VIOLET MINERAL PIGMENTS.
=Chromic Chloride.=—The violet modification of chromic chloride is
a compound which is little used, but deserves the highest attention
on account of its durability and beauty of shade. Up to the present
chromic chloride has been almost exclusively used for colouring paper
hangings under the name of chrome bronze, which it has received in
consequence of its property of imparting a peculiar metallic lustre
to paper upon which it is rubbed. It is also possible to fix this
substance upon fabrics, which thus acquire a similar metallic lustre.
Pure chromic chloride forms beautiful peach-blossom coloured scales,
which can only be obtained by treating chromium oxide in a certain
manner with chlorine. The pure substance is practically insoluble in
water, but if the water contains but a trace of chromous chloride
the chromic chloride readily dissolves to a green solution. In the
preparation of this compound it is necessary to avoid the least trace
of chromous chloride, otherwise the product will begin to change when
it comes into contact with moist air.
Pure chromic chloride is obtained by proceeding according to the method
first given by Wöhler: the simple apparatus necessary is depicted in
Fig. 29. Pure chromic oxide is first made by any suitable process;
this is made into a paste with charcoal, starch paste and water, and
formed into small balls which are heated in a crucible to a white heat.
The residual intimate mixture of chromic oxide and carbon is loosely
placed in a crucible which stands in a furnace; a porcelain tube is
cemented into the bottom of the crucible, it passes through the ashpit
to connect with a chlorine apparatus. Upon the crucible another larger
one is placed, which has a small opening in the bottom and serves as
a receiver for the sublimed chromic chloride. The heating and current
of chlorine are started at the same time, the fire is regulated so
that the lower crucible glows brightly. This is continued for an hour,
during which a rapid current of chlorine must pass into the crucible;
the fire is then removed and a slow current of chlorine passed through
the apparatus until it is quite cold.
[Illustration: FIG. 29.]
If the operation has succeeded the lower crucible is found empty and
the interior of the upper covered with the magnificent peach-blossom
crystals of chromic chloride. A small quantity of the chromic chloride
is tested as to its behaviour towards water; if it remains unaltered
the whole may be washed, but if it dissolves to a green solution
it must be again ignited in a current of chlorine. The washing is
necessary to remove small quantities of aluminium chloride formed by
the action of chlorine on the clay of the crucible.
=Manganese Violet, or Nuremberg Violet=, consists of manganic
phosphate. It is obtained by fusing glacial phosphoric acid with pure
pyrolusite, boiling the melt with ammonium carbonate, filtering the
solution, evaporating to dryness, and again fusing the residue. After
boiling with water this forms a handsome violet powder; it is suitable
for use as a very durable artists’ colour.
Shades of this pigment inclining to blue are obtained when a ferric
compound is added in the first fusion; the more iron ore is used the
more intense is the blue tone.
=Tin Violet or Mineral Lake= is obtained as a violet mass by igniting
an intimate mixture of 100 parts of tin dioxide with 2 parts of chromic
oxide. This pigment is completely permanent in air; it is suitable for
printing paper hangings and for colouring faience.
=Copper Violet, Guyard’s Violet.=—According to J. Depierre, this
pigment is obtained by precipitating a solution of copper ammonium
sulphate with potassium ferrocyanide, washing and drying the
precipitate and heating in a porcelain dish. At 170° C. cyanogen and
ammonia are evolved, the mass takes up oxygen and becomes violet; it
then resists the action of dilute acids and alkalis, and as a pigment
has great covering power. On heating to 200° C. it turns blue, and at
240° to 250° C. becomes greenish.