t 309 ] 
SECT. XEVIIL 
Cobalt is particularly remarkable for giving 
to the glafs a blue colour, which is the zafFre or 
fmalt. To produce this, a piece of cobalt ore 
rnuft be calcined in the fire (Sed. xxx. xxxi.) and 
afterwards melted with borax. As foon as the glafs, 
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 difcontinucd, and the 
operator muft take hold with the nippers (Seft. 
xi. 6.) of a little of the glafs, whilft yet hot, and 
draw it out flowly in the beginning, but afterwards 
very quick, before it cools, v^hereby a thread of 
the coloured glafs is procured, more or lefs thick, 
on which the colour may eafier be feen againfl 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- 
P^P^: 
If this glafs is melted again with more of the 
cobalt, and kept in fufion for a while, the colour 
bepotries very deep ; and thus the colour may be 
altered, according to pleafure. 
SEC T. L. 
When the cobalt ore is pure, or at leaf!: contains 
but little iron, a cobalt regulus is almofl in-ftantiy 
produced in the borax, during the fufion : but 
when it is mixed with a quantity of iron, this lafl 
metal ought firfl to be feparated, which is eafily 
performed, fince it fcorifies fooner than the cobalt ; 
therefore, as long as the flag retains any brown or 
Jblack colour Sed. xlviii. it muff be feparated, 
X 3 and 
