Chap. 24.] Smalt or Powder Blue. 135 



The regulus of cobalt is of a whitifh grey or flcel 

 colour, hard, brittle, of a dull clofe-grained frafture, 

 and moderate fpecific gravity. It has about the fame 

 degree of fufibility as copper ; does not eafily become 

 calcined ; and its calx is of fo deep a blue colour as 

 to appear black. Cobalt expoied to heat does not 

 melt till it is well ignited. It appears to be very 

 fixed in the fire, and it is not known whether it can 

 be volatilized in clofe vefiels. If it is fuffejed to 

 cool flowly, it cryftallizes in needle-formed prifms, 

 placed one on another, and united in bundles, Cobalt, 

 melted and expofed to the air, becomes covered with 

 a dull pellicle, which is a calcination analogous to the 

 rufting of iron. 



The richeft mines of cobalt are in Saxony. The 

 ore which is worked there in the large way contains a 

 confiderable quantity of arfenic, which is driven off by 

 heat, but ii collected in long channels of wood, and 

 preferved for fale, as was intimated before. After the 

 ore has been kept fome time in the furnace, there 

 remains a dark friable fubftance, which is the cobalt in 

 the form of a calx, and called zaffre. This is mixed 

 with the ordinary ingredients of glafs, and melted with 

 a violent heat, fo as to produce the common blue 

 powder called fmalt, which is a pounded glafs. Some 

 of this is mixed with flints and alkaline falts, and 

 then fold under the name of, fapphire to the manufac- 

 turers of porcelain and common Delft ware, for ting- 

 ing their glazing blue. 



Powder blue, or azure, is obtained by grinding 

 fmalt in mills, and afterwards wafhing it in water. 

 This laft operation is performed in a cafk filled with 

 water, and pierced with three openings at different 

 heights. The water of the uppermoft cock carries 

 K 4 out 



