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- 261

as in the cafe of calcined flints, and the foftcft kind
of natural fand ; where one hundred and forty
pounds of fait are required to an hundred and fifty-
pounds of calcined flints; and only one hundred
and thirty pounds of fait to two hundred pounds of
the fand.

Of materials ufed as fluxes in the compoution of
glafs :—The materials ufed for the fluxes in the
compoution of manufactured glafs are lead, pearl-
afhes, nitre, fea-falt, borax, arfenic, fmiths clinkers,
and wood-afhes. containing; the earth and lixiviate
falts as produced by incineration.

Lead is the prefent moft important flux in the
Britifh manufactures of what is called flint glais;
but it muff be brought, by previous calcination, to
the ftate of minium, or what is called red lead.
This, ufed in a due proportion, makes a tougher
and firmer glafs than can be produced from falts
alone, and is vet procured at a very fmall expence.
But all the glafs formed of lead is tinged originally
with yellow, and therefere requires the addition of
nitre to burn and deftroy the fulphur or phlogiftic
matter it contains, in order to bring it to a more
colourlefs ftate ; which addition of nitre enhances
again the cofl of glafs fo compofed, that would other-
wife be extremely low. There is another reafon,
likewife, for the addition of nitre, or fome other
fait, to operate as a flux in the glafs compounded of
lead, which is. that there may not be a necefhty of
ufing beyond a certain proportion of it; for, if
glafs have much lead in its compoution, it will fuf-
fer a corrolion by the air, which gives a greyifh
dulnefs to its furface that is very injurious both to
its beauty and utility. It is needlefs here to teach

the
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