SAP COLOURS.
Under the designation of ”sap colours” several pigments are brought
into the market in the condition in which they are ready for immediate
use in printing. The term is practically restricted to lake pigments
which form a transparent layer when dry, thus the sap colours may be
defined as dissolved lakes mixed in a viscous medium, such as thick
gum solution. In general the sap colours are not much used, since when
wetted with water they again dissolve, which is not the case with
pigments ground in oil. Yet for certain purposes they are commonly
used, as in the manufacture of playing cards.
In the manufacture of these pigments only colouring matters soluble
in water can be used, the number of which is restricted. According to
a particular method a tin lake is first made, which is decomposed by
a strong base, such as ammonia, so that the colouring matter again
goes into solution. The very deep coloured liquid is then mixed with a
thickener and an indifferent white substance such as flour or starch,
formed into thin sticks, and sent into the market.
A decoction of buckthorn berries is used for the yellow sap colours.
It is considerably evaporated, and then mixed with 2 to 3 per cent. of
alum, after which starch paste or gum Arabic and sugar are added in
proper quantity, and the mixture evaporated at a low temperature so
that it is not browned by over-heating.
When a tin lake is employed in the preparation of a sap colour, the
precipitate obtained by adding stannic chloride to the solution of
the colouring matter is washed, and, without drying, treated with a
small quantity of strong ammonia; generally about 10 per cent. of
the volume of the precipitate is sufficient to dissolve the lake. It
is always better to leave a portion of the precipitate undissolved
than to modify the colour by the addition of too much ammonia. The
precipitate should be dissolved in a glass vessel; the liquid is well
stirred, covered over, and allowed to stand until the undissolved
portion of the precipitate has settled. The ammoniacal solution of the
colouring matter cannot be concentrated by evaporation. Thus, before
the precipitate is dissolved it must be well freed from water by long
draining. To give the solution the proper consistency about an equal
quantity of thick gum solution is added, and then sufficient starch
to form a paste which can be rolled into thin sticks, which are then
dried upon boards at a gentle heat. Sap colours are made in exactly the
same way from any other colouring matter adapted to the purpose. In
using these colours it is simply necessary to bring them into water;
if they contain only gum and sugar in addition to colouring matter
they completely dissolve, but if they contain starch the solution is
incomplete. As a rule starch is only added to give the colour the
consistency requisite for printing.
Red sap colours can be made either from red wood lake or from cochineal
carmine; the former is used in ammoniacal solution, the latter as a
solution of the pure carmine in ammonia. The colour is prepared from
red wood by allowing a decoction to stand for several days, adding 2
to 3 per cent. of alum, evaporating and thickening with gum solution.
This colour is cheap, but cannot compare in beauty with that obtained
by treating red wood tin lake with ammonia. The solution of genuine
carmine in ammonia produces a sap colour which leaves nothing to be
desired in regard to shade, but on account of the cost of carmine
it can but rarely be used. A solution of pure indigo carmine mixed
with gum solution is used as a blue sap colour; it can be shaded by
additions of red or yellow colours. Chinese blue may be used instead
of indigo carmine; it dissolves in a solution of oxalic acid. When
Chinese blue, which has been thoroughly washed, is mixed whilst still
moist with a little saturated oxalic acid solution, the blue dissolves,
and the solution may then be made into a paste by the addition of
thickening materials.
Green sap colours are made, as a rule, by mixing yellow and blue. All
shades can be thus obtained. A green colour of different composition
is obtained by boiling the violet solution of chrome alum with gum
solution until the colour changes to green. Violet colours are obtained
by mixing red and blue. Sepia produces a brown sap colour.
It has been proposed to use glucose in place of the much dearer gum
Arabic, and also to use malt extract as thickener. These substances
absorb water from the air, and thus the colours prepared with them
never become quite hard, but always remain pasty. For some purposes
this is a decided advantage, since such colours can be readily taken up
by the brush and easily rubbed up with water, but they cannot be formed
into cakes as is usually done with water colours. When the colours are
to be made into cakes they must be thickened with gum Arabic alone,
without glucose or malt extract. The cakes must be dried with great
care; if they are dried too quickly they will crack or even fall to
pieces.