I 3°9 ] 



SECT. XLVIII. 



Cobalt is particularly remarkable for giving 

 to the glafs a blue colour, which is the zaffre or 

 £nak. To produce this, a piece of cobalt ore 

 muft be calcined in the fire (Sect. xxx. xxxi.) and 

 afterwards melted with borax. As foon as the giafs, 

 during the fufion, from being clear, feems to 

 grow opaque, it is a fign, that it is already tinged 

 a little •, the fire is then to be difcontinued, and the 

 operator mufl take hold with the nippers (Sect. 

 xi. 6.) of a little of the glafs, whilfl yet hot, and 

 draw it out flowly in the beginning, but afterwards 

 very quick, before it cools, whereby a thread of 

 the coloured glafs is procured, more or lefs thick, 

 on which the colour may eafier be feen againft the 

 day or candle-light, than if it was left in a globular 

 form. This thread melts eafily if only put in the 

 flame of the candle, without the help of the blow- 

 pipe. 



If this glafs is melted again with more of the 

 cobalt, and kept in fufion for a while, the colour 

 becomes very deep ; and thus the colour pity be 

 altered, according to pleafure. 



SECT. L. 



When the cobalt ore is pure, or at leafl contains 

 but little iron, a cobalt regulus is almofl inftantly 

 produced in the borax, during the fufion : but 

 when it is mixed with a quantity of iron, this laft 

 metal ought firft to be feparated, which is eafily 

 performed, fince it fcorifies fooner than the cobalt; 

 therefore, as long as the flag retains any brown or 

 black colour Seer, xlviii. it mufl be feparated, 



X 3 and 



