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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i5o THE ARTIST'S ASSISTANT.

Ultramarine (the preparation of which we have
before given) is ufecl in enamel, where very bright
blues of a lighter teint are wanted ; and fometimes
indeed, in other cafes, by thofe who do not under-
Hand the tight ufe of zaffer and fmalt. But there
are few inftances where zaffer, when perfectly good,
fluxed with borax and a little calcined flint, or Ve-
netian fflafs, to take off the fufible oualitv of the
borax, will not equally well anfwer with the beft
ultramarine. The ultramarine requires no prepara-
tion when ufed in enamel painting, previoufly to its
being mixt with the proper flux, and what relates to
its general qualities, and the means of diftinguifhing
its eoodnefs or genuinenefs, we have, alon^ with its
preparation, before taught, p. 54, and the following :

Ultramarine afhes are ufed, where light femi-tranf-
parent blues are wanted. But they are fo frequently
adulterated with precipitations of copper, which, of
course, turn green on fluxing, that it is very necciTarv
to be cautious, in the ufe of any parcel not previ-
oufly tried.

Zaffer is ufed for producing blue, green, purple,
and black colours in enamel. It is an earth, obtain-
ed by calcining a kind of ftone, called cobalt ; and
when it is mixed with any kind of vitreous bodies,
it vitrifies, at the fame time affuming a ffcrong blue
colour; but for the moft part verging to the purple.
It is to be had in a ftate proper for ufe, oi thofe co-
lourmen who make it their particular bunnefs to
fupply the glafs-makers with colours. The goodnefs
of zaffer can fcarcely be known but by an sCtual
trial of it ; and comparing the effeft of it with that
of fomc other known to be good, and ufed in the
lame proportion.

Magnefia
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