EMERALD GREEN.
There are many versions as to the discovery of this pigment. According
to one it was first made in Schweinfurt by Russ and Sattler; another
version gives it that Mitis of Vienna first made it on the large scale.
It is possible, as has been the case with many chemical products, that
this pigment was simultaneously discovered by both. The method of
making the pigment was at first a secret possessed by few. Not until
the publication of a method by Liebig in 1822 was the preparation
of emerald green generally known. In recent times the industry has
supplied other colours which are less poisonous. The time should not be
far distant in which this handsome but highly poisonous pigment shall
no longer be found in commerce.
The circumstance that the poisonous nature of the pigment and cautions
as to its use were given great publicity, brought about the discovery
of the countless names under which it has been sold. It was endeavoured
by re-naming the pigment to pass it off as a different substance. All
the pigments which have been sold under the names of Mitis, moss,
patent, new, king’s green, etc., are either pure emerald green or
mixtures of it with barytes or gypsum, which additions were intended to
alter the shade of the green and make it appear to the buyer as a new
pigment.
In chemical composition emerald green is a compound of copper arsenite
and copper acetate. It has the following formula:—
3(CuO.As₂O₃) + Cu(C₂H₃O₂)₂.
This compound rarely comes into commerce in the pure state; it is
generally mixed with chrome yellow or lead sulphate, which are added
with the object of producing shades between yellowish-green and dark
green. Emerald green is a compound of constant composition. When made
by different processes it has the same properties. Under the microscope
it is seen to consist of crystals; when the pure substance is ground
it becomes paler, the intensity of the colour being diminished by the
breakage of the crystals. The colour of emerald green is not altered
by artificial light. This is a most valuable property; the majority of
green pigments appear yellowish-green in artificial light.
Emerald green can be made on the large scale by two processes; these
differ according to the copper salt used, which is either the acetate
(verdigris) or the sulphate. The latter is by far cheaper than
verdigris, which is now seldom used for this purpose.
=Manufacture of Emerald Green from Verdigris.=—By boiling for several
hours, 70 parts of ordinary verdigris are dissolved in water; in a
second vessel 100 parts of arsenious acid are dissolved in 1,500 parts
of hot water; the verdigris solution is filtered through a cloth into a
larger wooden vessel and mixed with two-thirds of the arsenic solution.
The mixture is well stirred and allowed to stand for some hours. A
precipitate of dull green colour is formed. This consists of copper
arsenite. After three to four hours the remaining one-third of the
arsenic solution is added; the precipitate then changes into the double
salt of which emerald green consists.
The use of such dilute solutions as are given in these directions is
essential when a product of particularly good colour is required.
Emerald green is a crystalline substance which is deeper in colour in
proportion to the size of its crystals. Large crystals can only be
formed from very dilute solutions; when strong solutions are used,
the emerald green has never the deep colour possessed by the product
of dilute solutions. In the place of ordinary (basic verdigris) the
so-called distilled verdigris (neutral copper acetate) can be used,
of which seventy-six parts are required for the quantity of arsenious
acid given. The emerald green made from ”distilled” verdigris is of
a yet deeper colour than that made from ordinary verdigris, but is
considerably dearer. Such emerald green is commercially described, very
incorrectly, as distilled emerald green.
The method given by Liebig is as follows: 4 parts of basic verdigris
and 3 parts of arsenious acid are separately dissolved in boiling
vinegar; the liquids are mixed and evaporated until a precipitate of
emerald green separates.
According to Ehrmann and Kastner, 70 parts of verdigris and 56 to 63
parts of arsenious acid are separately dissolved, the boiling solutions
mixed and boiled until the dirty green precipitate changes into a
bright green. In consequence of the rapid formation of the precipitate
due to the boiling, the colour is pale green; if a dark-coloured
product is required, the solutions are allowed to cool before mixing.
After several days a very dark-coloured precipitate separates from the
liquid; it consists of comparatively very large crystals of emerald
green.
