Series editors' preface - Wood Tools

(1990) and CCI notes 9/7 (1993). The components
of some proprietary silver corrosion
removal products have been analysed by
Selwyn and Costain (1991).
Silver may have been deliberately patinated.
Other deliberate decorative effects include
niello, an enamel-like mixture of metallic
sulphides that was intentionally fired into
recesses on silver and gold objects. Such patination
adds historical and monetary value to the
object as a whole and should not be removed.
The presence of niello can be obscured by
silver tarnish. Proprietary products such as silver
dip will severely damage or destroy niello.
Although silver corrosion products are stable
they do not protect the underlying metal,
which will continue to corrode as long as
there are reactants. The reactive species are
hydrogen sulphide (H 2S), carbonyl sulphide
(COS), which is the reactive component of
organic sulphides, and sulphur dioxide (SO 2).
Removal of corrosion products
Silver is soft and easily damaged by mechanical
corrosion removal methods. Silver plating
is very vulnerable and any method of tarnish
removal is potentially damaging. Surfaces are
normally blackened by silver sulphide, and
copper oxides and sulphides. In some cases,
light coatings of these corrosion products may
be mistaken for gilding due to thin-layer
optical interference phenomena. True gilding
will not be removed by acidic thiourea,
however, and this can be used as a spot test.
Tarnish may be removed from silver using
electrochemical reduction, polishes or chemical
dips.
Electrochemical reduction, described in CCI
Notes 9/7, reduces the corrosion products
back to silver, which is redeposited on the
surface. The dull, matte finish that results often
requires polishing to produce an acceptable
appearance. Care is needed to ensure that the
final finish remains in balance with the object
as a whole.
Polishing tarnished silver with a mild
abrasive is the traditional method of removing
corrosion products. Precipitated chalk, mixed
with ethanol to the consistency of a paste, may
be used for this purpose. The advantages of
polishing are that sulphide is left in low spots
heightening the visual detail of chasing and
engraving. By the same token, abrasives wear
Conserving other materials I 699
away high spots, change the surface topography,
obliterate detail and reveal sub-surface
copper oxides (fire scale) over time.
Silver polishes may be used to clean in situ
though care must be taken to avoid leaving
residues on adjacent material (Figure 15.13).
Minimal pressure, mild abrasives and fine particle
size will minimize the immediate and
cumulative effect of abrasive polishes. Wharton
et al. (1990) found that it is less damaging to
clean silver with a mild abrasive for a longer
time than a harder, more aggressive material
for a shorter time. Selwyn and Costain (1991)
ranked common proprietary silver corrosion
removal products according to the amount of
silver removed, the degree of scratching caused
and rate of retarnishing. They found that multipurpose
polishes were not suitable for removing
tarnish from silver as they contain harsher
alumina abrasives, rather than the softer silica
found in silver polishes.
Figure 15.13 Ebony clock case, London, c.1680–1700,
with pierced and engraved silver mounts. In order to
avoid metal cleaning residues, mounts may be removed
or vulnerable areas protected, e.g. with Melinex/Mylar