(8) Equal quantities of flowers of sulphur and bichromate of potash are thoroughly mixed, and heated to redness in a crucible, producing a mixture of oxide of chromium with sulphide and sulphate of potash. The latter are dissolved out by washing repeatedly with hot water, leaving the chromium oxide as a finely comminuted dense powder of an intense green colour.
(9) A modification of (8) consists in adding small successive quantities of flowers of sulphur to a boiling concentrated solution of potash bichromate. From this results a gelatinous oxide of chromium, which is washed with boiling water, dried, and calcined in a crucible at a red heat.
(10) Hydrochloric acid decomposes bichromate of potash, forming a soluble chloride of potash which can be removed by washing, and a residue of chromium oxide, which is washed on a filter and dried.
There remain for description two or three processes in which phosphoric acid plays a part, but the greens made by these methods do not possess the freshness of the others, and it is difficult to see what advantages can attend this modification.
(11) According to Arnaudon, 149 parts of bichromate of potash are thoroughly incorporated with 128 parts of crystallised neutral phosphate of ammonia, and the mixture is heated in thin layers to a temperature between 338° and 356° F., which brings about intumescence, change of colour, and disengagement of water and ammonia; the heating is continued for half an hour, but must not be allowed to exceed 392° F. When the development of the green colour is complete, the product is washed with hot water to remove soluble salts, and the residue constitutes an impalpable powder of chromium oxide, forming a leaf-green pigment.
(12) Dissolve 10 lb. of bichromate of potash and 18 lb. of phosphate of soda in boiling water, and add to the boiling mixture 10 lb. of thio-sulphate of soda solution and a little hydrochloric acid. A precipitate of phosphate of chromium is gradually thrown down as the boiling is maintained.
For general utility no class of pigments can exceed the several forms of Guignet’s green. It is capable of affording a great variety of tints, all absolutely permanent under reasonable conditions. No ordinary agent will decompose them, and they will stand almost any test to which they may be subjected without losing colour. They are quite insoluble in acids and alkalies, and are not affected even by the extreme heat of the glass furnace. They possess good covering power, do not suffer in brightness or purity under artificial light, and are equally useful as oil or water colours, besides being admirably adapted for fresco and silicious painting, and employed in making green glass and in calico printing. They can be mixed with any other pigment. Adulteration with Brunswick or Prussian greens is often practised, but may be discovered by a portion being dissolved on boiling with caustic soda, the solution giving a precipitate of chrome yellow on adding acetic acid, and (a separate portion, of course) Prussian blue with hydrochloric acid and perchloride of iron.
Lokao.—This pigment, which is also known as “Chinese green,” was first met with as a sediment left after dyeing cotton cloths with the barks of one or more species of buckthorn, notably Rhamnus chlorophorus and R. utilis, and passing in China under the general name of Lo-Kao. This sediment is spread on blotting paper and thus dried, forming thin cakes. Latterly, the juice afforded by the berries of the same trees is extracted by pressure, absorbed by alum, and dried in the same form of little cakes. When first introduced into England it was highly valued as affording a pure green, even in artificial light. Its price on the London market in 1861 was 7s. 6d. an ounce. So long ago as 1853 it was imported into France and used for dyeing silk. The colouring principle appears to consist of a glucose (lokaose) and an acid (lokaonic acid). In 1864, Chauvin obtained an identical colouring matter from Rhamnus catharticus, or the common buckthorn, a shrub which grows wild in most parts of Europe, and found a ready market for the pigment at 37s. a pound. This was simply the article known as sap green ([see p. 132.])
Malachite.—This is one of the names applied to mountain green (q.v.).
Manganese Green.—Several formulæ have been published for making a green pigment from manganese, as follows:—