Series editors' preface - Wood Tools

colours are ideal for under-saturated base coats,
and can be glazed with a resinous medium to
adjust colour or gloss. They may be used to
retouch a (non-water soluble) unvarnished
matte surface.
Water colours are available in pan and tube
form. Pan water colours have a comparatively
high glycerine content, which makes them
more sensitive to changes in RH and may
impair their adhesion to a varnished surface
(Caley, 1997). Water colours are notorious for
changing hue when varnished and require
either wetting out with white spirit or the use
of a thin saturating varnish layer. It is possible
for conservators to make their own slightly
glossier water colours using fresh gum arabic,
which can give better saturation. Producing a
water colour with the right body, viscosity and
gloss when dry may require some experimentation.
The organic binding medium used in
water colours is hygroscopic and there is the
potential for fungal growth in conditions of
high RH. Aged water colour retouchings often
have a white or cloudy appearance when the
varnish lying over them is very thin and has not
prevented contact with moisture or pollutants.
Tempera Egg tempera, utilizing egg yolk as
the binding medium, has been used as a
retouching medium (Kempski, 2000; Lank,
1990). Whilst freshly applied tempera can
remain soft for some time, aged tempera forms
a very durable paint layer. Potential problems
with reversibility mean that it must be isolated
from the original surface. Tempera is particularly
useful for smooth opaque and bodied base
coats, over which resinous glazing can be
applied. Whilst this effect can also be achieved
with synthetics, it is more easily executed in
tempera.
Making paint tablets
Although hand mixing of paint does not
achieve the same consistency of wetting as
industrial milling, conservators often formulate
their own paint using a range of pigments,
binding media and diluents. The role of these
constituents in paint is considered in section
4.4.3. Whichever binder is selected, it is essential
to ensure that pigment particles are as thoroughly
wetted in medium as possible.
Traditionally, a muller and slab were used to
grind pigment into a binding medium thinned
Principles of consolidation, aesthetic reintegration and coatings 585
with diluent (e.g. linseed oil thinned in turpentine).
The paste thus produced was mixed with
the paint vehicle (medium and diluent) and
was then ready for use. This ensured both that
pigment was evenly ground and that each particle
was thoroughly wetted by the medium.
Whilst muller and slab are rarely employed for
the preparation of small amounts of paint, the
essential principle of ensuring that pigment
powder and medium are thoroughly mixed
remains. Using an artist’s spatula and a firm surface
such as a tile, many conservators work the
dry pigment, binding medium and diluent thoroughly
together to ensure the best possible dispersion
and wetting of pigment by binding
medium. It is important to ensure that binding
medium and diluent are also well mixed to
ensure that pigment has not been wetted only
by solvent. Paint films characterized by welldispersed
pigment will appear more saturated
and are usually more physically stable than
those in which pigment is poorly dispersed.
Whilst some conservators opt to mix pigment
and binding medium on the palette, others prefer
to use paint in tablet form. There are advantages
and disadvantages for each method.
Mixing paint on the palette can be faster for
more extensive areas of loss, as it is easier to
make a larger amount of evenly coloured paint.
Preparation on the palette may produce a better
dispersion of pigment in medium because
the paint is ‘worked’ more thoroughly. Mixing
paint on a palette is messier than using tablets
and requires a bigger palette to accommodate
larger paint ‘puddles’. Tablets are less wasteful
of paint and are less messy in use. They are
harder on brushes and it is comparatively easy
to cross-contaminate colours, particularly
whites.
Paint tablets can be made by the conservator
using most of the binding media described
above in the section on materials for retouching.
Dry resin is weighed, placed in a muslin
bag and suspended in solvent to prevent the
formation of a slow dissolving semi-solid mass
at the bottom of the container. Dissolution will
be fastest with the use of a magnetic mixer. In
the absence of a mixer, the solution should be
stirred periodically until the resin has dissolved.
The proportion of resin to solvent is dependent
on the molecular weight of the resin and the
intended final use. Low molecular weight resins
are often used at a 20–50% w/v concentration