Series editors' preface - Wood Tools

and those that must withstand use. Surfaces that
must resist wear and tear require harder,
tougher coatings. As a general rule coatings
with a glass transition temperature (Tg) above
the ambient temperature do not imbibe dirt
(see Plate 3) and are harder but more brittle
than coatings with a lower Tg. This hardness is
the basis for the use of furniture varnishes
based on sandarac and shellac (see section
13.7.2). However, ‘hard’ coatings tend to dry
with substantial shrinkage stresses, and thus the
furniture conservator may need to balance the
effect of such stresses on an underlying decorative
surface (see Figure 8.6) against the need
for a harder wearing coating (Whitmore et al.,
1999). Mastic and dammar are not usually
durable enough for use on furniture that has a
functional as well as decorative role, although
they may have been added as a secondary component
to furniture varnish formulations.
Photochemical degradation is the main agent
of deterioration of varnishes and causes
changes in optical, physical and chemical properties.
Optical changes include yellowing,
increase in opacity and loss of gloss. Varnishes
that tend to yellow more quickly contribute to
a cleaning cycle of frequent varnish removal
and reapplication and thus increase the risk of
damage to the decorative surface from leaching
and abrasion. Physical changes include an
increase in brittleness and the development of
craquelure, crazing or crocodiling (see section
13.8). Chemical changes include crosslinking or
a shift toward more polar solubility parameters
(oxidation), particularly through the formation
of carboxylic acid groups (de la Rie, 1988b).
The use of polar solvent blends to remove discoloured
varnish often has the potential to
damage the decorative surface underneath (see
Solvents under section 16.6.3).
The application of varnish can leach paint
films in a similar manner to that which occurs
during the removal of varnish with solvents.
The leaching effect of a varnish solution on an
oil paint film is dependent on the polarity of
both solvent and resin components of the varnish.
The leaching of oil paint films by varnish
solutions is discussed in more detail in section
16.6.7.
The chemistry of natural resins is discussed
by Mills and White (1977), their degradation by
de la Rie (1988b). Samet (1998b) considers varnish
and coatings for paintings in considerable
Principles of consolidation, aesthetic reintegration and coatings 587
(a)
(b)
(c)
Figure 12.7 Gloss and matteness
(a) Glossy surface (e.g. a newly French polished
surface). Specular reflection of light occurs when the
angle of the reflected light is the same as the angle of
incident light. The greater the degree of specular
reflection, the glossier a surface appears
(b) Matte surface (e.g. underbound paint with exposed
pigment particles). Diffuse reflection of light occurs
when light is randomly scattered from an irregular
surface. The greater the degree of diffuse reflection, the
more matte the surface will appear
(c) Semi-matte surface (e.g. an aged varnish). Semimatte
surfaces combine both specular and diffuse
reflection of light
depth. The principles of adhesion outlined in
Chapter 4 also apply to the application of coatings.
The chemical properties of many of the
materials discussed below are considered in
Chapter 4 and their deterioration is discussed in
Chapter 8.
12.4.2 Saturation and gloss
Saturation and gloss are inter-related optical
properties of a surface. Saturation describes the
degree of intensity or vividness of a colour
(Mayer, 1991). A glossy surface has a high
degree of specular (mirror-like) reflection of
incident light, whilst a matte surface has a high
degree of diffuse reflection of light (Figure
12.7).
The molecular weight, refractive index and
gloss of a varnish will affect the saturation of a
painted or decorated surface below. Saturation
also depends on the ability of a varnish to wet
a surface thoroughly, i.e. to displace the air on