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

as they cannot be prepared with advantage in this
country, and are to be had at a reasonable price, by
thofe who may have occafion to ufe them in mak-
ing glafs, I fhall wave entering more particularly
into the detail of the procefs, by which they may
be beft and moll profitably produced, as not proper-
ly falling either within the defign or the intention
of this work. The goodnefs of pearl-afhes mufh be
diftincruifhed by the equal and white appearance of
them ; as it confifts in their purity, and their hav-
ing been calcined for a long fpace of time, of which,
the whitenefs and equal appearance are marks, un-
lefs in the cafe of fome parcels that contain lumps of
a bluifh caft, produced by calcination ; which dif-
colouring is not, however, any proof of their be-
being bad ; but any brownifh caft in particular
parts, or greynefs in the whole, is a certain criterion
of their not being good. This mult, however, be
confined to fuch as are perfectly dry, which can only
well be on the opening the cafks they are brought
over in ; for, if the air have accefs to them, they
foon deliquiate, and look brown or greyifh, from a
femi-tranfpareney they acquire in that dcliquiating
ftate. There is one, and the moft common adultera-
tion, which is made in thefe fairs, that is not eafily
diftinguifhable by the appearance; it is, the addi-
tion of common or fea fait, to them, which is fome-
times copioufly made ; this is not, however, very de-
trimental in the application of them to the forming
glafs. But it is, neverthelefs, a difadvantage con-
siderable enough in large concerns, to buy one
thing for another at fix times its current price.
As it is expedient, therefore, to know how to dif-

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