Series editors' preface - Wood Tools

into consideration the importance and vulnerability
of extant patina and whether a change
in appearance will be acceptable.
Benzotriazole (BTA, C 6H 5N 3) is a common
treatment for archaeological copper alloy
objects and for bronze disease. BTA is a
copper complexing agent designed to form a
polymeric film of copper and BTA, tying up
free copper at the surface of the metal so that
it is unavailable for further corrosion reactions.
The polymer layer may also slow down the
diffusion of O 2 and other aggressive species.
It is incorporated into some lacquers, e.g.
Incralac, applied to historic objects.
Zinc dust has been proposed as a method
of stabilizing bronze disease without overall
treatment. Zinc will form stable and adherent
layers of basic zinc hydroxide chlorides that
can seal off areas of local corrosion. Active
corrosion on bronze objects were treated with
zinc dust, exposed to artificially high relative
humidity, then to five years of uncontrolled
tropical environment in India without renewed
activity (Sharma et al., 1995).
Mechanical removal of corrosion
products
Conservators often combine chemical and
mechanical methods for the removal of corrosion
products from copper and brass.
Stabilization, for example, is usually improved
by mechanical corrosion removal methods
because corrosion crusts have been reduced,
and penetration of BTA is better. The selective
mechanical removal of corrosion products
of brass and copper can be difficult because
the corrosion products are hard, whilst the
surrounding metal is comparatively soft.
Abrasive removal of corrosion products often
leads to excessive polishing of the surface
adjacent to the corrosion products.
Original brass may have had a brushed or
satin finish. Corrosion can disrupt this finish
so that, after removal of corrosion products,
the surface appears a characteristic yellow but
roughened. In the past such surfaces were
often restored by abrasive polishing, but this
practice has become less acceptable and is not
compatible with the conservation principle of
minimal intervention. The historical and
monetary value of many bronzes in enhanced
by the effects of age and corrosion. Many
bronzes, for example those from cultures with
Conserving other materials I 693
a strong reverence for the past, e.g. China or
Rome, should not be polished.
Problems associated with mechanical
removal of corrosion products mean chemical
methods are often used as a first stage treatment.
Where metal is not detachable or is
adjacent to vulnerable material, e.g. metal
inlay, it may be necessary to avoid chemical
methods. Fine particle abrasive powders that
may be useful include charcoal, jeweller’s
rouge (particularly useful after chemical
removal of corrosion products), rottenstone
and finely ground pumice powder. Proprietary
polishes that contain coarse particles or
ammonia should be avoided (e.g. Autosol).
Start with softer abrasives and move to harder
types if these are unsuccessful (see Table 10.3).
Abrasives may be applied using a hydrocarbon
solvent as a lubricant. On flat surfaces, some
conservators have used abrasives applied to
the face of a French polishing rubber, in
combination with solvent, to allow moderate to
heavy pressure to be applied evenly. It may be
impossible to remove residual abrasive particles,
for example from boulle surfaces, though
some conservators add a small amount of
pigment to disguise such deposits.
Chemical removal of corrosion products
Chemical methods are often effective in
removing corrosion products or softening
them sufficiently to allow mechanical removal.
Vacuum impregnation with benzotriazole has
made it feasible to retain corrosion product
crusts even if aggressive chlorides are present,
but many treatments in the past relied on stripping
the metal surface so that chlorides and
chloride-containing products could be
removed. When corrosion products are
relatively light and their removal is likely to
improve the appearance and stability of the
object, chemical removal of corrosion products
can be an option. The are several potential
problems associated with chemical removal of
corrosion products from copper alloys including
etching of the surface of adjacent or
underlying metal, stress corrosion cracking,
dezincification and the deposition of insoluble
metal complexes.
Stress corrosion cracking
Stress corrosion cracking (SCC) is a progressive
fracture mechanism that occurs in metals