ULTRAMARINE.

161

sufficient quantity to form with the sulphur polysulphuret of sodium. Thisand the formation of another sulphuret (Na 2 S) are obtained by observing thefollowing proportions:—

Dry kaolin.

IOO

Calcined Glauber’s salt ..

• •• 41

83— IOO

Calcined soda.

• -• 41

—

Charcoal or pit-coal.

• *7

r 7

Sulphur .

13

—

This mixture is ignited without access of air, and a white mass is obtained,which becomes green by exposure to air, and blue by being calcined in contactwith air. The mixture is placed in fire-clay crucibles, and well rammed down,the crucibles being maintained at a high temperature, with a limited supply ofair for eight to ten hours, the ignition being completed at a white heat. Thecrucibles when cooled are found to contain a semi-fused grey- or yellow-greenmass, which is repeatedly lixiviated with water. The green ultramarine con-tains, according to Stolzell’s analysis (1855), * n 100 P arts —

Alumina.30*11

Iron. 0*49

Calcium . 0*45

Sodium . 19*09

Silica. 37'46

Sulphuric acid. 0*76

Sulphur . 6*o8

Chlorine. 0*37

Magnesia, potash, phosphoric acid .traces

94-81

Oxygen . 5*19

100*00

2. The conversion of green into blue ultramarine is effected by roastingthe green ultramarine and sulphur at a low temperature, with access of air.Sulphurous acid is formed, a portion of the sodium oxidised into soluble sul-phate being washed out; the sulphur originally present in the green ultra-marine remains combined with a smaller quantity of sodium. The roastingis generally effected in an iron cylinder, somewhat similar to a gas-retort,sulphur being added until the required depth of blue is obtained. The ultra-marine is then pulverised, lixiviated, dried, and assorted as to quality.

/3. Soda-ultramarine is prepared from the following proportions of ma-

terials:—

Kaolin. 100 100

Sulphate of soda . — —

Soda. 90 100

Carbon. 6 12

Sulphur . 100 60

Resin. 6 —

The ignition is effected in the manner previouslydescribed. By increasing the pro-portion of soda and of sulphur, blue ultramarine may be formed at one operation.

7* Silica-ultramarine is prepared in a similar manner, silica being added tothe former materials to the amount of 5 to 10 per cent of the kaolin. Blue