308 Chromat of Lead . 
was separated by filtering ; the water was again boiled in a clean 
vessel, and used hot, as above directed. 
After the lotions are concluded, the filter may remain in the 
funnel for 24 hours to drain ; the funnel being covered with paper, 
to prevent the pigment being injured by dust, &c. ; the paper will 
then bear being raised without danger of tearing, and may be laid 
on a chalk stone to dry, with a sheet of bibulous paper laid on its 
surface. 
My second process, which furnishes the finest specimens, is 
as follows: — The mineral is treated as in the former process, ex- 
cept that instead of adding nitric acid to separate the earths from 
the concentrated solution; I add a saturated solution of muriat of 
ammonia , an abundant precipitate immediately appears, the muriat 
of ammonia is added until it ceases ; the whole is then thrown on 
a filter : by this means I free the solution of all the earths. Into 
the filtered liquid I add nitric acid until it is sensibly sour, and boil 
immediately ; whilst boiling, a few additional drops of acid are ad- 
ded, to ascertain whether the liquid contains any carbonic acid , if 
an effervescence ensues, or small globules of air are perceived to 
rise from below the surface, more acid is added until they cease, 
when clean acetate of lead in powder is immediately added, as long as 
chromat of lead is precipitated. The chromat is then separated 
by filtration, and the washing, See. conducted as in the former pro- 
cess. 
This last process (if properly conducted) always produces deep 
orange colours of great beauty. Of six specimens which I prepa- 
red, no difference could be observed in the shades, by the most 
discriminating eye ; but it is indispensable to success, that there 
be no delay, after the carbonic acid is expelled by adding nitric 
acid and boiling : as the muriatic acid of the muriat of ammonia, 
decomposes part of the chromic acid, oxymuriatic acid gas is a- 
bundantly evolved, and the solution changes rapidly to a dark yel- 
lowish green, the decomposition however is only partial ; yet the 
least delay in adding the acetate of lead would render it general, 
and thus defeat the previous labour. 
With respect to the permanence of the colour, I cannot say 
much from my own experience ; the opinion of artists I find dif- 
ferent on the subject. I will however observe, that no inference 
should be formed from the specimens which have been offered to 
the public ; they are all contaminated with carbonate of lead , a 
pigment which is well known to blacken, the instant it comes in 
contact with sulphuretted hydrogen gas, or the effluvia from pu~ 
