AZURE

the fine blue pigment, commonly called smalt, is a gla coloured with oxide of cobalt, and ground to an impalpable powder. The manufacture of azure, or smalt, has been lately improved in Sweden, by the adoption of the following proce :— The cobalt ore is first roasted till the greater part of the arsenic is driven off. The residuary impure black oxide is mixed with as much sulphuric acid (concentrated) as will make it into a paste, which is exposed at first to a moderate heat, then to a cherry-red ignition for an hour. The sulphate thus obtained is reduced to powder, and di olved in water. To the solution, carbonate of potash is gradually added, in order to separate the remaining portion of oxide of iron; the quantity of which depends upon the previous degree of calcination. If it be not enough oxidized, the iron is difficult to be got rid of. When, from the colour of the precipitate, we find that the potash separates merely carbonate of cobalt, it is allowed to settle, the supernatant liquor is decanted, and precipitated, by means of a solution of silicate of potash, prepared as follows :— Ten parts of potash are carefully mixed with fifteen parts of finely ground flints or sand, and one part of pounded charcoal. This mixture is melted in a crucible of brick clay, an operation which requires steady ignition during 5 or 6 hours. The ma , when melted and pulverized, may be easily di olved in boiling water, adding to it, by little at a time, the gla previously ground. The filtered solution is colourle , and keeps well in the air, if it contains one part of gla for 5 or 6 of water. The silicate of cobalt, which precipitates upon mixing the two solutions, is the preparation of cobalt most suitable for painting upon porcelain, and for the manufacture of blue gla . See Cobalt . b.