Series editors' preface - Wood Tools

638 Conservation of Furniture
particularly prone to this problem. Crocodiling
of varnish layers may also be the result of the
inclusion of non-drying bituminous materials
such as asphaltum. The surface of varnishes
that incorporate this material dry, whilst the
interior does not. The non-drying interior is
pulled into islands as the surface dries and
contracts (Figure 13.15). It is impossible to
reverse this effect. Treatment options will be
determined in part by the nature of the object.
In some cases, for example where such a finish
contains information about an original decorative
scheme, such a defect may be acceptable.
In other cases, for example where the crocodiled
varnish is an unwanted later restoration
and can be separated from the original, partial
or complete removal may be an option.
It may be difficult to match gloss or areas
surrounded by cracked or crazed original
surface coating. A crazed surface finish can
sometimes be simulated using a two-part
system that consists of a slow-drying undercoat
and a fast-drying upper coat. A wide
variety of materials may be utilized but the
size and distribution of the cracks will vary
and experimentation will be necessary to
produce the desired result. Commercial
products, such as ‘Vernis à Vieillir et
Craqueleur’ by Lefranc and Bourgeois, are also
available. Techniques that attempt to match
craquelure vary in their success. Fairbairn
(1988) described casting a silicone rubber
mould with the pattern of craquelure and
using it to cast a varnish infill that was then
adhered with a thin coat of animal glue.
13.9 Polishing or dulling a
varnished surface
Gloss can be increased if the surface is rubbed
over with progressively finer grades of
abrasive. This was the method used to polish
spirit varnishes that had been applied with a
brush to a high gloss. This method requires
the use of a hard resin, such as shellac or
sandarac. A lubricant, such as white spirit, may
be used if required.
Examination of historic objects with aged
coatings will demonstrate the variation in gloss
present on an object. Recessed areas, for
example, may be matte whilst high points may
have a high sheen. Sensitivity to such varia-
Figure 13.16 Photomicrograph of cross-sections of a
matte paint film applied to a porous wood substrate:
(left) excessive absorption of binder into the substrate
has left non-wetted particles exposed on the surface;
(right) the inclusion of a thixotropic agent to ensure
adequate wetting of particles of matted paint
tions will help in matching a new surface to
the old. Gloss may be adjusted by using a mild
abrasive to roughen the surface or by the
addition of a matting agent such as fumed
silica to the varnish. Some fumed silicas act as
both matting agents and thixotropes.
Thixotropes thicken the lacquer and prevent
excessive penetration into absorbent substrates
(Figure 13.16). If excessive absorption of a
transparent matted varnish occurs, the matting
agent may be visible as a white deposit in the
pores of the wood (Figure 13.17). The use of
non-thixotropic fumed silicas requires either
Figure 13.17 A clear lacquer with a non-thixotropic
fumed silica applied to bare wood. Excessive absorption
of the binder into the porous substrate has left a white
deposit in the open grain of the wood (left). Such
deposits can be prevented by the addition of a
thixotropic agent or by the pre-sealing the surface
before applying the matted coating to the wood (right)