbonates and paper items
are among other classes of material that may
be damaged in this way. A great variety of
materials including rubber, wool (especially
felt) and adhesives used in particle boards and
plywood may give off volatile sulphides,
possibly encouraged by sulphur metabolizing
bacteria. These can bring about the rapid
tarnishing of silver including that found in
photographic images. PVA emulsions may
initially give off acetic acid vapours and the
degradation of PVC may result in locally high
concentrations of hydrogen chloride and
hence hydrochloric acid, an extremely strong
acid. Although the major problem in museums
may be the removal of sulphur dioxide, nitrogen
oxides and ozone, there are some
recorded cases of really serious damage
having been caused by local pollution of this
sort. In addition, many hundreds of volatile
organic compounds have been detected in
show case environments (Martin and Blades,
1994) of which little is known concerning the
damage they may cause. A general rule is to
avoid organic materials in the construction and
furnishing of cases for objects containing
sensitive materials and, where this is not possible,
to test such display materials carefully
before use. However, if the objects being
displayed are themselves predominantly of
wooden construction this rule can be applied
with far greater discretion. In a few cases, the
materials combined in a single object may
markedly react with each other. Testing of
materials for short-term prediction of long-term
effect is notoriously difficult but several materials
testing services are available, including the
General review of environment and deterioration 265
Oddy test offered by the British Museum and
the Purafil coupon service.
Control of gaseous pollutants A lesson that
can be learned from the second law of
thermodynamics (see section 6.2.1) is that
pollutants should be trapped at source. If the
randomization process is allowed to take over,
by which pollutants are widely dispersed, then
any process used in an attempt to reduce this
disorder will itself result in the creation of
even more disorder.
Two methods of removing pollutants are
commonly used in museums. The first of these
is a water spray, which is routinely used in air
conditioning systems as part of the RH control.
Since both sulphur dioxide and nitrogen
dioxide are freely soluble in water, it will
effectively remove these gases, provided that
the water does not become too acidic. An
alkali could be used provided it was not
volatile but there are technical problems, such
as the possibility of spray blockage, with this
technique. Caustic alkalis would not be a good
choice with NO 2 since NO is produced in the
reaction and this is not very soluble in water
but can recombine with oxygen to reform
NO 2. Ozone is soluble in water but the
solution formed is unstable, hence water spray
is not satisfactory for ozone removal. In
addition to the above mentioned gases, water
spray should be very effective in removing
hydrochloric acid and chlorides where these
are present. Some volatile pollutants in cases
can be absorbed on to materials such as
activated charcoal or other, pollutant-specific,
scavengers.
All substances are capable of retaining a film
of gas on their surfaces but the amount
retained is usually very small. Charcoal, having
an enormous surface area absorbs gases in a
remarkable manner. Specially prepared
activated carbon filters can be obtained with
surface areas of up to 700–1000 square metres
per gram. The carbon granules are held in a
mesh frame which is obliquely presented to
the air flow. This type of filter is capable of
removing up to about 95% of the sulphur
dioxide in a typical industrial air sample at a
single pass. Efficiency with nitrogen dioxide
removal is more variable but can be up to 90%
under similar conditions. The material is also
very effective in reducing ozone to trace levels