Series editors' preface - Wood Tools

772 Conservation of Furniture
gilding, decorative elements composed of composition,
or the presence of water or spirit soluble
surface layers. Material that can be safely
applied to one type of surface may seriously
damage others in areas that are immediately
adjacent. A great deal of skill is also required.
Materials that can be used safely by an experienced
gilding conservator may prove disastrous
in less skilled hands. The conservator should be
sensitive to the different reactions of burnished
and matte gilding to a cleaning solution – in
general burnished gilding is more tenacious
and will clean more easily than matte gilding.
Cleaning should be incremental to avoid overcleaning
and detrimental aesthetic consequences.
It is important to start with a very mild
solution and to test cautiously, advancing to
more ‘active’ materials only if necessary.
The application of tinted varnish to water
gilding was common in seventeenth and eighteenth
century England. Removal of discoloured
varnish can be inappropriate if it
represents an original decorative finish.
Solvents can be used for removing discoloured
shellac from water gilding but it is essential to
pre-test on an inconspicuous area and to adopt
a cautious approach.
Water gilding that has been burnished to a
high lustre is unlikely to have received any
coating, which would have interfered with the
intended decorative effect. It is therefore particularly
vulnerable to damage from abrasion
during cleaning. Aliphatic or aromatic hydrocarbon
solvents, applied with soft cotton wool,
can be used to remove surface dirt from water
gilding. Such solvents will also remove waxbased
toning that may have been applied in
previous restorations. The more hydrogen
bonding plays a role in the solvent or solvent
mix used the more likely it is that the water
gilded surface can be damaged. Very polar solvents
that exhibit no hydrogen bonding, such
as dichloromethane, can be used to clean
water gilded surfaces. In some cases, water
gilding may be surface-cleaned with a detergent
in a solvent. Hydrocarbon solvent-based
emulsions that incorporate a small amount of
detergent and water can be a useful tool.
Gelled cleaning mixtures may offer advantages,
but any appreciable water content is likely to
cause damage to water gilding. Cellulose
ethers can be used to formulate water-free systems
(see section 11.6.2).
A traditional technique of surface cleaning,
which is essentially an early form of gel cleaning,
involves applying a very weak parchment
or rabbit skin size to the surface, allowing it to
cool and gel before rolling or peeling it from
the surface. This technique requires that the
gold and gesso are stable and well adhered,
otherwise consolidation will be necessary
before such a treatment can be attempted.
The mordant used for oil gilding is based on
linseed oil and the constraints this imposes on
cleaning are similar to those found in the cleaning
of oil-painted surfaces. The mordant is susceptible
to swelling and leaching as a result of
exposure to polar solvents such as IMS or acetone.
Oil gilding can be surface-cleaned with an
aqueous solution, sometimes with the addition
of a little detergent. The mordant may be
saponified if the pH of aqueous cleaning solutions
exceeds 8.5, whilst calcium carbonate in
the gesso is susceptible to damage from a pH
below 5.5. The use of deionized water with the
addition of a small percentage of a non-ionic
detergent may be effective in some cases. The
use of a few drops of ammonium hydroxide in
water for cleaning oil gilded surfaces has been
reported in case histories but is not recommended
as a general rule. Microscopic examination
can reveal that gold has been removed
with ammonium hydroxide even when this is
not apparent from visual examination.
Ammonium citrate is very useful as a general
cleaning agent for oil gilding (Figure 16.26).
The pH may be adjusted, and buffered if necessary,
to keep it in a safe range for the gilding
that is being cleaned. As gilding ages the
oil size becomes more polar, so the upper limit
of the pH range of the ammonium citrate solu-
Figure 16.26 Cleaning of a gilded frame with a pH
buffered solution of ammonium citrate