Furthermore, it is not stated whether the compound forms more readily in
an acid or neutral solution, or whether it can or cannot be formed in a
neutral one; now both these points are important to notice in describing
its manufacture. As regards the former I shall notice it presently, and,
as far as my knowledge extends, the pigment will not form when the
solution is neutral.
The operation is conducted in the following manner in the factory: The
requisite quantity of sulphate of copper is placed in a large wooden vat,
and hot water added to dissolve it; the requisite quantity of arsenic
(arsenious anhydride) and carbonate of soda, the latter not in quantity
quite sufficient to neutralize the whole of the sulphuric acid set free
from the sulphate of copper on the precipitation of the copper as
arsenite, are placed in another wooden vessel; water is then added, and
the formation of the arsenite of soda and its solution are aided by the
introduction of steam into the liquid. When complete solution has been
effected the arsenic solution is run off into the vat containing the
solution of the sulphate of copper, arsenite of copper being at once
precipitated. The necessary quantity of acetic acid is afterward added. In
_warm_ weather the formation of the aceto-arsenite soon commences after
the addition of the vinegar; but, even in that case, it takes a week or
more to have the whole of a big batch of arsenite converted into the
aceto-arsenite; and perfect conversion is necessary, as the presence of a
very minute quantity of unchanged arsenite lowers very much the price of
the emerald pigment, and a by no means large quantity renders the pigment
unsalable, owing to its dirty yellowish-green color.
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