Series editors' preface - Wood Tools

732 Conservation of Furniture
oily dressings within the leather cause it to
darken and become embrittled. In time, components
such as free fatty acids that are solid
at room temperature may crystallize out on the
surface of the leather to form spews (or spues),
which are often visible as white spots. The previous
use of saddle soap, which has a pH of
9–10, may cause significant deterioration
because it destabilizes the acidity level of the
surface of the leather (Haines and Calnan,
1988). In the long term this lowers the shrinkage
temperature at the surface of the leather
and causes severe cracking. In some cases the
cracked leather may cup and expose the flesh
below.
Cleaning
As well as the dirt encountered on many
objects (e.g. dust, soot, fungal spores etc.),
there are particular types of soiling associated
with leather. Oils that have been applied in the
past to dress the leather, as well as fats and oils
from within the leather itself, can migrate to
the surface and are known as known as ‘fatty
spues’. Writing and other horizontal surfaces
may have stains associated with their use such
as ink, water, wine, food etc. Staining may
result from residues of metal polishing compounds,
particularly around metal studs. White
residues mixed with small amounts of abraded
metal are not uncommon, whilst components
from metal polishes which are absorbed into
the body of the leather may leave a dark halo
around the studs. Staining from metal corrosion
products is not usually a significant problem.
Unvarnished surfaces are prone to a build up
of ingrained oily dirt, e.g. from handling and
fingermarks. Past maintenance treatments such
as wax, oil and leather dressings are often discoloured
and serve to bind dirt to the leather.
It is essential to evaluate the type of soiling
that is present and consider the effect of a proposed
treatment on both unwanted dirt or
coatings and on the leather itself. Leather is a
porous material and acts as a sponge, drawing
liquid and solubilized dirt deeper into the body
of the leather. This behaviour is minimized
when the surface of the leather is freshly
tanned and intact, but is exacerbated in old
and abraded leather. Dirt may be lifted from
the surface using swabs slightly dampened
with solvent. As a general rule the surface
should not be flooded with solvent or other
cleaning solutions. Polar solvents (e.g. acetone)
can affect the chemical additives applied to the
leather in the tanning process, known as tannage.
Non-polar solvents are markedly less
likely to damage the leather.
Loose dirt may be removed with a soft brush
and a vacuum cleaner. This is usually safe for
leather with an intact or abraded surface. Dry
cleaning methods, described in section 11.2.4,
may be used if the surface is sufficiently robust.
Such treatments should always be pre-tested in
an inconspicuous area because in some cases,
particularly when the leather has a low pH, the
fibre bundles may have deteriorated to such an
extent that even light pressure will remove the
whole surface. Non-polar hydrocarbon solvents
may be used to remove wax or oily
deposits from intact or abraded leather. The
use of a swab slightly dampened with solvent
should reduce the likelihood of solubilized
material being absorbed into the body of
uncoated, porous leather. Leather with an
intact surface may be cleaned with a swab
slightly dampened with water. Water is problematic
for abraded leather because it can
cause stains, tidelines and can catalyse acidic
deterioration. Further, capillary action may pull
solubilized soils from the surface into the body
of the leather. Fungal accretions may be
removed by vacuuming or swabs dampened
with IMS or ethanol swabs, having pre-tested
these in an inconspicuous area. Mould stains
cannot be easily removed.
Chemical stabilization
The porous nature of leather results in the
absorption of both acidic and alkaline pollutants
which catalyse degradation. pH measurement
has been used to gauge the condition of
leather because acidic deterioration will occur
at a pH of 3 or below. If the leather has
absorbed alkaline pollutants a pH measurement
may indicate a higher overall pH but is
not necessarily an accurate indicator of chemical
stability. Whilst pH measurement may be
useful, a careful visual inspection of the overall
condition of the leather, checking for a dry
appearance, acrid smell, or powdery structure
is often a better indicator of overall condition.
Treatments intended to stabilize the acidity
of vegetable tanned leather and thus halt
chemical degradation have often used aluminium
salts such as aluminium sulphate, alu-