COLOURS: THEIR COMPOSITION AND PERMANENCE
One of the characteristics of the mediæval artist was that he had a
good knowledge of the different materials that he employed in his work.
One cannot help being struck by this fact when reading some of their
writings that have come down to us. They seem to have known all the
various properties of the different colours and materials that they
used. It is probable that the chief reason for this was that it was
absolutely necessary for them to be able to prepare the materials for
use, as it was practically impossible to buy them ready prepared.
The artist of to-day can buy so many things ready prepared for him by
the artists’ colourman that he is very liable to give little or no
attention to their composition and quality. It is as well, however,
that the artist should have some knowledge of the materials that he
uses, so that he may be able to select the best for his purpose.
It is important that he should understand something, at any rate,
about the composition of the various colours that he uses, and that he
should be able to distinguish permanent colours from those notoriously
fugitive. Of course, it must not be understood by this that it is
necessary for the artist to subject all his colours to chemical
analysis; if he buys his colours from any reliable artists’ colourman
he can rely on the colours being true to their name.
There are, however, some students who have not the slightest idea of
the character of the different colours that they are in the habit of
using. It is a great pity when good work is completely spoiled by being
executed with fugitive colours through the ignorance of the artist.
Good work should be as lasting as it is possible to have it, and this
is not practicable if one is not certain whether the colours are likely
to fade or change in any way.
The following notes as to the composition and permanence of the
different colours may be of service to the student.
YELLOW PIGMENTS
Aureolin, Cadmium Yellows, Chrome Yellows, Gamboge, Raw Sienna, Yellow
Ochre, Naples Yellow.
Of these colours, Aureolin is the most expensive, but it is a very
beautiful colour, and has the advantage of being permanent. It is
sometimes called Cobalt Yellow; it is prepared from cobalt and
potassium nitrites.
The Cadmium Yellows are sulphides of cadmium, and are in various shades
from pale yellow to orange; they are also permanent.
Chrome Yellows are all chromates of lead, and they darken very quickly
in an impure atmosphere, especially when used in water-colour
painting. These colours are very cheap, and, at the same time, very
brilliant, but they should be avoided in all cases where permanence
is desirable. They also produce serious changes when mixed with other
colours; for example, a green made by mixing chrome yellow with
prussian or antwerp blue is notoriously fugitive in character.
Gamboge is a gum-resin from the East Indian tree _Garcimia Cambogia_.
It forms a bright opaque yellow solution with water, requiring no
grinding or mixing in any way, owing to its natural gum. It is
fairly permanent and works well in water-colour, but is not quite so
satisfactory in body-colour painting.
Raw Sienna and Yellow Ochre are both natural earths containing iron
oxide. They are quite permanent.
Naples Yellow is generally prepared by mixing Cadmium Yellow with Zinc
White.
RED PIGMENTS
Vermilion, Rose Madder, Scarlet Madder, Alizarin Crimson, Crimson Lake,
Carmine, Indian Red, Light Red, Burnt Sienna.
Vermilion is one of the most important colours for the illuminator. It
is prepared from sulphide of mercury. There are various kinds, which
are termed Orange Vermilion, Scarlet Vermilion, Vermilion Extract,
Vermilion, etc. Probably that known simply as Vermilion is most useful
to the illuminator.
Most vermilions are manufactured in this country, but that known as
Chinese Vermilion is imported from China.
Although vermilion is indispensable to the illuminator, it cannot be
guaranteed as being absolutely permanent. Cennino Cennini, writing
about this pigment, says: “But remember that vermilion is not durable
when exposed to the air; it is more lasting on pictures than on walls,
because, by long exposure to the air, it becomes black when applied to
walls.”
It is a fact that vermilion does change in this way, but it is
debatable whether it is affected by the air. It is thought by some
that the action of the sun’s rays is a more likely cause of change.
There are two varieties of mercuric sulphide, the red and the black;
unfortunately the red is liable to change into the black. No chemical
change is necessary for this, as they are both identical from a
chemical standpoint. In the diffused light of a room this colour seems
to be quite permanent, but it is liable to turn black suddenly when
exposed to direct sunlight. There is conclusive proof that vermilion,
when not placed so that the rays of the sun come into direct contact,
is quite permanent by the fact that in the illuminated MSS. produced
centuries ago the vermilion is still bright.
