Series editors' preface - Wood Tools

use whenever possible and many conservators
dispose of such solutions after one month. The
increased potential for degradation of liquid
formulations has implications for pre-mixed
proprietary products.
12.4.4 Matting down varnishes
Gloss may be reduced by the addition of matting
agents. Matting agents reduce gloss by
increasing the proportion of diffusely reflected
light from the surface (see Figure 12.7). The
two main mechanisms are roughening the surface
(Figure. 12.9) and/or increasing second
order internal reflection of light within the varnish
layer (Figure 12.10).
Amorphous fumed silica (silica dioxide) is
used as a matting agent by many conservators.
Some fumed silicas are used in industry as
thickening agents (thixotropes) but agglomerates
must be well dispersed if they are to function
in this way (see section 13.9). Hydrophobic
and hydrophilic grades of fumed silica are produced
to match the polarity of the polymer in
which they will be mixed. As a general rule (but
with a few notable exceptions), well dispersed
fumed silica does not have a matting effect.
However, the inclusion of any type of poorly
dispersed particles in a varnish film will
increase second-order internal reflections, scattering
light as it passes through the varnish,
thus producing a matte surface. Excessive
poorly dispersed fumed silica can cause a pebbled
appearance.
Precipitated silica is used in matting agents
such as Acematt ® (Degussa-Hüls). The proportion
of silica required varies depending on the
degree of matteness required and the type of
silica used. Acematt ® TS 100, for example, can
significantly reduce gloss at around 4–8% concentration
in some coatings, whilst Acematt ®
OK 412 may require 8–12% to achieve the same
effect (Figure 12.11). The silica is added just
before the varnish is to be applied, and dispersed
using a high speed stirrer
(2000–2500 rpm) for about 15 minutes to break
down particle agglomerates. Alternatively, silica
powder can be added to a small amount of varnish
and worked with a palette knife. This will
break up large agglomerates and ensure the
particles are wetted by the varnish. When the
particles are fully dispersed, the mixture can be
added to the bulk varnish solution. As the
Principles of consolidation, aesthetic reintegration and coatings 591
Figure 12.11 Graphical representation of the
relationship between the degree of gloss and the
concentration of two types of matting agents (Degussa-
Hüls) in a modern acid cured coating. Gloss decreases
as more matting agent is added to the varnish
(a) (b) (c)
Figure 12.12 Diagrammatic representation of the effect
of film thickness on gloss, assuming the same
concentration of matting agent. On application, matting
agent particles are uniformly distributed in the varnish
film (a). The thickness of the coating decreases as
solvent evaporates (indicated by the arrow). A thicker
varnish layer (b) results in a glossier surface because
less matting agent is concentrated at the surface. The
higher concentration of matting agent particles at the
surface of a thin varnish layer (c) produces greater
scattering of light and reduced gloss
solvent evaporates from the varnish film, the
silica particles move to the surface, roughening
it and scattering light. The thinner the coating,
the more matting agent is concentrated at the
surface, the more light is scattered and the
more matte the surface will appear (Figure
12.12). Since it is usually easier to apply very
thin layers using spray equipment rather than
by brush application, it is not surprising that