Scientific American Supplement, No. 455, September 20, 1884 | Page 5

which is practically superior to any
other, both in the rapidity with which the color can be formed, and for
producing it at a less cost.
It is stated in Watts' "Dictionary of Chemistry" that it is "prepared on a
large scale by mixing arsenious acid with cupric acetate and water. Five
parts of verdigris are made up to a thin paste, and added to a boiling
solution of 4 parts or rather more of arsenious acid in 50 parts of water.
The boiling must be well kept up, otherwise the precipitate assumes a
yellow-green color, from the formation of copper arsenite; in that case
acetic acid must be added, and the boiling continued a few minutes
longer. The precipitate then becomes crystalline, and acquires the fine
green color peculiar to the aceto-arsenite." I do not know from personal
knowledge, but I have always understood that the copper salt employed
in its manufacture in France is the acetate. This would account, in my
opinion, for the larger crystalline flakes in which it is obtained in
France than can be produced by the English method of manufacturing it.
Cupric acetate is never employed, I believe, in England--the much
cheaper copper salt, the sulphate, being always employed.
In "Miller's Chemistry" it is stated it "may be obtained by boiling
solutions of arsenious anhydride and cupric acetate, and adding to the
mixture an equal bulk of cold water." Why it should be recommended
to add cold water, I am at a loss to understand.
In Drs. Roscoe and Schorlemmer's large work on "Chemistry," and in
the English edition of "Wagner's Handbook of Chemical Technology,"
edited by Mr. Crookes, the process as described by Dr. Ehrmann in the
"Ann. Pharm.," xii., 92, is given. It is thus stated in Wagner's work:
"This pigment is prepared by first separately dissolving equal parts by

weight of arsenious acid and neutral acetate of copper in boiling water,
and next mixing these solutions while boiling. There is immediately
formed a flocculent olive-green colored precipitate of arsenite of
copper, while the supernatant liquid contains free acetic acid. After a
while the precipitate becomes gradually crystalline, at the same time
forming a beautiful green pigment, which is separated from the liquid
by filtration, and after washing and carefully drying is ready for use.
The mode of preparing this pigment on a large scale was originally
devised by M. Braconnot, as follows: 15 kilos. of sulphate of copper
are dissolved in the smallest quantity of boiling water, and mixed with
a boiling and concentrated solution of arsenite of soda or potassa, so
prepared as to contain 20 kilos. of arsenious acid. There is immediately
formed a dirty greenish-colored precipitate which is converted into
Schweinfurt green by the addition of some 15 liters of concentrated
wood-vinegar. This having been done, the precipitate is immediately
filtered off and washed."
As I have already stated, the copper salt used in the manufacture of this
pigment in England is the sulphate, and it is carried out pretty much
according to Braconnot's method as described by Dr Ehrmann; but any
one would infer, from reading his description of the manufacturing
process, that the compound, aceto-arsenite of copper, was formed
almost immediately after the addition of the acetic acid, a higher or
lower atmospheric temperature having no effect in hastening or
retarding the formation. 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