OTHER GREEN CHROMIUM PIGMENTS.
=Guignet’s Green= is a chromium oxide made in the dry way; it is
obtained by grinding 1 part of potassium bichromate with 3 parts of
pure boric acid and water, drying and heating to a dark red heat
with access of air. The hot mass is brought into water; by boiling
repeatedly with water it is freed from the boric acid which it
obstinately retains. In order to remove the last portions of boric acid
it is necessary to boil with sulphuric acid and then with caustic soda;
this treatment may be omitted for technical purposes, since the small
quantities of boric acid contained in the pigment are not harmful.
Guignet’s green is distinguished by the great resistance it offers to
chemical reagents; it is largely used in painting and calico printing.
Pale shades are obtained by additions of barytes.
=Emerald Green.=—This pigment must not be confounded with the more
important copper compound previously described, to which the name
emerald green is usually applied. It consists of chromium hydroxide
obtained by precipitating a solution of the green modification of a
chromium salt by zinc hydroxide. After careful washing it has a dull
green shade; it is a very durable pigment.
=Chrome Green Lake= is a mixture of chromium oxide with alumina; it
is obtained by precipitating a solution of alum and a chromium salt
by a soda solution. The precipitate, which contains aluminium and
chromium hydroxides, on ignition takes a paler shade in proportion to
the quantity of alumina it contains. For the chromium salt a solution
of potassium bichromate, which has been allowed to stand with sulphuric
acid and alcohol until if has acquired a pure green colour, may be used.
=Turkish Green.=—This pigment, which retains its fine green colour in
artificial light, is prepared in a peculiar manner: 40 parts of alumina
free from iron, 30 parts of cobalt carbonate and 20 parts of chromium
oxide are thoroughly ground in a mortar. The mixture is placed in a
porcelain tube, which is exposed to a strong white heat whilst pure
oxygen is led through. The oxygen may be replaced by air if it is
previously heated and under pressure.
Another recipe for Turkish green is to intimately mix 4 parts of
freshly precipitated alumina with 3 parts of cobalt carbonate and
2 parts of chromium oxide, heat the mixture in a crucible at a
white heat, powder and levigate. Turkish green has a characteristic
bluish-green colour; the shade is inclined to blue or green by
increasing the quantity of cobalt carbonate or chromium oxide.
=Leaf Green= is a pale green, very durable pigment similar to chrome
green lake. It is obtained by igniting mixtures of chromium oxide with
pure aluminium hydrate; the paleness of the shade is in proportion to
the amount of alumina.
=Chromium Phosphate Pigments.=—Several chromium phosphates are used as
pigments; the more important are described here.
=Arnaudan’s Green= is chromium metaphosphate; it is obtained by
intimately mixing 128 parts of neutral ammonium phosphate with 149
parts of potassium bichromate by long grinding, and carefully heating
the mixture at 170° to 180° C., but not higher, until the mass is pure
green. It is then brought into hot water and thoroughly washed; it has
a very handsome green colour, which is not altered in artificial light.
Ammonium arsenate may be used in the place of ammonium phosphate; a
handsomer green is then obtained, which is, however, very poisonous.
=Plessy’s Green= is essentially a chromium phosphate mixed with
variable amounts of chromium oxide and calcium phosphate. It is
obtained by boiling a solution of 1 part of potassium bichromate in 10
parts of water with 3 parts of a solution of acid calcium phosphate and
1 part of sugar, until the whole has become deep green. The chromic
acid is reduced by the sugar, so that the precipitate contains chromium
phosphate, chromium oxide and neutral calcium phosphate.
These two chromium phosphate greens are very stable towards chemical
reagents, and very durable towards atmospheric influences.
=Schnitzer’s Green.=—Thirty-six parts of sodium phosphate are melted
in its water of crystallisation, 15 parts of potassium bichromate
and 14 parts of Rochelle salt are then added. A large dish must be
used for the fusion, since the mass effervesces. The colour changes
gradually from yellow to green. When it is pure green the heating is
stopped, and, after cooling somewhat, as much hydrochloric acid is
added as the mass will absorb; then, after standing some time, it is
washed with cold and finally with boiling water. On account of their
great durability, the chromium phosphate pigments are well adapted for
paperhangings, calico printing and oil painting.
=Chromaventurine= is a glass coloured green by chromium oxide. It is
hardly used as a painters’ colour in the ordinary sense of the term;
it is applied in porcelain painting as an under-glaze colour, and is
largely used for colouring glass.
Chromaventurine is most simply made by the process of Pelouze: 250
parts of quartz sand, 100 parts of soda, 50 parts of calcium carbonate
and 40 parts of potassium bichromate are fused together. The presence
of iron would have a very harmful effect on the shade of the product.
Quartz sand generally contains a small quantity of ferric oxide, which
should be extracted by treatment with strong hydrochloric acid when a
product of the best colour is required. The calcium carbonate should
also, as far as possible, be free from ferric oxide.
=Chrome Blue= (Garnier).—A mixture of 48·62 parts of potassium chromate
with 65 parts of fluorspar and 157 parts of silica is fused in a
crucible lined with coal-dust.