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])

THE ARTIST'S ASSISTANT. 59

the colour in it, muft be put into another glafs of
the fame kind with as much frefh oil as will rife
five or fix inches above the colour. This glafs
muft be treated in the fame manner as the firft:
obferving when the colour has fubfided, the oil
muft be poured off, and a frefh quantity put in its
place. This having been likewife poured off, the
; colour muft then be well walked, to free it from
4 the remaining oil, firft in boiling water, and af-
; terwards in fome of the lixivium abovementioned,
; made boiling hot alfo. As much of the lixivium
: being poured off, when the colour has fubfided, as
; can be feparated from it that way, the colour muft
; be thoroughly freed from the remainder by fre-
: quent ablutions with clean water: after which
; the water muft be taken off by the means above
£; directed for the ultramarine, till the matter be of
t: a proper degree of moifture for grinding. It muft
" then be thoroughly well ground on a porphyry,
11 and wafhed over ; that all the harder and infuf-
<! hciently calcined parts may be reduced to an im-
" palpable powder ; in order to which, the remain-
Lag groffer parts, after the finer have been feparated
': bv the walking over, muft be again ground till
<: the whole be perfectly fine. The fame means muft
'; be afterwards ufed to bring the afhes to a dry pow-
(; der that were before direfted for the ultramarine."

PRUSSIAN BLUE,

Is the earth of alum, combined with the fixed
fulphur of animal or vegetable, coal; and may be
made from almoft any animal fubftance ; but it is
generally made of the coal of blood only. It is

H 2 ufeful