Most authorities are of the opinion that genuine Chinese vermilion is
more permanent than the English variety. There are two methods employed
at the present day in the manufacture of this colour--the wet and the
dry process. The dry method is used in China and the other in Europe.
The wet method is more economical, but it is generally admitted that
vermilion produced by the dry process is more permanent than the other
kind. Unfortunately the Chinese vermilion that is obtainable in this
country is often rather poor in quality.
Rose Madder and Scarlet Madder are lakes prepared from the madder root.
These are very beautiful colours and are permanent under ordinary
conditions. They should not, however, be exposed to direct sunlight.
Alizarin Crimson is a permanent crimson with a coal-tar origin.
Crimson Lake and Carmine are lakes prepared from cochineal; they are
quite fugitive and should not be employed for serious work.
Indian Red is a variety of iron oxide and is permanent.
Light Red and Burnt Sienna are prepared by burning Yellow Ochre and Raw
Sienna; they are both quite permanent.
BLUE PIGMENTS
Cobalt Blue, Cerulean Blue, Ultramarine, Ultramarine Ash, French
Ultramarine, New Blue, Prussian Blue, Antwerp Blue, Cobalt Violet,
Purple Madder.
Cobalt Blue is a fine colour prepared from cobalt oxide and alumina.
This is quite permanent under ordinary conditions.
Cerulean Blue, made from cobalt and tin oxides, is also a permanent
colour.
Genuine Ultramarine is a beautiful permanent colour obtained by
grinding the _lapis lazuli_. Cennino Cennini, in his treatise, gives
interesting particulars concerning the method then used to prepare
this colour. Ultramarine Ash is a second quality of this same blue.
Unfortunately the great cost of genuine ultramarine debars the
majority of artists from using this colour. Happily, under the name
of French Ultramarine, it is now made synthetically. By this means a
good permanent colour is produced at a cheap rate. New Blue is a pale
variety of French Ultramarine.
Prussian Blue is ferrocyanide of iron. Antwerp Blue is a weaker variety
of the same colour containing alumina. These colours are not altogether
reliable, as they are subject to change.
Cobalt Violet is a purple colour made from cobalt, and is quite
permanent.
Purple Madder is usually prepared from the madder root, and is
permanent under ordinary conditions. Sometimes, however, it is prepared
from Crimson Lake, in which case it is fugitive.
GREEN PIGMENTS
Chromium Oxide, Viridian, Emerald Green.
Chromium Oxide is, as its name implies, an oxide of chromium; this
is an opaque variety. Viridian is also an oxide of chromium, but is
transparent. Both these are quite permanent.
Emerald Green is aceto-arsenite of copper, and a somewhat dangerous
colour to use; it is darkened by impure air, but this is not so serious
as are its effects upon other colours when mixed with them. If used at
all, it should be used quite by itself, as if it is mixed with other
colours it is sure to have a bad effect on them. It turns some colours
black very quickly.
BROWN PIGMENTS
Raw Umber, Burnt Umber, Sepia, Vandyke Brown.
Raw Umber is a natural earth, containing oxide of manganese; it is
quite permanent. Burnt Umber is the same colour burnt, by which it
becomes darker and richer in colour.
Sepia is generally prepared from the ink of the cuttle-fish, although
occasionally a natural earth is substituted. Vandyke Brown is also a
natural earth; both these colours are permanent.
BLACK PIGMENTS
Ivory Black, Lamp Black, Indian Ink.
Ivory Black is made from ivory and bone charred to blackness.
Lamp Black is a smoke-black, being a finely divided soot formed by the
incomplete combustion of hydrocarbons.
Genuine Indian Ink does not come from India, as might be supposed, but
from China. There are, however, several other varieties in liquid form.
It is generally admitted that carbon-black forms the bulk of all these.
All these black pigments are quite permanent.
WHITE PIGMENTS
Zinc White, Flake White.
Zinc, or Chinese, White is an oxide of zinc. This pigment is quite
permanent and should be used always when a lasting white is needed.
Unfortunately this white does not photograph its true value, so for
process work it is best not to use this.
Flake White is prepared from lead carbonate and hydrate, and may be
used for process work in place of zinc white. It, however, should not
be used for work that is required to be lasting, as when used as a
water-colour it soon turns black. As, however, it has more body than
zinc white, it photographs better. A white is sometimes prepared for
process work from barium sulphate. This is permanent, but does not work
quite so well from the brush.