Series editors' preface - Wood Tools

700 Conservation of Furniture
Many silver polishes contain a tarnish
inhibitor. North (1980) reported that removing
the tarnish layer that had formed after the
application of a tarnish inhibitor required more
effort and resulted in greater removal of silver.
Tarnish inhibitors will fail eventually and the
tarnish that then forms may be uneven. Lins
(1989) found that a film of tarnish inhibitor
resulted in a silver surface that could not be
wetted easily, polished evenly or adequately
lacquered.
Silver dips are usually composed of a
mineral or organic acid (e.g. sulphuric or
formic acid) and a complexing agent such as
thiourea (Brenner, 1953). It should be noted
that silver sulphide has a pK sp of 49.2, which
far exceeds the K f of most chelators with the
metal ion (see section 11.5.5). Thiourea, for
example, binds silver with a K f of only 13.1,
thus the removal of silver sulphide is a result
of acidic conditions rather than chelation.
Although dips will dissolve tarnish, they tend
to leave a rougher, more granular surface with
a larger surface area than polishes (Selwyn
and Costain, 1991). They may be applied to
mounts in situ, using a swab, or mounts may
be removed from the object and immersed.
Contact with silver dip should be minimized
to limit acidic attack of the surface of the
metal. Silver dips have the advantage of being
quick and sometimes avoid the need for
mechanical removal of corrosion products.
However, dips tend to be an all or nothing
process and the ultra clean appearance
produced may not be desirable. In some cases
it may be necessary to use polish after an
object has been dipped. Silver treated with dip
retarnishes more quickly than that cleaned
with polishes.
Reshaping
Silver/copper alloys are metastable. Copper, as
well as unintentional metallic impurities, will
often migrate to the grain boundaries over
time, leading to a loss of malleability called
precipitation hardening. Because of this effect,
it is considered unwise to attempt mechanical
straightening of objects over 100 years old,
and in practice, less time may be needed to
make an object unsuitable for cold reforming.
The techniques of polymer casting and
electrotyping are both suitable for supplying
missing parts on silver objects.
Prevention of tarnish
Given that the primary mechanism for the
formation of tarnish on silver is exposure to
atmospheric pollutants, the exclusion or
absorption of such pollutants should be given
a high priority for furniture that incorporates
silver components.
Coatings
Lacquering is probably the best means of
protecting silver from tarnishing while on
open display in period rooms, houses etc.
Various organic coatings have been used but
many conservators prefer the appearance of
cellulose nitrate lacquers (e.g. Frigilene,
Agateen) on highly polished historic silver.
They level very well and a skilfully lacquered
object cannot be distinguished from an
uncoated piece.
15.3.12 Lead
Lead may be encountered in furniture, for
example in lead linings to urns or wine
coolers, or in the form of lead-based repairs.
Lead is relatively inert. Inorganic acids may
form salts on the surface that stop or slow
further corrosion. Organic acids, particularly
acetic acid, are the chief agents of deterioration
in collections. Organic acid attack will
produce fuzzy white deposits of lead formate
(methanoate), lead acetate (ethanoate) and
ultimately, lead carbonate. Whilst lead carbonate
is considered stable under most circumstances,
lead acetate and formate are not,
because they may convert to lead carbonate.
Care should be taken when removing these
corrosion products, and in the disposal of both
removed material and cleaning materials such
as swabs, as they are a health hazard. Thickett
et al. (1998) considered the risk posed to metal
artefacts by carbonyl pollutants.
Lead was often added to bronze casting
alloys to improve the flow characteristics. Lead
does not form an alloy with bronze, but segregates
out as the metal cools. These lead
pockets or inclusions can be preferentially
attacked by organic acids under some display
and storage conditions.
Lead exposure in the workplace is subject
to health and safety legislation. Protection
measures against exposure are geared towards
reducing escape of lead dust, fumes or vapour