Series editors' preface - Wood Tools

230 Conservation of Furniture
like). Shape affects the way particles pack
together and therefore affects hiding power.
Rod shaped particles can act like reinforcing
bars in concrete but can also project through
the surface thereby reducing gloss but also
providing a mechanical key for subsequent
paint layers. Lamella particles, as found in
aluminium and mica pigments, can overlap
one another in paint films and confer
increased water resistance on the paint film.
The tinting strength of pigments is a measure
of the amount of a coloured pigment required
to achieve a particular strength of colour. It is
typically measured as the strength of colour
achieved by one part of coloured pigment in
twenty parts of pure zinc oxide. The tinting
strength of a pigment is independent of its
hiding power and even quite transparent
pigments can have high tinting strengths.
5.8.3 Dyes
Dyes are organic molecules containing
functional groups that absorb particular
wavelengths of light giving rise to perceived
colour (see section 5.8.1). The particle sizes of
dyes are often on a molecular level allowing
them to interpenetrate and stick to, or in some
cases chemically react with, a variety of
substrates. Their aggregates on fibres are too
small to scatter light and therefore there are
no white dyes. Unlike most pigments, most
dyes can be prepared in solution. There is,
however, some ambiguity of terminology in
that some pigments, for example indigo and
Prussian blue, are used as dyes. Some
dyestuffs can also be made into pigments of
larger particle size by precipitating them out
of solution so that they are incorporated into
an inorganic crystalline material such as
aluminium hydroxide. These dye pigments are
generically called lakes. Lakes are bright
colours with low covering power and were
especially useful in lightly pigmented paint
layers or glazes. Lakes have a reputation for
being highly fugitive and in may cases have
only survived unchanged where protected
from light. However, very high quality alizarin
crimsons have been produced since the 1920s
that remain permanent under conditions that
cause older or inferior varieties to fail.
Dyes may be animal, vegetable or mineral
in origin. Vegetable dyes are cheap and
ubiquitous and may be extracted from leaves,
roots, seeds, flowers and from woods that are
particularly rich in coloured extractives such as
Brazilwood (Caesalpinia braziliensis and
related spp.), logwood (Haematoxylon campechianum)
and Sanders wood (Pterocarpus
santalinus). Dyes of animal origin include lac,
kermes and cochineal (carmine) from insects
and Tyrian purple extracted from shellfish of
the genus Murex. Mineral dyes and stains
include iron browns and blacks, and
blue–green copper acetate (verdigris).
The age of synthetic dyes began with the
synthesis of Perkin’s Mauve by William Perkin
in 1856. Many more colours were synthesized
from a chemical derived from coal-tar (aniline)
before the end of the century. Today there are
many thousands of synthetic dyes available,
some of which, such as CIBA Orasol dyes and
BASF Basantol dyes, show excellent stability.
The use of dyes is as ancient as the use of
pigments and many dyes have been sold as
stains for wood. Dyes are far less stable than
pigments, however, and tend to fade in colour
or even disappear in extreme cases. Dyes have
been used to colour all porous substrates, and
penetrate well due to their small size. They
bond to the substrates by various mechanisms
including hydrogen bonding (e.g. direct dyes),
non polar forces, due to matching shapes of
dye and substrate molecules, ionic forces, and
covalent bonding (e.g. reactive dyes).
Most natural dyes are dependent on a
mordant (from the Latin word ‘to bite’) to help
fix them to the substrate. Traditional mordants
were compounds of iron, copper, tin, chrome
and aluminium such as alum (potassium
aluminium sulphate) and copperas (ferrous
sulphate). Tin and chromium salts yield particularly
bright colours but are relatively modern,
their use dating from about 1630 and 1850
respectively. Many single dye sources will
yield a variety of colours depending on the
mordant. Madder root for example will yield
orange with tin, maroon with chrome, yellow
with copper and brown with iron. Further
information on dyes is given in Chapter 3 and
in the bibliography to that chapter.
5.8.4 Stains
The terms stain and staining are used to
describe any sort of material and process used