THE COLOURING MATTER OF LAC.
From the punctures of the insect _Coccus lacca_ on certain East Indian
trees, especially those belonging to the genus _Ficus_, flow at the
same time resin and colouring matter in such quantities that the
insects are frequently enclosed and large red masses are formed on the
trees. The insects live in some measure in these masses, the females
lay their eggs in the spaces and the larvæ are said to feed on the red
sap contained in the mass. When the larvæ have left the incrustation,
generally in November, it is broken off from the branches and
collected. It is now known as stick-lac; it consists of large masses
of resin of a fine deep red colour. In the interior the cells of the
insects may still be perceived. On chewing, it becomes soft and colours
the saliva a deep violet. When stick-lac is boiled in water a portion
of the colouring matter dissolves. Good stick-lac contains 10 per cent.
of colouring matter and 80 per cent. of resin.
Seed-lac has the same origin as stick-lac, but frequently the greater
part of the colouring matter has been extracted; it rarely contains
more than 2·5 per cent., and is consequently of little value for
colouring purposes.
=Lac Dye.=—The colouring matter of stick-lac is generally separated
from the resin in India and comes into the market under the name of
lac dye. It is made by stirring coarsely powdered stick-lac in large
vessels for several hours with warm water. Almost the whole of the
colouring matter dissolves, whilst the resin remains as a ruby red mass
which is melted and brought into the market under the name of shellac.
The solution of the colouring matter is evaporated in shallow vessels
in the sun or boiled down in pans; the residue is made into cakes. At
present soda solution is used instead of pure water to extract the
colouring matter; a larger yield is obtained. Lac dye usually contains
45 to 50 per cent. of colouring matter, 25 per cent. of resin, and, in
addition, earthy substances which are to be regarded as intentional
admixtures.
In the preparation of lac dye by Stephen’s method coarsely powdered
stick-lac is boiled with soda, and the remainder of the colouring
matter contained in the resin extracted by repeated boiling with
water. All the extracts are then united and precipitated by alum. The
colouring matter separates as a lake which still contains a large
quantity of resin.
According to the patented process of Henley, the colouring matter of
seed-lac is extracted by hydraulic presses. The lac is filled into
press bags which are placed between iron boxes heated by steam. On
applying pressure the melted resin, shellac, comes through the bags,
whilst the colouring matter remains behind. If this method were
employed on the large scale, presses similar to those used in stearin
candle works would be suitable.
When lac dye is treated with hydrochloric acid the colouring matter
dissolves; the solution can be used to dye wool, which it colours a
beautiful red. The colouring matter is very similar in appearance to
that of cochineal, but has the appearance of much greater fastness. It
appears also to be similar in composition to carminic acid, but little
is known as to its chemical constitution.
When lac dye is to be used for painting it must be freed from resin;
this is accomplished by treating the finely powdered mass for a long
time with boiling alcohol and separating the solution of resin from
the undissolved colouring matter, which is then dried. In this way a
lake is produced known commercially as Vienna red and little inferior
to carmine lake. Unfortunately the price of lac dye, although it is
imported in large quantities from India to England, is so high that it
can only be used for artists’ colours.