Series editors' preface - Wood Tools

a substance exists as a hybrid of two different
structures (e.g. malachite green).
Besides the simple absorption of light of a
particular wavelength, the colour of a dye or
pigment depends on several other factors
including the width and profile (steepness) of
the absorption band, the probability that light
of the correct wavelength will actually be
absorbed (the molar extinction coefficient), the
change in refractive index with wavelength
and the wavelength(s) at which maximum
absorption occurs. The importance of these
factors is further discussed by McLaren (1983).
5.8.2 Pigments
In contrast to dyes, true pigments are insoluble,
have comparatively large particle sizes
and are generally bound in a medium to
create paints. Surface coating pigments may be
required to fulfil a variety of functions among
which providing colour is the most obvious
but not the only requirement. The ability of a
paint to obscure previous colours may also be
important as may the effect pigments can have
on the strength, durability and adhesion of
paint films. Pigments may also provide
increased protection against corrosion of
metals, reduce gloss and modify the flow and
application properties of paints.
The use of pigments began in remote prehistory
with the exploitation of various brightly
coloured natural materials from animal,
vegetable and mineral sources. Coloured
material obtained from flowers, seeds, nuts,
berries, bark, roots and wood were mostly
impermanent but madder and woad are more
durable examples (Goodwin, 1965). Yellow,
red and brown coloured earths and clays and
carbon blacks from various sources are also
known from the early archaeological sites.
Coloured minerals of heavy metals such as
malachite, azurite, cinnabar, lapis lazuli,
orpiment and realgar were known and used,
at least in the areas where they naturally
occurred, from very early times. The history of
pigments continues through the introduction
of various semi-synthetic and synthetic
colorants. The beginning of the development
of modern synthetic pigments dates to the
early years of the eighteenth century but many
synthetic pigments were in fact known before
this time, including Egyptian blue, artificial red
Other materials and structures 221
and yellow oxides of lead, lead white (basic
lead carbonate) and verdigris (basic copper
acetate).
Pigments embrace a wide variety of different
chemical types with consequent wide
variation in properties. They include inorganic,
organic and metallo-organic compounds, and
organic/inorganic composites. In some cases
they are pure homogeneous materials in other
cases, mixtures of different materials and
composite materials. Among the inorganic
pigments are found oxides, carbonates,
chromates, phosphates, silicates and sulphides,
of heavy metals and transition metals. The
properties of pigments that affect their performance
and ability to function in the ways
discussed above depend on chemical reactivity
and a variety of physical properties including
refractive index, particle size, particle
morphology, surface energy, density and
thermal stability.
Chemical properties
There are several ways in which the chemical
properties of pigments govern their performance,
including solubility, reactions with each
other and with paint media, colour fastness,
stability and toxicity. Pigments are generally
insoluble but not all pigments are equally
insoluble in all media. In some cases, a later
application of paint may pick up the colour of
an earlier one. Organic reds are particularly
prone to this fault of bleeding and white
applied over a cured red paint film may turn
pink on, or shortly after, application. Pigments
may be acidic or basic and may therefore react
with other basic or acidic materials including
media or other pigments in mixtures. They
may affect the formulation by modifying pH
in emulsions or by the ease with which they
can be wetted out. Zinc oxide, for example,
is amphoteric, that is it can act as an acid or
a base. As a base it reacts with resins containing
a high proportion of acidic groups to form
soaps. Because zinc is divalent this leads to
cross linking of the resin and an increase in
the viscosity of the paint in storage. Oxides of
heavy metals in general form sufficiently
strong bases to cause saponification of free
fatty acids in oil media to form soaps. This is
believed to be one of the reasons why oxides
of lead in oil form such compact, elastic and
durable paint films. Release of chromates on