Series editors' preface - Wood Tools

alent to low aromatic (c.17–20% aromatic)
white spirit but has a slower evaporating rate.
A range of aliphatic petroleum spirits are
available and vary in boiling range from
around 30–40 °C to 120–160 °C. Petroleum
spirit blends are not necessarily interchangeable.
In the case of those supplied by Merck
UK, the OEL varies according to the presence
of n-hexane (40–60 °C – < 2%; 60–80 °C – 40%;
80–100 °C – < 0.1% n-hexane). Prolonged
exposure to n-hexane can cause serious
damage to health and carries a risk of
impaired fertility. Thus the blends that contain
any proportion of n-hexane have a significantly
lower OEL than those without it.
Given the range of solvents included under
generic names, it may be helpful to use and
refer to named solvents whose properties can
be specified (e.g. ‘petroleum spirits 30–40 °C’,
rather than ‘petroleum spirits’ or ‘ligroine’).
Information such as boiling range, aromatic
content and evaporation rate may be obtained
from the manufacturer and can often be found
in material safety data sheets. The properties
of some common hydrocarbon solvents are
listed in Table 11.1.
Turpentine is a volatile oil derived through
the distillation of wood or resins from certain
conifers. It is a naturally occurring cyclic
hydrocarbon. Gum turpentine comes directly
from processed pine sap, most often a species
of an American southern pine, whilst wood
turpentine is derived from extracts after the
wood has been harvested. Turpentine has a
boiling point range of around 154–170 °C,
consists of unsaturated cyclic hydrocarbons
(also called turpenes) and may also contain
resin acids. Mills and White (1994) discuss and
illustrate the structure of some of the components
of turpentine. Many turpenes are
reactive and oxidize with exposure to air, light
or heat, leaving insoluble polymerized
residues. As a result turpentine has been
largely replaced by white spirit in most conservation
studios, but there may be cleaning
problems for which it is appropriate.
Hydrocarbon solvents such as petroleum
distillates, white spirit, turpentine and xylene
contain hydrocarbon structures similar to those
in oily dirt, soot and waxes. They are good
solvents for these materials and generally poor
solvents for polar natural resins. The combination
of varying proportions of alkanes,
Principles of cleaning 509
Figure 11.8 A japanned papier mâché tray c.1865. The
inlaid mother-of-pearl is embellished with fine painted
and gilded details. A natural resin varnish, visible under
UV, was usually applied selectively to the decorated
areas. The painted and varnished mother-of-pearl, inlaid
into the black japanned background, was disfigured by
a multitude of dirt spots. Solvent tests showed that an
aromatic hydrocarbon solvent would remove these
spots, but at considerable risk to the delicate paint
layers on the mother-of-pearl. By mixing an aliphatic
hydrocarbon solvent with an aromatic hydrocarbon
solvent to give progressively increasing aromatic content,
it was possible to find a mixture with sufficient aromatic
content to dissolve to dirt without damaging the
decoration
cycloalkanes and aromatic components within
a hydrocarbon solvent determines its solubility
parameters (see Table 11.1 and Figure
11.10). Solvent tests for the removal of dirt
from a surface varnished with a natural resin
may start with an aliphatic hydrocarbon,
progress through aliphatic/aromatic mixtures
with an increasing aromatic content, to a pure
aromatic hydrocarbon solvent if necessary
(Figure 11.8).
Chlorinated hydrocarbons
The non-symmetrical addition of chlorine to a
hydrocarbon increases the contribution of
dipole bonding to the solvent. A chlorinated
solvent used in furniture conservation is
dichloromethane.
Cl
Cl
CH 2
dichloromethane
The solvent effect of dichloromethane is
dominated by dipole bonding and as a result it
is an effective solvent for many materials,