Series editors' preface - Wood Tools

Table 10.3 Abrasives – continued
ness 3 or less by a copper coin. A harder
material will scratch and wear away a softer
one – the greater the difference between the
two materials the more pronounced the effect
will be. The properties of common abrasives
can be found in Table 10.3. The Knoop
Hardness Scale is a series of values arrived at
by an indenting instrument. It is more accurate
than the Mohs scale, but many abrasive particles
are too small to be tested this way.
Abrasives work by scoring the surface – the
larger and harder the particles the deeper are
these score lines. On wood, it is essential to
work abrasive papers along the grain so that
the lines scored into the surface are parallel
with the grain. Sanding marks across the grain
will be visible on raw timber, through any
transparent finish that is applied, and will take
up a disproportionate amount of any stain that
is applied directly to the wood. It is necessary
to work through the grades of paper (i.e. 80,
100, 120, 150, 180) so that each removes the
score lines left by the previous and replaces it
with progressively shallower lines, parallel to
the grain, which become imperceptible to the
eye on a wooden surface at about 180 grit. Grit
sizes of 240 and above are appropriate for
denibbing or abrading transparent finishes. The
score lines that may be present on original
surfaces may be used to identify the abrasive
material – the deep and parallel lines left by
Dutch reeds, for example, are quite distinctive.
Principles of conserving and repairing wooden furniture 453
Group Abrasive name and Hardness Sizes Notes (names given in brackets are
composition Mohs Knoop available trade names)
Charcoal
and ash
Organic
abrasives
Horsetails: Equisetum
hyemale, ‘Dutch rush’ or
‘shave grass’ is a hollow
reed-like plant with nodes
and a silaceous cuticle
Shark skin
used as fine abrasives. Holzapffel stated
that elder-wood charcoal was used by
engravers to polish plates. Lamp black in
brick form, called ‘satin rouge’, has been
used to polish silverware. Fine ash of
various types has been used as a final
polish for gemstones. Rice chaff ash has
been used as a final polish for Oriental
lacquer
Dried and tied up in bundles, horsetails
have been used to polish scagliola,
plasterwork and meerschaum. Small,
flattened sections of the cuticle have been
used by gilders to polish gesso and bole.
The rough sandpaper-like skins have been
used as an abrasive for wood, especially
in Asia and Oceania, but also in Europe
Hand sanding will be necessary for most
conservation work but it is possible that there
may be situations where machine sanding is
appropriate. There are several types of power
tool sanders available but the defining factor in
both cabinetmaking and conservation is that
the abrasive action is parallel to the grain. Belt
sanders may produce a satisfactory finish but
those with a circular motion, such as disc
sanders and orbital sanders, will produce
surface defects similar to those found when
hand sanding across the grain.
All sanding involves generating fine particles
of wood dust. Prolonged exposure to wood
dust has been connected to nose and throat
cancer as well as lung complaints. Measures
should be taken to collect dust as it is generated
and the use of personal protective equipment
such as dust masks is essential. In some
countries, for example in the UK, control of
wood dust in the workplace is subject to
government legislation. All dust must be
removed from the surface of the wood before
any finishing process commences.
The process of using abrasive papers
compresses the grain. It may be necessary to
raise the grain by wetting the surface and
allowing it to dry before using 180 grit paper
on the roughened surface. It may be necessary
to repeat this process a second time if a bleach
or water stain is to be applied directly to the
timber.