Series editors' preface - Wood Tools

514 Conservation of Furniture
branching) of molecules and the secondary
bonding between them. Increasing temperature
will reduce viscosity. Solvents commonly
used in conservation are not very viscous and
are sometimes gelled to increase their viscosity
and hence application control. Viscosity is
discussed in detail by Moncrieff and Weaver
(1992).
Surface tension and capillary action
Surface tension is a direct measure of intermolecular
forces and describes the strength of
attraction between the molecules in a liquid.
Whether a solvent wets onto a surface well and
spreads out, or whether it wets poorly and
forms droplets or beads, will depend on
whether the liquid molecules are more strongly
attracted to the surface or each other. Solvents
that are predominantly bonded by van der
Waals forces wet surfaces more effectively than
those characterized by hydrogen bonding.
Surface tension can be reduced by the addition
of a surfactant (see section 11.5.4).
Capillary action occurs when liquids rise or
spread spontaneously through very fine tubes
or pores – a common example is water and
blotting paper. Capillarity is important in
solvent cleaning because fine pores in a
decorated surface may trap solvents and delay
evaporation, and because capillary action is
the basis for cleaning using poultices.
Toxicity
All organic solvents are potentially toxic, some
are carcinogenic and some are teratogens and
cause damage to fetuses. Manufacturers in the
USA and UK are required to provide users
with information about the physical and
chemical properties of solvents (and other
materials), the health and safety implications
of their use and any special precautions that
may be required. This information is provided
in the form of Material Safety Data Sheets. The
relevant health and safety regulations should
be read, understood and conformed with
before any organic solvents are used.
In the UK the Control of Substances
Hazardous to Health (CoSHH) Regulations set
out occupational exposure limits to solvents
and other hazardous materials. The Health and
Safety Executive publish an annual booklet
(EH40), which lists occupational exposure
limits (OEL) and maximum exposure limits
(MEL), both of which are legally enforceable
under CoSHH (see section 9.7.4). Also included
is a technical supplement on calculating
exposure and, in the appendices, an evergrowing
list of carcinogens. The maximum
exposure limit is the maximum concentration
of an airborne substance, averaged over a
reference period (long-term exposure limit or
LTEL is eight hours, short-term exposure limit
or STEL is 15 minutes), to which employees
may be exposed by inhalation under any
circumstances. MELs are set for substances that
may cause the most serious damage to health,
such as cancer or occupational asthma, and for
which safe levels of exposure cannot be determined
or exposure cannot be practicably
controlled. The occupational exposure
standard (OES) is the concentration of an
airborne substance, averaged over a reference
period (LTEL 8 hours, STEL 15 minutes) at
which, according to current knowledge, there
is no evidence that it is likely to be injurious
to employees if they are exposed by inhalation
day after day at that concentration. Both are
expressed in parts per million (ppm) or
milligrams per cubic metre (mg.m –3 ). As a
general rule, the smaller the number the more
harmful the substance. It is possible to convert
mg/m 3 to ppm using the following formula
(Health and Safety Executive, EH40):
OEL in ppm = OEL in mg/m3 24.05526
molecular weight
Similarly:
OEL in mg/m3 =
OEL in ppm molecular weight
24.05526
For example, the OEL for petroleum spirits
60–80 °C (Merck UK) is 70 mg/m 3 . The
constituent that is responsible for this comparatively
low OEL is n-hexane (MW 86.18). To
convert mg/m 3 to ppm:
70 24.05526
= 20 ppm (rounded up
86.18 from 19.53)
In the USA, permissible exposure limits (PELs)
for toxic and hazardous substances are set by
the Occupational Safety and Health
Administration (OSHA), a federal government