The Artist's Assistant, In the Study and Practice of Mechanical Sciences: Calculated for the Improvement of Genius. Illustrated with Copper-Plates — Birmingham, [ca. 1785]

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1 cm

is very fmall. It is brought from the Eaft-Indies,
under the name of tincal, and the refinement of it in
a perfeft manner, is hitherto known but to few per-
fons in Europe, who carefully keep it a fecret. The
knowledge of it, however, is not important to the
art of making glafs, as it is always procured for that
purpofe in a refined ffate. and not ufed in very large
quantities. The purity of it may be afcertained by
the largenefs and clearnefs of the cr\ fta's, for when
it is had in that ftate, it may always be concluded
good. The previous preparation of borax for the
compofition of glafs, is to calcine it with a gentle'
heat, which converts it to a flaky, feathery kind of
fubftance, like calcined alum ; after which it fhould
be ground to powder, and is then fit to be commixed
with other ingredients : this calcination of borax
fhould be with a gentle heat, and in a very lar^e vef-
fel proportionably to the quantity, for it fwells and
rifes in inflated bladders, fo as to occupy a very great

Arfenic is alfo a powerful flux, but muft not be
added, neverthelefs, in too great quantity ; for, though
when once vitrified perfectly, it greatly promotes the
fame change in other fubftanccs, yet, when added in
a redundant proportion, it turns the glafs milky, or
opake, and keeps it in that ftate a confiderable time
before it will duly affimilate, from whence the due
vitrification is greatly retarded, fo as to occafion an
intolerable lofs of time and fuel; though the glafs
in all fuch cafes would become clear, if continued
long enough in the fire, yet on this principle of its
flownefs in vitrifying when added to compofitions of
glafs jit a large proportion, it. is ufed for giving an
opake white colour to glafs, as we fhall fee below.

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