MADDER LAKES.
The madder lakes are equally as permanent as the madder colouring
matters dyed upon fabrics. On this account and because of their
handsome colour they are highly prized by painters. The best qualities
of madder are necessary to produce fine lakes; it is more convenient
to use garancin or madder extract. Only when the colouring matter is
tolerably free from the foreign substances which accompany it can
a pure red madder lake be produced; the poorer qualities of madder
produce a lake which is not a pure red and is never bright. The good
repute of the madder lakes of certain makers is in great part due
to careful choice of the raw materials. This choice and care in the
process are the secrets of the manufacturers who produce good madder
lakes.
The author has found that the finest lakes can be obtained without
difficulty from the alizarin, separated in the before-mentioned manner
from madder by means of sulphuric acid. The small cost of obtaining
crude alizarin by this process enables it to be used on a commercial
scale. The labour involved in the process is far outweighed by the
excellent qualities of the lakes produced.
A very bright lake is obtained directly from madder by treating with
sulphuric acid (thus garancin is really used), digesting the mass
for several hours with a solution of alum absolutely free from iron,
filtering and adding at first a small quantity of soda solution; a
little madder lake is precipitated which is of the finest shade.
When this lake has settled the liquid is poured off and again mixed
with a little soda solution. The lake now precipitated is inferior
to the first, if the madder is not of specially fine quality. By
this fractional precipitation of the colouring matter, madder lakes
varying in beauty are produced; the separate fractions often differ
considerably in shade, this is because the later portions are more and
more contaminated by the foreign substances contained in the madder.
From inferior madder lake, which could only be sold at a low price,
a lake of the best quality can be produced in the following manner:
The lake is finely powdered, mixed with acetic acid and left to stand
twenty-four hours; the madder lake dissolves in the acid to a fine red
solution, whilst the impurities remain on the filter when the solution
is filtered. The clear solution is mixed with a large volume of water
free from lime, and the acetic acid neutralised by soda, when pure
madder lake separates. The addition of soda is not continued until all
the lake is precipitated, but only until the liquid is still slightly
red. Hydrochloric acid may be used instead of acetic; it is much
cheaper, but must be absolutely free from iron.
It has already been stated that the crude alizarin prepared as
described above is well suited as a source of madder lake, since it is
free from the majority of the substances which injure the purity of
the shade. It is treated with a boiling solution of alum and filtered
whilst hot; madder lake is then precipitated by cautious additions of
soda, the first portions precipitated being the best.
=Madder Carmine= is not often found in commerce; it consists of the
almost pure lakes of alizarin and purpurin. The process by which
this valuable pigment is produced is based upon the great stability
of the madder colouring matters. The following is the process on the
large scale: Good madder, very finely ground, is spread out in small
heaps in a room, the temperature of which is about 16° to 18° C.; the
heaps are moistened with water and left for several days. The mass
ferments and gives off a peculiar smell; in the fermentation not only
is the glucoside decomposed, but also many of the other compounds
contained in the madder, consequently the mass becomes dark and
considerably diminishes in weight. The end of the fermentation process
may be recognised by a little practice with tolerable certainty by
the disappearance of the peculiar smell; the mass is broken up and
transferred to a lead-lined vessel, in which it is mixed with three or
four times its weight of ordinary sulphuric acid. The acid is allowed
to act for several hours, the plant fibres are almost completely
charred, and the mixture becomes black. When the charred residue has
settled, the liquid is filtered through pure quartz sand or powdered
glass, and mixed with a large quantity of water. The carmine, which is
insoluble in water, separates as a red powder; it is then washed and
dried.
The colour of madder carmine is of such beauty that it can only be
compared with that of good cochineal carmine, but it is incomparably
more permanent than the latter and may be used in all varieties of
painting.