BLUES. Cobalt Blue.—A mixture of 8-10 parts alumina, freshly precipitated and freed from water, and 1 part arseniate or phosphate of cobalt, slowly dried, and heated to dull redness.
Paris Blue.—(a) A thorough mixture of 2 parts sulphur and 1 part dry carbonate of soda is gradually heated in a covered crucible to redness or till fused ; a mixture of silicate of soda and aluminate of soda is then sprinkled in, and the heat is continued for an hour ; the little free sulphur present may be washed out by water. (b) An intimate mixture of 37 parts China-clay, 15 parts sulphate of soda, 22 parts carbonate of soda, 18 parts sulphur, and 8 parts charcoal, is heated in large crucibles for 24-30 hours ; the mass is re-heated in cast-iron boxes at a moderate temperature till the desired tint appears, and is finally pulverized, washed, and dried. (c) Gently fuse 1075 oz. crystallized carbonate of soda in its water of crystallization ; shake in 5 oz. finely-pulverized orpi ment, and, when partly decomposed, as much gelatinous alumina hydrate as contains 7 oz. anhy drous alumina ; add 100 oz. finely-sifted clay, and 221 oz. flowers of sulphur ; place the whole in a covered crucible, and heat gently till the water is driven off, then to redness, so that the ingredients sinter together without fusing ; the mass is then cooled, finely pulverized, suspended in river-water, and filtered. The product is heated in a covered dish to dull redness for 1-2 hours, with occasional stirring. Colourless or brownish patches may occur, and must be removed.
Prussian Blue.—(a) A solution of 2 parts alum and 1 part sulphate of iron is made in water ; a solution of yellow prussiate of potash is then acidulated with sulphuric acid, and some of the first solution is dropped in till the precipitate falls slowly ; the latter is well washed on a filter, and dried. (b) Mix a solution of protosulphate of iron with one of red prussiate of potash ; wash
and dry.
Saxon Blue.—Diesolve 1 oz. sulphate of iron and 8 oz. alum in 1 gal. water ; add separate solutions of prussiate of potash and pearlash, until the precipitation ceases ; collect the precipitate after some time ; wash thoroughly, and dry.
Ultramarine.—The preparation of ultramariue from the gem lapis lazuli (see Gems, p. 1042) no longer survives. Artificial ultramarine, of which, some 10,000 tons are made annually, is composed approximately of 46.60 per cent. silica, 23.30 alumina, 3.83 sulphuric acid, 21.48 soda, 1.06 per oxide of iron, aud traces of lime, sulphur, and magnesia. The ingredients employed are sometimes China-clay, sulphate of soda, charcoal or pit-coal, and rosin ; or China-clay, soda, silica, sulphur, and rosin. Their proportions are a matter of secrecy, but may be deduced pretty accurately from the percentage composition just given. The raw materials are ground very fine, well mixed, pressed into muffie-furnaces, and calcined at a red heat for 12-36 hours, or until the sulphur is nearly burnt off. When the firing is complete, the furnaces are closed tightly, and the material is allowed to cool, requiring 5-6 days. The product is green ultramarine, which is then roasted with finely-powdered sulphur in pans under the influence of the air. After washing, it is ground in wet mills for 2-5 days, settled under the action of heat, repeatedly washed, classified, dried, bolted, and pa,cked.