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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https://digi.ub.uni-heidelberg.de/diglit/assistent1785/0071
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THE ARTIST'S ASSISTANT. ,57

<; not broken into fragments or fmall parts ; and
<; when io much of the colour is extracted, as to
render it ncceifary for the obtaining more, the
'; heat of the water muft be incre'afed to the greateft
*' decree. The quantities of the calcined matter
t; (which is now the ultramarine) that were firffc
" wafhed off, and appear of the fame degree of deep-
" nefs and brightnefs, maybe put together ; and the
" fame of thoie of the fecond degree ; the lafl wafh-
" ings making a third. The water being then pour-
<; ed off from each of thefe parcels, but on a lixivium
i: formed of two ounces of fait of tartar, or pearl-
£: afnes, diffolved in a pint of water, and filtered
': through paper after the folution is cold. This
'; lixivium muft be put on boiling hot, and the ul-
': tramarine ftirred well about in it ; and then the
{: mixture fet to cool. The powder being fubfided,
t: the clear lixivium muft be poured off, and clean
" water put in its place ; which muft be repeated
£! till the whole of the falts of the lixivium are
': walked away. The ultramarine muft afterwards
£: be dried; and will be then duly prepared for
<! ufe."

Ultramarine is fubje£t to be adulterated, on ac-
count of its great price—this is frequently done by
a precipitation of copper, made by alkaline fait,
and is very injurious ; becaufe the magiftery of cop-
per (if the ultramarine fophifticated with it be
ufed in painting, either with oil or water) will
change its hue and turn black. And, in enamel
painting, as foon as fluxed it will become a green,
and confequently make the effect of the ultrama-
rine vary from what is intended. This fraud may
be eafily detected by pouring fome diluted fpirit

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