Although the proportion of cobalt oxide contained in smalts is so variable, yet it is probably present as a compound of constant composition—the cobalt potassium double silicate of the formula CoO. 2SiO₂ + K₂O. 2SiO₂. The numerous published analyses of smalts lead to this conclusion. The small quantities of other metallic oxides which are found in smalts are to be regarded as chance impurities. Being a glass free from lime smalts is easily decomposed; in this respect it shows great similarity with water-glass, which is completely soluble in boiling water. Smalts comes into commerce in the form of powder. In the ”Blue Colour Works” in which smalts is made a number of qualities are distinguished; these are separated according to the fineness of the powder. Smalts of good quality is composed of particles of the same size and has a pure sky-blue colour. A shade inclining to red is generally to be ascribed to the presence of iron; material of this nature is of little value. Smalts is not easily attacked by chemical reagents. It has been above stated to belong to the easily decomposed glasses, and it is attacked by reagents which ordinary glass completely resists. Finely ground smalts is strongly attacked by boiling sulphuric acid, and, after fusion with soda, is soluble in acids. Water has some action upon smalts; by the long continued action of a large quantity the shade is injured, the colour acquires a greenish tinge. In the manufacture of smalts there are three principal operations:— 1. Preparation of the charge. 2. Fusion of the charge. 3. Grinding the fused mass. These processes are principally mechanical. Only as regards the composition of the charge is chemical knowledge required. =1. Preparation of the Charge.=—The cobalt is supplied by substances of different origin—as a rule, roasted speiss cobalt and cobalt glance are used; occasionally ores are employed without previous preparation. When pure cobalt salts (sulphate and nitrate) are used, a product of very fine shade is produced, but in practice these salts are not applicable on account of their cost. There remain therefore only the compounds of cobalt occurring in nature. On account of the foreign substances they contain, these require a special treatment before they can be used in the manufacture of smalts; the treatment varies according to the other elements present in the cobalt ores. Ores containing bismuth are first treated in order to obtain this valuable metal; ores containing arsenic are roasted in furnaces of special construction, which are connected with a series of stone or wooden passages in order to condense the arsenic trioxide completely.