Series editors' preface - Wood Tools

When a scalpel is used to pick away dirt,
the material either side of the blade is
compressed and the material below the blade
is in tension. A crack will develop ahead of
the blade as this tension is released. When
there is poor adhesion at the interface
between the dirt and the surface, the crack
will propagate along this interface, releasing
stress and breaking a fragment of unwanted
material away from the surface (Figure 11.6a).
If there is strong adhesion at the interface
however, there are two possible results. First,
if the dirt layer is stronger than the surface,
the crack will propagate into the substrate and
damage it. Secondly, if the dirt layer is weaker
than the substrate, the crack will propagate
within the dirt layer, partially removing dirt,
but also leaving some dirt behind on the
surface (Figure 11.6b). If the surface is
textured, rough or porous the result may be
particularly unsightly. In practice the situation
is more complicated, because the nature of
both dirt and substrate may vary, resulting in
changes in weakness and strength, adhesion
and cohesion, at different places and in different
layers (Figure 11.6c).
There is a risk that cleaving or picking
unwanted material from a surface can scratch
and damage the surface, and such damage is
not always visible to the naked eye. In some
cases such damage can be minimized or
avoided by using tools made from ivory, bone,
Perspex or Plexiglas. In some cases it may be
possible to remove unwanted upper layers
using pressure-sensitive tape (Webb, 1989).
11.2.3 Abrasives
Abrasives are not recommended for cleaning
decorative surfaces because they are relatively
indiscriminate and often damage the underlying
surface. Abrasive techniques are occasionally
used to reduce the thickness of an
unwanted layer such as overpaint or transparent
coatings on a flat surface. Fibreglass pens
are unsuitable for, and will scratch, varnished
and decorated surfaces. They may find
occasional use for the mechanical removal of
very hard accretions of surface dirt or corrosion
products from metals. 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.
Principles of cleaning 503
11.2.4 Dry cleaning methods
Dry cleaning methods have a long history in
the cleaning of decorative surfaces. The term
dry cleaning in the laundry industry refers to
the use of solvent cleaning for textiles. In the
context of the conservation of varnished and
decorated surfaces, it refers to cleaning
without the use of any liquid. When used
correctly, dry cleaning methods are nonabrasive
and rely on the adhesive properties
of the dry cleaning material to pick up dirt
and remove it from the surface. Cleaning does
not require a rubbing action as these materials
are usually rolled over the surface. Oil-free
bread is a traditional dry cleaning material
used for the removal of surface dirt but
residues will support mould growth or pest
attack. Specialized products are available that
Figure 11.7 Detail of painted decoration on a late
eighteenth-century clock case, during conservation. Dirt
was removed using a soft eraser