Series editors' preface - Wood Tools

590 Conservation of Furniture
12.4.3 Varnish formulation
The use of different solvents has been shown
to affect physical properties of films cast from
polymers, such as density and permeability
(Lawrence, 1990; Malkin, 1983). Solutions of
polymers in different solvents have different
viscosities, indicating the degree to which a
given solvent allows the polymer molecule to
extend. For example, at 25 °C, a 40% w/v solution
of Paraloid B72 in acetone has a viscosity
of 200 cps, toluene 600 cps and xylene 980 cps
(Lascaux technical data sheet R138/1996). The
more viscous the solution produced for a given
solvent–resin proportion, the greater the degree
of uncoiling of the polymer molecule in that
solvent. Good solvents produce films that have
interpenetrating polymer chains and a dense
structure, whilst poor solvents produce films
with aggregated chains and a porous structure
(Hansen, 1994).
Different solvents can also affect the mechanical
properties of polymer films. Paraloid B72
films cast from acetone (a comparatively poor
solvent for this polymer), for example, have an
extendibility of less than 10% strain to break. In
comparison, films cast from toluene, a comparatively
good solvent for this polymer, have an
extendibility of over 100% strain to break
(Hansen et al., 1996). The chemical compatibility
of solvent and substrate also has a role in
the final performance of the film. For example,
Schniewind (1998) noted that the adhesion to
wood of Paraloid B72 and PVAC AYAT delivered
in toluene were a fraction of that found
when these materials were delivered in ethanol
or acetone. Factors that affect the physical
properties of films of thermoplastic polymers
that are cast from solution include solution concentration,
retained solvent and film thickness
(Hansen et al., 1991).
Turpentine was the traditional solvent for
dammar and mastic but contributes to the photochemical
instability of varnish films (Feller et
al., 1985). A hydrocarbon solvent with an
equivalent K-B value (see discussion of hydrocarbon
solvents under section 11.3.1) and
evaporation rate should be substituted for conservation
applications. The properties of a
range of hydrocarbon solvents referred to in
conservation literature can be found in Table
11.1. Many traditional varnish formulations
contained linseed oil but this ingredient should
Figure 12.9 Photomicrograph of the roughened surface
of a coating that incorporates a matting agent (raking
light)
Second
reflection
First
reflection
Incident light
Varnish with
matting agent
particles
Substrate
(paint, wood,
etc.)
Figure 12.10 Second order internal reflections occur
when light is scattered within a varnish layer. Matteness
is increased as light is scattered by particulate matting
agent
be avoided due to its propensity to darken and
crosslink.
Many varnishes may be made up by the conservator.
Resin can be placed in a muslin bag,
suspended in solvent and agitated occasionally
until most or all of the resin has dissolved. This
process may be speeded up by use of a magnetic
mixer. Undissolved resin should be discarded
before the varnish is strained, e.g.
through a double layer of stockings or finely
woven silk, to remove detritus. Two to three
thin even layers of varnish may be required to
achieve even saturation and gloss.
Reactions that contribute to the degradation
of natural and synthetic varnishes occur faster
in the liquid varnish formulations that in solid
resins (de la Rie and McGlinchey, 1989). This
effect may be visible, for example, in unused
preparations that have yellowed in storage.
Coating mixtures used by the conservator
should therefore be mixed immediately prior to
Air