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])

58 THE ARTIST'S ASSIST A N T,

of nitre on a fmall quantity; which, if there be
any copper, will foon diffolve, and form a greenifh
blue folution.

ULTRA M ARINE ASHES.

After the ultramarine has been extracted from the
lapis lazuli, the reuduum or remains from this
pigment. And when the operation of extracting
the colour has not fucceeded well, a confidcrable
fhare of the ultramarine is left behind with the re-
crement, and greatly enhances the worth of the
afhes ; for of courfe the value of the latter is infe-
rior to the former, but it is ftill fubject to adultera-
tion, which may be difcovered by putting fome of
it into a fmall quantity of fpirit. of nitre, and if there
be any copper in it, it will be tinged green. It is
prepared as follows:

« Take the cement of the ultramarine, which re-
c: mains after the colour is extracted, and mix it
{C With four times its weight of linfecd oil. Let the

D

£: mixture be fet in a glazed pipkin over the fire,
*■* and when it is thus boiled a fhort time, put it into
<: a glafs veffel lufnciently large to contain it, of a
<: cylindrical figure : of which veilel the diameter
" muft be {mall in proportion to the length. But
" care muft be taken, that the matter when put into
c: this glafs be cool enough not to endanger the
': breaking it. This glafs muft then be put into a
': balneum marise, which mult be made as hot as
" pofiible without boiling, and kept there till the
colour appears to be all fublided to the bottom.
c: The oil muft then be poured off, till the colour
<: appears to rife with it; and the remainder, with

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