(1) Fuse together 3 parts boracic acid and 1 potash bichromate at a dull-red heat on the hearth of a flame-furnace. This forms a borate of chromium and potash, with evolution of oxygen. The mass is repeatedly washed with boiling water, which causes decomposition and consequent separation of hydrated chromium oxide and a soluble potash borate. The oxide is washed and ground very fine.
(2) In boiling a solution of a chromium salt slightly acidified, to which has been added an alkaline phosphate and sodium acetate, the whole of the chromium is precipitated as phosphate. This method succeeds with the green and violet salts, chlorides, sulphates, and acetates, but not with oxalates. It is also suitable for alkaline chromates, but in this case the action of the phosphoric acid must be combined with that of sodium thiosulphate (hyposulphite), which acts as a reducing agent. The solution of chromate, to which is added a sufficient quantity of phosphoric acid or a phosphate, then of acetate, and lastly of hypophosphite, and which has been slightly acidified, is boiled for about an hour; it deposits all the chromium as phosphate, with a little sulphur derived from the hyposulphite. The phosphate precipitated is a green hydrate. It may be washed with boiling water, or, preferably, with hot solutions, first of ammonium acetate followed by ammonium nitrate. On calcination it turns grey, and contains chromic oxide in the proportion of 51.86 per cent. Alumina can be readily separated from chromium by converting the latter into an alkaline chromate, acidifying the solution slightly with acetic acid, and adding excess of sodium phosphate.
The mixture is boiled and filtered, to separate the aluminium phosphate. When this is done, it is easy to determine the chromium by pouring into the liquid hyposulphite, and, if needful, a further quantity of alkaline phosphate, and boiling. The precipitate of chromium phosphate is then washed, ignited, and weighed. It yields an insoluble green colouring matter, which retains when dry a very fine shade, and may be used in painting in place of the dangerous compounds of arsenic and copper. This colour, which is perfectly inoffensive, may also be used in dyeing, as the insoluble green phosphate may be 'produced in the fibre. (Carnot.)
Cobalt-green is obtained by calcination of a mixture of oxides of zinc and cobalt. The first step is to prepare cobalt protoxide free from foreign metals. It is dissolved in 3 parts hydrochloric acid, and the solution is evaporated to dryness. The residue is dissolved again in 6 parts water, and a stream of sulphuretted hydrogen is passed through the liquor as long as precipitation takes place. The clear liquor, decanted from the sulphides of the foreign metals, is again evaporated to dryness, and the residue is dissolved in enough water to make 10 parts. This liquor is precipitated with soda carbonate, and if, after washing, the still wet precipitate of carbonate of protoxide of cobalt be mixed with zinc white, there is produced a reddish-violet magma, which, dried and calcined, constitutes a green mass, the colour of which is more intense in proportion as the cobalt solution has been greater.