Series editors' preface - Wood Tools

gases, vapours, liquids and ions. The application
of a coating may slow or prevent corrosion
caused by external factors such as
pollutants and moisture. However, if unstabilized
corrosion remains in pits or in the body
of the metal, corrosion will continue regardless
of the coating applied.
The addition of corrosion inhibitors may
enhance the ability of organic coatings to resist
corrosion. Some proprietary products contain
corrosion inhibitors, e.g. silver cloths, or
acrylic-based Incralac lacquer, which contains
benzotriazole (BTA), although the BTA may
volatilize from a protective film within a
couple of years. BTA has been recommended
for stabilizing cuprous chloride on archaeological
artefacts (Madsen, 1967; Sease, 1978).
The protective layer provided by the corrosion
inhibitor is chemically bonded to the metal.
The use of corrosion inhibitors on composite
or alloyed metals may be problematic because
whilst the corrosion inhibitor may be effective
for one metal it may attack the other. Some
corrosion inhibitors that are used for industrial
applications rely on a corrosion free surface
and may therefore require harsher surface
preparation than would be acceptable in
conservation. Some corrosion inhibitors, such
as phosphoric acid on ferrous metals, form
part of a multi-stage coating system, applied
to prevent physical disruption of the inhibitor
layer. Information on the principles and use of
corrosion inhibitors in conservation may be
found in Keene (1985). Before using corrosion
inhibitors, conservators should be aware of the
likely occurrence of the corrosion products
inhibited by a specific product.
The application of coatings to metal in
industry is usually preceded by cleaning the
surface using abrasion or solvents. In the
context of conservation, where it may not be
appropriate to abrade a surface, it is considered
sufficient to remove loose flakes of corrosion
and degrease the surface with white spirit
followed by ethanol. Acetone can be problematic
because its fast evaporation rate can cool
the metal sufficiently to cause condensation of
water vapour, visible as a white misty bloom
on the surface.
Method of application
The method chosen to apply a coating will be
dependent on several factors, including scale,
Conserving other materials I 685
ease of removability of hardware, temperature
and RH, the surface finish of the metal etc.
The surface of the metal must be thoroughly
degreased before lacquer is applied. Three
methods that may be applicable to the conservator
are brush coating, spray coating and
dipping. The success of all these methods is
largely dependent on operator skill. Brush
application is very versatile, whilst spraying or
dipping can produce coatings with good continuity
and even thickness. Lacquering metal is
a difficult skill to master, and requires practice.
Brush coating may be problematic because
brush marks produce a coating of variable
thickness. Thin areas tend to break down
before thick areas and preferential corrosion
occurs in a brush-stroke pattern that may be
difficult to remove. Other factors, such as the
solvent carrier, may also have an effect. A
slow-evaporating solvent may allow time for
the surface to flow and level but may not be
appropriate for vertical surfaces. The type of
brush used will also affect the lacquer layer
and specialist suppliers sell brushes manufactured
for lacquering metal. Traditionally these
were flat, wide, thin squirrel hair brushes that
minimized brush marks, though the expense
of squirrel brushes has led to the use of
cheaper mixed hair brushes.
Spray application of coatings is discussed in
section 12.4.8. A turn-table is necessary for
spray coating larger objects. Dipping can
provide a coherent and even coating, though
it may not always be practicable (Rymuza,
1989). Dipped objects may be hung on wires,
and drips collected with blotting paper as the
lacquer dries. Some experimentation may be
required to ensure that the coating is even,
rather than thin at the top (which will produce
a rainbow interference pattern) and thick at
the bottom.
Preferential corrosion
The problem of preferential corrosion is likely
to arise where there is uneven exposure of the
metal to the ambient atmosphere. This may
occur as a result of holidays (voids) in the
coating or where lacquer begins to degrade
(Figure 15.7). In such cases the coated metal
behaves as a more noble metal whilst the
uncoated metal is more reactive. An air corrosion
cell, which may lead to the formation of
disfiguring pits, can occur on the uncoated