Clay 47·2 Sulphate of soda 19·3 Carbonate of soda 19·3 Charcoal 8·1 Sulphur 6·1 ----- 100·0 Caustic soda is also sometimes used instead of carbonate. These mixtures (whether sulphate alone or sulphate and carbonate) are made with a view to have the soda present in sufficient amount to combine with one-half the silica contained in the clay, and to leave sufficient soda to form polysulphide of sodium with a portion of the sulphur. There should then remain enough soda and sulphur to produce ordinary sulphide of sodium (Na_{2}S). If either of the two mixtures be ignited out of contact with air, a white compound is formed, which is sometimes termed white ultramarine. On leaving this exposed to the atmosphere for some time it becomes green, and on further ignition, with free access of air, it is converted into ultramarine blue. In actual working the carefully prepared mixture of the above mentioned materials is heated for several hours to a high temperature in fire-clay crucibles, only a limited supply of air being allowed to enter, and the temperature being eventually raised to a white heat. The product of this operation, when cool, has a grey or yellowish-green appearance. It is washed several times with water, dried, reduced to a fine powder, and then represents the green ultramarine of commerce. Stölzel found that green ultramarine had the following composition:-- Per cent. Alumina 30·11 Silica 37·46 Sodium 19·09 Sulphur 6·08 Iron ·49 Calcium ·45 Chlorine ·37 Oxygen 5·19 Sulphuric acid ·76 Magnesia, potash, and phosphoric acid traces. ------- 100·00 Green ultramarine is transformed into blue by heating with about 4 per cent. of sulphur at a low temperature, with free access of air. Sulphur is afterwards added, if necessary, in small quantities at a time, and the heating is continued until the desired shade of blue is obtained. The mass is then powdered, the soluble matter (sulphide of soda, &c.) is removed by washing with water, and the blue is dried and assorted according to quality. (_b_) “_Soda_” _Ultramarine_ is sometimes made with soda alone (either carbonate or caustic), and at others with a mixture of soda and sulphate of soda. Rowland Williams found the following proportions of the respective ingredients to answer satisfactorily:--