Series editors' preface - Wood Tools

suppliers. Careful selection of solvent/s and
delivery systems can greatly increase the specificity
of a cleaning treatment and decrease the
risk to the surface that is to be retained. By
selecting solvents on the basis of, for example,
volatility and relative compatibility to a
surface, the speed and extent of action can be
controlled. Solvents can be used on a wide
range of dirt and surface types and can
penetrate into otherwise inaccessible areas.
Some of their disadvantages are their limited
effect on very polar dirt, toxicity, flammability
and expense. Factors influencing solvent
choice include volatility, viscosity, surface
tension (capillarity), toxicity, flammability and
solubility parameters.
Evaporation rates, vapour pressure and
density
The evaporation rate describes the rate at
which a material will change from liquid to
vapour, at a standard temperature and
pressure, in comparison to the evaporation
rate of a specific known material. The evaporation
rate of a solvent is dependent on the
size of the solvent molecules and the forces
of attraction between them. A homologous
series of compounds only differs from the next
compound in the sequence by the addition of
a CH 2 group, e.g. methane (CH 4), ethane
(C 2H 6), propane (C 3H 8) etc. The smaller the
molecule in any given homologous series, the
lower the boiling point, the higher the vapour
pressure and the faster its evaporation rate.
Van der Waals forces are the predominant
attractive force in hydrocarbon solvents. These
are weak and as a result low molecular weight
hydrocarbons are very volatile (e.g. methane,
ethane and propane). As the molecules
become larger they evaporate more slowly and
when very large, aliphatic hydrocarbons form
solid substances called paraffin waxes. An
aromatic structure induces slight polarity in
otherwise non-polar molecules and results in
a comparative increase in boiling point and
vapour pressure. Alcohols are polar molecules
that also have considerable hydrogen bonding
between them. As a result they are less volatile
than similar-sized hydrocarbons. Hydrogen
bonding is the dominant intermolecular force
in water. Thus even though water is a small
molecule, stronger intermolecular forces mean
that, comparatively, its boiling point is high
Principles of cleaning 513
and its vapour pressure is low. Liquids that
evaporate rapidly may be described as volatile.
The evaporation rate of a single solvent is
dependent on vapour pressure (in turn dependent
on temperature and pressure), surface/
volume ratio and the rate of air flow over the
surface (Wicks et al., 1992).
As a general rule, there is an inverse
relationship between the boiling point of a
solvent and its evaporation rate, i.e. the higher
the boiling point, the lower the evaporation
rate. Evaporation rates are usually expressed
as a ratio and therefore do not have units of
measurement. There are two widely used
standard solvents, diethyl ether (ether) and nbutyl
acetate (nBuAc), which are assigned a
relative value of 1 or 100. Unfortunately, the
scales are not interchangeable. A slow evaporation
rate on the ether scale corresponds to
a high number, i.e. the time a solvent takes to
evaporate is many times that which ether
takes. In contrast, a slow evaporation rate on
the butyl scale corresponds to a low number,
i.e. the evaporation rate is slower than that of
butyl acetate. Evaporation rates may also be
measured in ‘seconds to 90% evaporation’ (see
Table 11.1).
The vapour pressure of a solvent is a
measure of the pressure exerted by its vapour
when the solvent and its vapour are in dynamic
equilibrium, i.e. when the molecules leaving a
liquid to form vapour and those rejoining it are
in equilibrium. Vapour pressure is measured in
a closed vessel and at a standard temperature
and pressure. As a general rule, there is an
inverse relationship between the boiling point
of a solvent and its vapour pressure, i.e. the
higher the vapour pressure, the lower the
boiling point and the faster the evaporation
rate.
Organic solvent vapours are heavier than air
(i.e. their relative vapour density is higher than
air). This is particularly important for ventilation
and when dealing with spillages. Whilst
extraction should draw solvent fumes away
from the conservator, ventilation should be
designed to draw from, and tests for toxic
concentrations may need to be made at, floor
level.
Viscosity
Viscosity describes the resistance to flow of a
liquid. It is a result of the size and shape (e.g.