PURPLE OF CASSIUS.
Purple of Cassius, or gold purple, is not a pigment for painters’ use;
it is employed for colouring glass, for porcelain painting and for
coloured glazes. For these last purposes we know nothing which can
completely replace purple of Cassius, so that its preparation will be
described here. Its costliness prevents its general use.
As regards the chemical constitution of purple of Cassius very diverse
views are held. Chemists have not yet succeeded in explaining it;
some regard it as aurous stannate, others as stannic acid, in which a
particular form of gold of a red colour is contained in an extremely
finely divided condition. There are reasonable grounds for both
opinions. Theoretical considerations are of little interest to the
manufacturer, who is chiefly interested in the method by which he can
prepare a product which satisfies the demands of the consumer.
Purple of Cassius has been known for nearly two hundred years,
hence there are many formulas for its preparation. Before these are
enumerated it is necessary to give an account of the conditions under
which this substance is formed. In order to obtain gold purple,
a solution is required which contains both stannous and stannic
chlorides; if this solution is mixed with a very dilute solution of
gold chloride a precipitate is obtained which is generally brownish
red, and only acquires its fine red shade on igniting. The colour
shown by the purple after precipitation is no guide to the shade of
the finished product; a purple which shows a very fine colour whilst
wet often produces a pigment of much less beauty than an actually
ugly precipitate. These differences are to be ascribed to the varying
molecular condition of the purple, for the different varieties show
very small differences in chemical composition.
According to the directions of Fuchs, a fine purple is obtained by
mixing a solution of stannous chloride with sufficient ferric chloride
solution to give the mixture a green colour. Part of the ferric salt is
then reduced to ferrous salt, whilst the stannous chloride is partially
oxidised. At the same time a solution of gold chloride is made; this
must be quite free from nitric acid and contain 1 part of the salt in
400 to 500 parts of water. The tin solution is added drop by drop to
the gold solution with constant stirring; the mixture becomes turbid,
but the precipitate requires a long time to settle on account of its
fine state of division. The depth of colour shown by the purple varies
according to the strength of the solutions.
M. Müller states that the following process gives the best results: The
quantity of stannic chloride equivalent to 9 grammes of stannic acid
is dissolved in 200 cubic centimetres of water; potassium carbonate
is added to alkaline reaction, then 1 gramme of gold in the form of
chloride. Grape sugar is added to the mixture, which is made up to 300
cubic centimetres by water, and warmed until the brightest shade is
reached. When a very gelatinous mass is formed after the addition of
the potassium carbonate it is heated for a short time before adding the
gold solution and grape sugar.
Wächter gives directions for obtaining pale and deep shades. To obtain
the pale purple 5 grammes of tin are dissolved in _aqua regia_, the
solution evaporated to dryness on the water bath, the solid residue
mixed with a solution of 2 grammes of stannous chloride and dissolved
in 10 litres of water. To this liquid is added a solution of 0·5
gramme of gold in _aqua regia_, and immediately 50 grammes of ammonia
to neutralise free acid. The precipitate separates spontaneously from
the dark red liquid; its formation may be hastened by adding sulphuric
acid. The washed precipitate is mixed whilst moist by means of a
silver spatula with 20 grammes of lead flux, 2 grammes of red lead, 1
gramme of quartz sand, and 1 gramme of calcined borax. The mixture is
then dried; it produces a purple red colour. If 3 grammes of silver
carbonate are added, a clean, pale purple pigment results.
The deep purple is made by mixing a solution of 0·5 gramme of gold
in 10 litres of water with 7·5 grammes of stannous chloride solution
(specific gravity, 1·7) whilst stirring, and adding a few drops of
sulphuric acid. The washed precipitate is mixed with 10 grammes of lead
flux and 0·5 gramme of silver carbonate.
A rose-red shade of gold purple is obtained by dissolving 1 gramme
of gold and mixing the solution simultaneously with solutions of 50
grammes of alum in 20 litres of water, and of 1·5 gramme of stannous
chloride (specific gravity, 1·7). Ammonia is now added until all the
alumina is precipitated. In order to prepare from the dried precipitate
the mixture which will produce the colour on fusion, it is mixed with
50 grammes of lead flux and 2·5 grammes of silver carbonate.
=Magnesia Gold Purple.=—According to M. Müller, the colour of gold
purple is produced by covering the particles of a very finely divided
white substance with metallic gold. Thus, when calcined magnesia is
stirred up with water, gold chloride solution added, and the mixture
warmed to 100° C., the gold is precipitated upon the magnesia; the wet,
yellow powder acquires a reddish shade on drying, which at a red heat
becomes so beautiful a carmine-red that it surpasses purple of Cassius
in fineness of shade. A purple which contains 20 grammes of gold as
chloride to 84 grammes of magnesia has the most pure carmine tint; the
shade varies with the proportion of gold to magnesia. The different
shades of purple contain the following percentages of gold:—
_Percentage of Gold._ _Shade of the Purple._
33·5 Brownish-red (excess of gold).
25·0 Deep carmine-red.
20·0 Medium carmine-red.
10·0 Pale carmine.
5·0 A good rose-red.
3·0 Rose-red.
1·0 Pale rose.
0·2 Delicate rose.
0·1 Appreciable red tint.
=Alumina Gold Purple= is obtained, according to Müller, by adding
gold chloride and excess of potassium carbonate to alum solution and
boiling. The purple obtained in this way is as deep in colour with 10
per cent. of gold as a magnesia purple with 20 per cent. of gold, but
has a more bluish violet shade. If, in the preparation of purple, the
alumina is precipitated by potassium carbonate and the gold chloride
reduced by grape sugar, a different shade of purple is obtained to that
in the precipitation of which ammonia is used. The alumina gold purple
is specially adapted for the production of porcelain enamels.