Series editors' preface - Wood Tools

Plastics and polymers, coatings and binding media, adhesives and consolidants 135
Consolidants when present have usually been
added by a restorer or conservator. A wide
range of materials has been used to fulfil these
functions, with synthetic materials having been
added more recently to the range of naturally
occurring materials used historically. Both
groups are chemically complex, mostly
organic, polymers but natural materials tend to
be much more variable in composition and
properties than their synthetic counterparts.
These substances possess qualities of cohesiveness
and adhesiveness and share a range of
other properties, discussed below. In practice,
the same substance was often used for different
functions. An adhesive substance found to
be useful in joining wood is also likely to
adhere when applied in a thin layer as a coating,
to bind pigment particles together or
incorporate dyes and stick them to a surface as
a binding or paint medium. Therefore, while it
is convenient to discuss these categories separately,
it is important to realize that there is
considerable overlap between them.
To be useful, materials used as coatings,
binding media, adhesives and consolidants
need to be liquid at some point in their application
and then to solidify. There are four basic
mechanisms by which this occurs. Setting can
occur through change in temperature alone
(e.g. waxes), or through change of temperature
and loss of solvent together (e.g. animal glues).
It can also occur through loss of solvent or
liquid phase without change in temperature
(e.g. modern synthetic thermoplastics). Lastly,
setting can occur by chemical reaction (e.g.
thermosetting polymers, urushi, linseed oil).
Chemical reaction may involve loss of a small
molecule, such as water (e.g. urea formaldehyde)
or may proceed without loss of volatile
matter (epoxies, polyesters). Sometimes more
than one setting mechanism may exist for a
given chemical type. The way in which a
material sets plays a large part in the way it can
be used. Elimination products cause contraction.
Epoxies, because they do not produce
elimination products do not contract appreciably
on setting.
A basic (ideal) requirement for all coatings,
media, adhesives and consolidants is that they
should be stable, fully compatible with the
object material with which they are in contact
(that is there should be no adverse chemical or
physical interaction) and that they should
remain so. They should be durable under their
intended service conditions and should not discolour,
degrade, or crosslink. For conservation
materials added to objects these are important
considerations that govern our ability to undo
treatments and to retreat at a later date, for
example in the case of applying a coating to a
paint layer. Materials must be safe to use and
have the desired working properties to facilitate
handling and application. Sensitivity of
these materials to solvents remains important
after their application.
4.3 Coatings – functions and
properties
The general term coating is used here for any
fluid organic material used to provide a continuous
coat or cover on furniture and woodwork.
It includes both clear or lightly coloured
coatings commonly called varnishes and pigmented
coatings called paints. Coatings may be
applied to protect the surface of an object but
are often of the utmost importance in themselves
since it is the outer film or finish that
visually represents an object to the viewer.
Coatings are often encountered as systems of
surface decoration (e.g. gilding, lacquer) rather
than single materials. Therefore, a discussion of
the general nature, function and properties of
coatings is followed by an account of the structure
of some common types of surface decoration
and the preparations making up the layers
found in them. This discussion covers primarily
those coatings intended for interior use.
In summary, there are several properties of
materials that are generically important to their
function as coatings. Surface coatings are
required to provide protection against handling
and soiling and against damage caused by dust
and atmospheric pollution. They should also
impede the passage of water vapour, oxygen
and prevent certain wavelengths of electromagnetic
radiation. The coating must adhere
well to the surface it is to protect. The cohesion
and elasticity of surface coatings should
allow for all ordinary changes in humidity and
temperature of the air and preserve the elasticity
of paint films under them. The coating
should add to, rather than detract from, the
appearance of the object and its colour, clarity,
gloss and freedom from defects, such as bloom