Series editors' preface - Wood Tools

260 Conservation of Furniture
for many institutions is to arrive at hygrometric
half times which are sufficiently long for the
showcase RH to remain stable through a whole
season without maintenance or mechanical
aids. In any case, buffers can be a useful
adjunct to other control techniques and as a
back up in case these break down. Current
thinking suggests that rather than attempting to
maintain an absolutely stable and constant
environment throughout the year irrespective of
external conditions, it may be better to adjust
RH over a season, following natural seasonal
changes but in a gradual and controlled
manner. An example would be to allow RH to
change by 2% per month so that the mid winter
RH (in northern temperate climates) would be
12% below the mid summer RH, to which it is
gradually allowed to return over the intervening
six months. For further discussion of RH
see Thomson (1986) and the references given
at the end of section 6.2.
6.2.5 Pollution
Pollution is defined by the Oxford English
Dictionary as ‘that which makes physically
impure, foul or filthy, dirties, stains, taints or
befouls’. Unlike the previously discussed
agents of deterioration, pollution can take
many forms. The problem may be considered
under the headings of particulate pollution
and gaseous pollution.
Particulate pollution
The size of particulates – solid particles
suspended in air – ranges from about 0.01 μm
to about 100 μm and is of great importance.
Some examples of the sizes of different particulates
are as follows:
Polymerized plant material, automobile
exhaust and tobacco smoke 0.01–0.1 μm
Soot, oil smoke 0.1–1 μm
Sea salt 1–10 μm
Coal dust, ash, pollens 1–100 μm
Particles from about 1 μm upwards are
sometimes called primary particulates or
mechanical particulates because they have all
been formed by direct mechanical action or
sent directly into the air as particles. Those
below 1 μm are known as secondary particulates
as they are formed by chemical processes
between gases and vapours which have been
introduced into the air.
The burning of fossil fuels in power stations,
internal combustion engines and elsewhere
produces a great deal of sooty and tarry material.
This will usually have become acidic
through adsorption of sulphur dioxide and may
contain iron and other heavy metals in varying
proportions. The continuous process of
erosion, brought about by rain, wind, temperature
changes and mechanical action, of all
exposed material is another source of particulate
materials. Areas near the sea may be
affected by salt crystals blown into the air as
spray. Inside buildings, the breakdown of the
fabric of the building itself gives rise to a wide
range of particulates, including very fine
(0.01 μm) alkaline cement dust, plaster dusts
and fibres from furnishing fabrics. People in
buildings also give rise to very significant levels
of particulate pollution in the form of dead
skin, hair and textile fibres from clothing.
The Clean Air Act (1956) did much to
reduce levels of particulate pollution in
London from around 1000 μg/m 3 in the early
1950s to around 40 μg/m 3 in the late 1980s.
Background levels over the whole of the
industrialized countries may be expected to be
in the region of 20 μg/m 3 . The natural particulates
produced from plants and from aromatic
materials of plant origin which have been
polymerized by sunlight could account for
about half this figure.
All particulate material which settles on an
object will make it dirty and impair the visual
qualities of the surface thus making cleaning
necessary. However carefully cleaning is
carried out, the possibility of damage cannot
be excluded; removal of some of the object
along with the dirt is almost inevitable and
repeated cleaning will cause deterioration to a
varied but certain extent. Sea air laden with
salt spray may find its way indoors in coastal
areas where it will deposit small amounts of
salt which can maintain a high local RH. This
can greatly accelerate the corrosion of metals,
especially bronze, and may find its way into
ceramics and stone, storing up trouble for the
future when it may recrystallize in a lower RH
and damage the surface of the object. Acidic
particulate matter attacks many materials,
including all forms of cellulose. The alkaline
dust from cement may cause damage to oil