INDIGO CARMINE.
Indigo carmine is used in the laundry, for blue inks and as an artists’
pigment; in chemical composition it is a mixture of the sodium salts
of the two indigotin sulphonic acids described in the last chapter.
In working on the large scale it is particularly important that the
indigo should be finely powdered and completely free from moisture. It
is a laborious process to powder indigo in mortars, and loss always
occurs through the formation of dust. Thus when large quantities of
indigo are to be powdered it is advisable to use rotating cylinders.
These are strong casks bound with iron hoops and provided with a
well-fitting slide, through which the indigo and a number of iron balls
are introduced. When these casks are rotated for a sufficient length of
time the indigo is converted into the finest powder without any loss.
=Indigo Mills.=—Special mills have been designed for powdering indigo.
They may be advantageously employed for other materials also. The
construction of a well-designed colour mill is represented in Fig. 36,
in which indigo may be very finely powdered without loss. It consists
of a cast-iron pan, which can be closed by a well-fitting cover.
Through the cover passes the axis of the rotating part, which is moved
by the bevel cog-wheels and the handle. Above the cog-wheel fastened
to the axis is a cross piece carrying heavy balls at the ends, which
serve to keep the motion regular. In the interior of the pan four
vertical rods are attached to the axis; these are placed at such a
distance that they drive before them the heavy balls in the circular
depression of the pan. The action of this apparatus is very simple.
After the introduction of the indigo through an opening in the cover
the axis is put in rotation, the vertical rods attached to the axis
roll the balls before them, and thus break up the lumps of indigo. The
larger pieces of indigo oppose considerable resistance to the rotation,
which should at first be slow. When the lumps have been broken down the
speed of rotation can be considerably increased. After the operation
has been continued for a sufficient length of time the indigo is
converted into an impalpable powder.
[Illustration: FIG. 36.]
To facilitate the removal of the ground indigo, an arm carrying a brush
of fine hair should be attached to the horizontal arm of the axis.
If this brush is placed so that it sweeps the bottom of the circular
depression in which the balls move, all the indigo powder in this
depression will be carried in one direction. In the depression is an
opening closed by a slide during the grinding. The brush expels the
ground indigo through this opening when the axis is slowly rotated;
it is collected in a vessel below. The larger and heavier are the
balls in this mill, the more rapidly will a given quantity of indigo
be powdered, and the greater will be the power required to drive the
machine. By means of a pulley on the axis of the cog-wheel the mill can
be driven by mechanical power.
The powdered indigo is exposed for a long time to a temperature of 120°
C., to completely dry it. For this purpose it is spread out on sheets
of tin plate in a thin layer and heated so long as a test portion
decreases in weight. The powder whilst still warm is at once mixed with
the sulphuric acid; 4·5 kilogrammes of fuming sulphuric acid and 1
kilogramme of ordinary sulphuric acid (66° Bé) are used to 1 kilogramme
of indigo. Heat is developed in the operation, and in order to prevent
the indigo from being charred the vessel in which the mixture is made
is placed in a larger vessel filled with cold water. The mixture is
stirred with a glass spatula until it is quite uniform. The vessel
is then maintained at a temperature of about 50° C. (not more than
60° C.) for 7 to 8 days. After the lapse of this time the indigo is
completely dissolved, and the vessel is found to contain two distinct
layers, the upper fluid and the lower a paste. The contents are mixed
with cold water, about 10 kilogrammes of water to 1 kilogramme of
indigo. A solution of 10 kilogrammes of common salt is then added in
small quantities at a time. Indigo carmine is insoluble in strong salt
solutions; it separates as a deep blue precipitate, which is allowed
to settle and then filtered. If it were attempted to wash out the
remainder of the salt solution retained by the carmine, a considerable
quantity of the latter would dissolve as soon as the salt solution
became diluted to a certain extent. To avoid such loss the precipitate
is allowed to drain completely on the filter, and then spread out upon
bricks, which absorb the water, so that the carmine forms a stiff
paste.
Indigo carmine, from which the salt has not been completely washed out,
dries completely when exposed to the air. This may be prevented by the
addition of glycerine, which is a very hygroscopic substance, and so
keeps the carmine always moist, thus preventing the crystallisation of
the salt.
Indigo carmine easily dissolves in water to a deep blue liquid. It
has tremendous colouring power, in which respect it is only equalled
by genuine cochineal carmine. As mentioned above, indigo carmine is
extensively employed in the manufacture of ink. The so-called alizarin
inks show a very pale colour as they flow from the pen, and only become
deep black after some time. To make these inks equally visible at the
first indigo carmine is added, by which they acquire a handsome blue
colour.
Indigo carmine may also be made by digesting 1 part of dry powdered
indigo for 24 to 36 hours with 4 parts of strong sulphuric acid,
diluting with water, quickly filtering through a linen cloth, and
adding 4 parts of common salt. The precipitate is then collected and
dried on gypsum or clay plates.
=Blue Lake.=—When alum is added to a solution of indigo sulphonic acid,
and then soda solution, a blue precipitate is formed, which, when dry,
resembles Chinese blue in appearance. It is used in painting. This
indigo blue lake is superior to other blue lakes by reason of its
fastness to light. It is rather dear, and is mixed with starch, so that
it may be sold at low prices; the mixture is formed into slabs and sold
as ”new blue,” ”indigo extract,” etc. It is chiefly used in the laundry.
Under various names a number of blue pigments are sold consisting of
ordinary qualities of indigo mixed with Prussian blue, smalts, chalk,
etc. These are used in the laundry, and also for distempering.