Series editors' preface - Wood Tools

Plastics and polymers, coatings and binding media, adhesives and consolidants 141
formation temperature (MFFT) below which
the droplets do not flow together and therefore
do not form a satisfactory film. The MFFT is
important in relation to working properties,
hardness and susceptibility to creep (especially
important where emulsions are employed as
adhesives). Emulsions can be prepared with
varying sizes of dispersed particles. Small particles
give good penetration and binding of
pigment particles whereas large particles tend
to remain more on the surface. This can be an
advantage where consolidation which minimizes
darkening of a chalky friable surface is
required. The observations made about coatings
in general apply equally to films formed
from emulsions.
Before using emulsions as conservation coatings
it is wise to investigate their composition
since they are prepared by commercial emulsion
polymerization processes and may contain
undesirable additives. For an emulsion to form
a satisfactory film it is necessary that the Tg of
the polymer should lie some degrees below
the ambient temperature. One way to achieve
this is by use of a fugitive plasticizer in the formulation.
Such plasticizer is slowly lost by
evaporation in the final stages of drying causing
Tg to rise and improving hardness. The
nature of such plasticizers and other materials
such as dispersing and coalescing agents
should be known before use. Once dry, these
films cannot normally be re-dispersed in water
but require a polar organic solvent for removal.
The nature of solvents needed for removal
should be known before the material is
applied. Emulsions are more thoroughly
treated in the section on consolidation in
Chapter 12.
Each time a varnish is removed there is a risk
of solvent action on the underlying paint,
including leaching, swelling and partial dissolution
of the paint film. All coatings change
chemically as they age, to an extent that is
largely predictable based on the known chemistry
of the coating. It is therefore important to
consider carefully at the time of application of
a coating, the likely requirements for its
removal and the effect this will have on the
paint. A resin that is prone to chain scission, or
the breaking apart of the molecule into smaller
units, will cause the resin to loose some of its
cohesive strength. A polymer that crosslinks as
the coating cures and ages becomes insoluble
making it difficult to remove. Polymers that are
chemically stable, undergoing minimal chain
scission or crosslinking over time, are the most
likely to be long-lived coatings. It is easier to
predict the behaviour of a pure substance than
of a formulation or mixture containing several
materials.
4.4 Coatings – structures and
preparations
The basic elements of decoration considered
from a structural viewpoint are the support, the
ground, the ‘paint’ or other decorative layer,
and the varnish. The relationship of these layers
is shown diagrammatically in Figure 4.3.
Each component in a decorative scheme has a
particular function to fulfil. The desirability of
properties varies with different components
but adhesion to adjacent layers, internal cohesion,
hardness, brittleness, strength, sensitivity
to light, temperature and relative humidity,
gold leaf
bole
(a)
gesso
binding medium
void
pigment
particles
(b) (c)
gesso
varnish
paint
gesso
wood
substrate
Figure 4.3 (a) Diagrammatic representation of the
layer structure of a painted and gilded surface,
consisting of wood substrate and gesso ground, with
paint layer/s and gilding (bole and gold leaf) covered
by a varnish layer. (b) Underbound or ‘lean’ paint often
appears matte. It has minimal binding medium that is
often only barely sufficient to hold pigment in place.
(c) Well-bound, medium rich or ‘fat’ paint usually
appears glossy. The pigment particles are fully wetted
by the medium