Series editors' preface - Wood Tools

In the past, abrasives such as rottenstone or
tripoli powders were used, however there are
many much finer abrasives available today.
Abrasive particle size, hardness and the use of
lubricants is considered in section 13.9.
Micromesh polishing cloths are a good alternative
to traditional abrasive powders. This product
was developed for removing scratches from
acrylic panels. A cushioned cloth holds particles
of uniform size, which avoids both abrasive
particles lodging in the craquelure and
scratches caused by the inclusion of stray large
particles. Micromesh cloths are available from
1500 to 12 000 grit, allowing the conservator a
great deal of control over the amount of material
removed and the final finish.
16.7.4 Removal of overpaint and later
varnishes
Japanned furniture has often suffered from frequent
applications of varnish – in one case up
to thirty-three layers of overvarnish were found
(Wachowiak and Williams, 1994). Removal of
non-original varnish from japanned surfaces is
often complicated by the similarity in solubility
parameters between the original material and
subsequent additions. This can be exacerbated
if nineteenth-century oleo-resin varnishes have
been applied over seventeenth- and early eighteenth-century
resinous japanning. In some
cases, for example where the upper layer is
more aged and oxidized than a lower layer, the
lower layer will dissolve more readily in solvents
than the upper, unwanted layer.
The question of whether to remove non-original
restoration varnishes often involves balancing
aesthetic considerations, for example where
an original design is obscured or distorted by
multiple layers of discoloured varnish, against
the potential risk of damaging the original layers.
If such layer removal treatments are undertaken,
analytical techniques such as microscopy
(both visible light and UV) can help characterize
or identify components in the layer structure
and allow a more selective treatment.
The presence of water-sensitive metal leaf or
powders often precludes the use of aqueous
solutions. A more common approach is the use
of solvents. The process of choosing the appropriate
solvent is outlined in section 11.3.3. The
advantage of a fast-acting solvent is that the solvent
will be in contact with the surface for a
Conserving other materials II 757
shorter period of time, however, the solvent
may act too quickly resulting in damage to the
underlying surface. A slower solvent will give
the conservator more control over the dissolution
of the varnish, however, the underlying
surface will be exposed longer which may
cause damage by swelling (Michalski, 1990; see
also section 11.3.3). Using a solvent gel may
slow penetration of the solvent into the substrate
and thus provide more control over the
rate of dissolution of the varnish layers (Plate
6). Solvent gels are discussed in section 11.6.
Enzymes can be very useful to remove an oil
varnish that has been applied over a natural
resin coating. A water-based system using
enzymes may be safer than solvents in many
cases (Wolbers et al, 1990). Enzymes are discussed
in section 11.5.6.
At first glance often the mechanical removal
of later varnishes seems to be an attractive
option. This is especially true if the layers are
not well bonded to one another or a layer of
dirt separates them. The principles and problems
associated with cleaving are discussed in
section 11.2.2. This method of ‘pinging’ off dirt
or unwanted upper layers is limited by differences
in adhesion and cohesion between different
layers on the surface and by the skill of
the conservator. Abrasives can be used to
remove an unwanted varnish layer, subject to
the limitations outlined in the discussion on
cleaning above. Abrasives are considered in
sections 11.2.3 and 13.9.
16.7.5 Consolidation
Materials and techniques for consolidation, outlined
in Chapter 12, can be used on japanned
surfaces. Many japanned surfaces have been
applied over a protein-based ground layer.
Often this gesso is a thin layer merely filling the
pores of the wood. However, it is this layer that
is the cause of much damage. Protein glues are
hygroscopic and, in comparison to many synthetic
adhesives, brittle. Fluctuations in RH
cause corresponding expansion and contraction
of the adhesive. The resultant stress often
causes adhesive failure between the gesso
ground and the substrate.
The other main form of cleavage occurs
between the layers of varnish themselves. This
may have been caused by a change in the formulation
of the varnish or contamination of the