4. Classification of pigments
Pigments may
be classified according to colour, use, permanence, etc. It is
customary, however, to classify them according to origin, as follows:
A. Inorganic
(mineral)
1. Native earths: ochre, raw umber, etc.
2. Calcined native earths: burnt umber, burnt sienna, etc.
3. Inorganic synthetic colours: cadmium yellow, zinc oxide, etc.
B. Organic
1. Vegetable: gamboge, indigo, madder, etc.
2. Animal: cochineal, Indian yellow, etc.
3. Synthetic organic pigments.
General Characteristics of These Groups.
Inorganic synthetic colours made with the aid of strong heat are
generally of the greatest permanence for all uses, while those requiring
delicate or very accurately balanced processing are less so.
The synthetic counterparts of the red and yellow earths are more
brilliant and, if well prepared, superior in all other respects to the
native products. In general, pigments derived from natural sources are
less permanent than the average synthetic colour. The synthetic organic
pigments are characterized by a great brilliance and intensity. Many of
them are remarkably permanent, but others, particularly the older ones,
are fugitive and have the defect of bleeding in oils. Many require the
addition of inert bases during manufacture.
The native earths
used as pigments occur all over the world, but there is always some
special locality where each is found in superlative form or where
conditions have been established which permit its being purified to a
greater or more uniform extent than is economically possible elsewhere.
Substitutes for French ochre, Italian sienna, etc., are offered for
reasons other than the purpose of supplying the best available products.
The natural
impurities in some red earths are of such a character as to be harmful;
therefore the synthetic red oxides are preferred to them. The impurities
or noncolouring constituents of the highest grades of ochre and the
other permanent earth colours seldom present the same disadvantages.
The synthetically
prepared colours of American makes are equal if not superior to any
others. Cobalt yellow, cerulean blue, cobalt violet, cobalt green, and
Naples yellow are made in very limited amounts almost entirely for
artists' use and therefore for economic reasons are not produced in this
country. European books still describe defects in pigments that have
long since been overcome by American manufacturers.
5. The term 'Lake'
A lake is a
pigment made by precipitating or fixing a dye upon an inert pigment or
lake base. The process may be compared to that of dyeing cloth, and a
high degree of skill is required to produce good results. Lakes are made
in a great range of hues and strengths.
A toner is an organic pigment in its most concentrated form, containing
no inert pigment; for satisfactory performance in artists' colours it
often requires added inert pigment to contribute bulk to the paint and
to decrease excessive tinctorial power.
Alumina hydrate is
the usual base for clear, transparent lakes such as are used as glazing
colours, in printing inks, etc.; while blanc fixe is the best base for
those to be used in heavy paints and for similar purposes where more
body or opacity is required. Cheaper lakes, less clear in tone, are made
on clay, barytes. etc. Green earth is valuable as a base for green
lakes, as it is a species of clay which has a strong power of absorption
for dyes.
A lake is also
occasionally made with a coloured pigment as a base; for example, Tuscan
red, a pigment made for industrial use, is made with alizarin on an
Indian red base.
The dyestuffs used are synthetic products, although a few of the older
extracts of vegetable and animal origin still survive for some special
purposes - usually only because of their low cost, for lakes made from
modern organic colours are greatly superior in every paint requirement
to those obtained from the natural colouring extracts.
Prior to the eighteenth century, lake usually meant red lake only. The
term comes from the Indian lac. Scum or sediment from the dyers' vats,
called lacca and consisting of dyed particles of shreds, fibers, dust,
and other impurities, was collected and used as a pigment in Italy in
early times.
6.
Listing
of pigments and their origins
Alphabetical
listing and origins of pigments.
<<
page 1
