The preparation of ultramarine from lapis-lazuli 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 peroxide of iron, and traces of lime, sulphur, and magnesia. The ingredients employed are China-clay, soda sulphate, charcoal or pit-coal, and rosin; or China-clay, soda, silica, sulphur, and rosin. Their proportions may be deduced from the percentage composition just given. The raw materials are ground very fine, well mixed, pressed into muffle-furnaces, and calcined at a red heat for 12 to 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 to 6 days. The product is green ultramarine, which is roasted with finely-powdered sulphur in pans under the influence of the air. After washing, it is ground in wet mills for 2 to 5 days, settled under the action of heat, repeatedly washed, classified, dried, bolted, and packed.
All are carefully chosen and prepared. A little magnesia and lime in the clay is not objectionable; but more than 1 per cent, of iron oxide should not be admitted. The clay, after calcination, should correspond to the formula A1203, 2Si08. Often clays do not present this composition, but the mechanical operations to which they are submitted remove most of the foreign materials. The preparatory working, in order to remove mechanical impurities, is effected by levigation. The washed clay is dried, slightly calcined, and immediately ground to fine powder. The floating is done by hand or power. After decanting the water, the pasty mass is collected, pressed in sacks, and dried by heat, or in the air upon porous slabs of plaster of Paris. The calcination is effected in an ordinary reverbera-tory furnace, at a temperature not above the beginning of a cherry-red heat. The clay is pulverized under stamps, or in an edge runner, and passed through a series of fine metallic sieves.
If the soda sulphate be employed in the anhydrous state, it should contain no free acid, and be free from iron and lead. When such cannot be had, take Glauber salt which contains no free hydrochloric acid; it is dissolved in water, and the excess of sulphuric acid is saturated with milk of lime, which at the same time precipitates the iron oxide. The clear liquor is decanted and crystallized. The lime sulphate and excess of lime remain in the deposit. The crystallized soda sulphate is slowly dried on slabs of fire-clay in the bed of a reverberatory furnace. The product is anhydrous sulphate of soda. The clear liquors may be directly evaporated, without crystallizing, in a pan always kept full of fresh liquor. At a certain degree of concentration, an anhydrous sulphate is precipitated, which is "fished," and deprived of all adhering water by slight calcination in the reverberatory furnace. The anhydrous sulphate is ground and sifted, and kept in closed vessels. This salt may be bought of manufacturers, but it is difficult in ultramarine works to dispense with the apparatus for this treatment, because, during operations, there are produced washing liquors containing sulphate of soda, which ought to be evaporated.
The soda carbonate is also used in the anhydrous state, and can be bought as pure and dry as desired. A little soda sulphate in it presents no inconvenience. This soda carbonate is powdered like the sulphate, and kept in the same manner.
Works which do not directly use sodium sulphide in solution should be provided with iron evaporating kettles, heated by the waste heat of the furnaces. The liquors are evaporated to dryness, and constantly stirred towards the end of the operation. The sodium sulphide is powdered, and kept like the sulphate and carbonate. The sulphur employed is in refined rolls, ground and sifted. The necessary carbon may be derived from bituminous coal or wood charcoal. The impurities of the large pieces are removed by sifting, and of the small fragments by levigation; the floating charcoal is removed and dried. The caking kinds of bituminous coal are preferred, provided their percentage of ash is smalt. The two kinds of coal are reduced to very fine powder by trituration with balls in a revolving cylinder, or by grinding with water in granite mills. The settled powder is collected, drained, dried upon shelves, again ground, and sifted.
When the materials are used dry, it is advantageous to weigh small quantities at a time, mix them, pass through sieves of medium fineness, stir again, sift anew, and so on, until the proper result is arrived at.
The proportions of the raw materials vary, but care should be had - (1) That the soda, as sulphate or carbonate, be sufficient to saturate half the silica in the clay; (2) that the proportions of sulphur and soda be such as to produce a bisulphide or polysulphide of sodium; (3) that in the mixture there remain enough sulphur and sodium to form a mono-sulphide of sodium, when all the green ultramarine resulting from the silica and alumina is extracted.
German manufacturers compose their mixtures differently from the French. The latter employ only soda carbonate, while the former use only the sulphate, or a mixture of sulphate and carbonate. The results appear identical. In the case of the sulphate, more carbon and sulphur are employed. With the carbonate, no carbon is required, and a great deal of sulphur is needed. It appears that the German mode is the more economical.