WATER COLOURS.
The colours thus called are so prepared that they easily mix with water
to such a condition that they can be applied with the brush. Special
pigments are not required for this purpose, ordinary dry colours are
simply mixed with a binding medium soluble in water, and the paste is
generally pressed into cakes. In commerce very different qualities
of water colours are found, ranging from cheap children’s playthings
to the most costly colours used by painters in water colour. Those
pigments which are not already in very fine powder, in consequence
of the process by which they are made, such as chrome yellow, must
be subjected to a very careful process of levigation. To simplify as
far as possible this laborious process, it should be conducted by
grinding the pigment as finely as possible, well stirring it in a tub
with water, allowing to rest for several minutes, and then running off
the liquid into a second tub, from which it is again drawn off into a
third after several, minutes. The liquid remains in the last vessel
until it is quite clear, when the deposit is collected. This is now so
fine a powder that a powerful microscope is required to distinguish
the separate particles. The residue in the first and second vessels
consists of the coarser particles; it is again ground with fresh
material. In this process regard must be given to the specific gravity
of the material: the higher it is the shorter is the time during
which the liquid is allowed to remain in the tubs, since the coarser
particles of heavy substances settle very rapidly. In dealing with the
light lakes, such as the alumina lakes, the liquid in which the lake
is suspended must remain much longer at rest. Two vessels may be used
instead of three, without danger of the precipitate containing coarse
particles.
Gum Arabic and tragacanth are used as binding materials for the
pigments. Dextrine is also much used in place of the expensive gums;
for this purpose only pure white dextrine should be used, since the
brown colour of ordinary dextrine would injure the shade of the
colours, especially of pale colours. Gum Arabic and dextrine, which
are readily soluble in water, require no special preparation before
their solutions are mixed with the pigments. The solutions are made by
treating with water; they are then allowed to stand for several days in
a tall vessel so that impurities may settle. If the solutions are very
turbid, they are filtered through closely woven linen.
Gum tragacanth requires rather different treatment. It is not
completely soluble in water, in which it only swells up to a great
extent. It is prepared by leaving it for several days in water, and
when it has swollen, rubbing the slimy mass in a mortar until it is
completely uniform.
The levigated pigments are allowed to dry in the air to a soft paste,
which is mixed with the proper quantity of gum Arabic and tragacanth
solutions. As a rule the two gums are used together. The colouring
matter and binding medium may simply be ground together, but long
grinding would be required to produce a completely homogeneous mixture.
The costly manual labour is therefore as far as possible replaced by
machinery, by which means a cheaper and also more uniform product is
obtained. The machines by which the pigment is ground with the binding
material are of simple construction. They consist of rollers placed in
pairs one above the other and moving in opposite directions. The two
rollers of each pair are connected by cog-wheels, in such a way that
the lower moves more slowly than the upper. In consequence of this
arrangement, the rollers, in addition to crushing, exert a grinding
action upon the viscous mass passing between them. As a rule, after the
colour has passed through one pair of rollers, it goes through a second
and third, from the last of which it is taken off by a scraper. It is
now completely uniform.
The binding medium must be of such consistency that, after grinding
with the pigment, a fairly stiff paste results which is suitable for
pressing. The cakes of colour are pressed out in an ordinary spindle
press, which should be so constructed that the stamp in the down stroke
comes upon the paste beneath it and stamps out cakes upon which the
engraving of the die is clearly shown. If the cakes crack when slowly
dried, the medium contains too much gum Arabic. If the impression
of the die upon the cakes is not sharp, and if they remain rather
elastic when completely dry, too much tragacanth has been used. It is
rather difficult to grind a stiff paste, many manufacturers prefer to
grind the colour in a rather more fluid condition, and then to allow
the paste to thicken by drying to the consistency necessary for the
production of good cakes.
The moulds in which the cakes are made must be very carefully worked in
metal, so that the description and trade mark print clearly upon the
cakes. The cakes should be dried at the ordinary temperature or but
little higher. They are placed upon smooth boards, and care is taken
that the temperature of the drying room remains uniform, which is the
condition requisite for the production of the fewest cracks. When it is
intended to place on the market only faultless and well-stamped cakes
they must be sorted when dry, and the cracked ones rejected; these can
be worked up in the next operation.
The dry cakes are given a good appearance by coating them with a weak
solution of gum, and then drying. According to the price at which these
colours are to be sold the cakes are given a different character. The
finest colours are generally made into larger cakes and packed in
handsome boxes, whilst ordinary cheap colours are made into small lumps
or circular plates, flat on one side and somewhat convex on the other,
and packed in boxes of soft wood.
=Moist Water Colours.=—Instead of grinding water colours with gum
Arabic or tragacanth and bringing them on the market in the dry state,
they may also be sold in a condition resembling that of oil paints.
This may be accomplished by using very viscid glucose syrup instead
of gum Arabic, and grinding the pigments in this exactly as in oil.
Glucose is very hygroscopic, the colours prepared with it remain moist,
and may be spread out upon the palette like oil colours. It is then
only necessary to wet the brush in water and mix the mass with it in
order to obtain colour of the proper consistency.
Moist water colours have also been known as ”honey colours,” since this
mixture of sugars was formerly used for their preparation; it also is
hygroscopic. Honey is no longer used; the much cheaper glucose answers
the same purpose. On account of their semi-fluid nature, moist water
colours are put up in tubes just as oil colours. They are thus rather
dear, but are little used.