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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THE ARTIST'S ASSISTANT. 265

(late it is commixed with fome proportion of com-
mon fait; il is refined by perfons who make it their
proper bufinc<~s, and bought for the purpofe of glafs-
makirg. in the date of fait petre, on which account, it
is unneceffarv to give the procefs for refining it here.
If it be ob'ained in cryftals of fuch a fize, that the
figure of them may be diftinguifhable, there is no
hazard of any adulteration, but what would be very
apparent, as no heterogeneous matter can be made a
proper part of fuch cryftals ; and, therefore, if they
appear bright and colourlefs, the goodnefs cannot be
doubted.

Sea fait is alfo frequently ufed as a flux in the mak-
ing glafs of various kinds, and it has a very ftrong
power in promoting vitrification, even in fome ob-
durate bodies ; but ufed in a large proportion, it does
not produce fo ftrong and tenacious a glafs as lead?
or even the alkaline falts of vegetables, and is there-
fore only taken in aid of the others, when admitted
as an ingredient : it fhould be brought to a dry ftate
by decrepitation, that is, keeping it in a moderate
heat till it ceafes crackling, before it be put with
other ingredients into the fufing heat, otherwife, by
the little explolive burfts of its parts, it will drive
fome of the powdered matter out of the pot; it mufl
not, after fuch decrepitation be again expofed to the
air, for if it be, it will regain its former quality of
crackling in a fhort time.

Borax is the moft powerful flux of all the falts, or
indeed of any known fubftance whatever, but on ac-
count of its great price, can only be admitted into the
compofition of glafs, defignedfor looking-glafs plates,
or other purpofes, where a considerable value can be
let upon the produce3 or where the quantity wanted

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