Series editors' preface - Wood Tools

680 Conservation of Furniture
whole surface or applied selectively. Deionized
water rinses may be used to remove
soluble corrosion products and residues from
chemical cleaning agents (Fiorentino, 1994).
Mechanical removal of corrosion
products
The mechanical removal of corrosion products
is generally more selective and controllable
than chemical methods, but chemical and
mechanical methods are often used in combination.
It is often useful to remove the bulk
of heavy corrosion by mechanical means
before chemical removal of corrosion products
or stabilization is carried out. Mechanical
methods must be selected on the basis of the
condition and the delicacy and level of detail
on the object.
Mechanical removal is usually undertaken
with the aid of a low magnification microscope.
It is helpful to examine metal surfaces
under magnification before treatment, as original
surface treatments may be detected in the
corrosion layer, for example gold overlay on
iron may not be visible to the naked eye. This
type of treatment of corrosion products may
utilize a scalpel or other sharp tool to pick
thick brittle corrosion layers from a surface.
The point of the tool is used to pick off small
flakes of corrosion. Although a scalpel can be
used if corrosion layers are very thick, care
must be taken to avoid scratching underlying
or adjacent metal. Bone and ivory tools are
often useful because they are hard enough to
remove many corrosion products, but soft
enough not to damage the metal surface. A
variety of abrasive materials may be used,
such as Garryflex (an abrasive block) or
proprietary polishing compounds that utilize
abrasive powders in paste form. Glass fibre
bristle brushes are equivalent in hardness to
hardened and tempered steel. They are
inappropriate for the removal of corrosion
products from brass and other copper alloys,
gold and silver, but may find occasional use
for very resistant corrosion products. Gloves
should be worn when using these pens as the
glass fibres break off in minute pieces that
cause painful irritation if lodged in the skin.
Abrasives polish metal by scratching the
surface to remove a small amount of metal.
Abrasive powders vary in hardness and particle
size. Harder particles will remove more
metal, but particle size will determine whether
the scratches produced will be visible. Thus
fine alumina (Mohs hardness 9) will remove
more metal than jeweller’s rouge (iron oxide,
Mohs hardness 5.5–6) but if the particle sizes
are the same, the final surface appearance will
be similar. The abrasive effect of particles on
metal will be ameliorated by a lubricant such
as solvent or wax. Transparent Plexiglass or
Perspex may be used to carry out an empirical
evaluation of the degree of scratching
produced by different materials. Properties of
modern and traditional abrasives can be found
in Table 10.3. Buffing wheels should not be
used to polish metal fittings from furniture
because they rapidly erode the surface.
Proprietary polishing compounds often
combine mechanical and chemical removal of
corrosion products, for example proprietary
silver and brass polishes may contain chemically
active agents such as ammonia or amines
and may have a pH of up to 10. In addition
to fatty acids, surfactants and other additives,
they often contain very hard abrasive material
such as quartz (SiO 2) and alumina powder
(Al 2O 3, corundum). Conservators usually prefer
to split the mechanical and chemical processes
to give greater control of the corrosion
removal process.
Mechanical removal of corrosion products
has several limitations. Unless the surface is
abraded to the depth of corrosion pits, corrosion
products cannot be completely removed.
In many cases the pits are too deep for this
to be practicable and, if it is essential to
remove all corrosion products, chemical
methods may be required. In cases where the
underlying metal is very soft and is obscured
by corrosion products, e.g. lead acetate (lead
ethanoate), mechanical removal of corrosion
products can cause significant damage. It is
possible to do a great deal of damage using
abrasives, which are usually harder than the
underlying metal. It may be difficult to remove
all abrasive particles or polish residues, though
a small amount of pigment or dye may be
added to disguise such deposits. Wood
adjacent to metal fittings has often been
discoloured by deposits of metal cleaning
pastes and the residues of metal within them.
Such damage may be prevented by removing
hardware when corrosion removal is required,
or inserting Melinex/Mylar or Parafilm