=Manufacture of Emerald Green from Copper Sulphate.=—There are two
processes. Either the solution of copper sulphate is mixed with an
acetate, by which it is converted into copper acetate, and then with
arsenious acid, or copper arsenite is precipitated from the solution
of copper sulphate and then converted into emerald green by treatment
with acetic acid. If acetic acid can be cheaply obtained, the latter
process is to be recommended.
Braconnet gives the following directions: 3 parts of copper sulphate
are dissolved in a little boiling water, the hot solution is mixed with
a hot solution of 4 parts of arsenious acid and four parts of potassium
carbonate; the precipitate is then treated with 3 parts of pyroligneous
acid. The dull-coloured precipitate at first formed is changed by the
acetic acid to a bright green. The colour is more quickly produced
when the liquid is heated nearly to boiling for several hours. When
the desired shade appears the boiling is stopped and the precipitate
at once separated from the liquid, by long contact with which it would
give up arsenious acid and thus lose in shade.
According to Fuchs, 5 kilogrammes of lime and 25 kilogrammes of copper
sulphate are dissolved in acetic acid, then a boiling solution of 25
kilogrammes of arsenious acid is added. The precipitate is at once
formed; it is immediately separated from the liquid, washed, and dried;
the top liquor, with the addition of arsenious acid, can be again used
to precipitate the copper lime solution.
Emerald green has an extremely bright colour which surpasses other
green mineral pigments in beauty. The larger are the microscopic
crystals of which it consists the deeper is the colour. In order to
obtain the desired deep shade of emerald green it is necessary to
use very dilute solutions; from strong solutions the precipitate is
instantaneously produced, in which case it is impossible to obtain
large crystals. The covering power of emerald green, in consequence
of its crystalline nature, is less the deeper the shade. It cannot be
regarded as a permanent colour: very dilute alkalis and acids attack
it. By the action of sulphuretted hydrogen it is discoloured, owing to
the formation of black copper sulphide. It ought not to be used upon
lime, which withdraws acetic acid and produces yellowish-green copper
arsenite.
This pigment should not be used for distempering or for colouring wall
papers. By the action of the atmosphere on the arsenic it contains,
arseniuretted hydrogen is formed—an extremely poisonous gas, very small
quantities of which are sufficient to cause serious illness in the case
of susceptible people. In spite of its poisonous nature, emerald green
is still largely used. All possible shades of green can be produced
from it, yellowish-green or pale green, by admixtures of yellow
pigments, such as chrome yellow, or white pigments, such as barytes
or white lead. Many qualities of green known as palette green, Basle
green, etc., are generally composed of such mixtures.
=Mitis Green or Vienna Green= is obtained by dissolving verdigris in
acetic acid and adding a boiling solution of white arsenic in water;
acetic acid is then added until the precipitate dissolves. On boiling
the clear solution a green precipitate forms, which, when dry, is
a deep bluish-green of characteristic shade. This pigment is now
practically disused because of its poisonous nature, which is equal to
that of emerald green.
=Copper Stannate.=—This green pigment, also known as Gentele’s green,
can be made by two methods. Gentele gave the following process: 59
parts of tin are converted into stannic chloride by solution in _aqua
regia_; a solution of 125 parts of copper sulphate is added and copper
stannate is precipitated from the mixture by caustic soda, on washing
and igniting it acquires a pretty green colour. It may also be made by
fusing 59 parts of tin with saltpetre; the potassium stannate is then
dissolved in dilute caustic soda and copper sulphate solution added;
the precipitate is washed and ignited.
Copper stannate is tolerably durable, in this respect it considerably
surpasses emerald green. Only sulphuretted hydrogen has any
considerable action upon it, turning it to a dirty brownish-green hue.
=Kuhlmann’s Green= is a basic chloride of copper, obtained by heating
2 equivalents of lime with a solution of 3 equivalents of copper
chloride. It is important that the copper salt should be present in
excess. In shade Kuhlmann’s green is very similar to emerald green,
with which it agrees in retaining its colour by artificial light. It is
a somewhat less pure green than emerald green; the difference is only
perceived when the two pigments are directly compared. When, however,
it is considered that it is far more permanent than emerald green and
cheaper, a more extended use is indicated than is yet the case.
=Elsner’s Green= is a species of lake, made by mixing a solution of
copper sulphate with a decoction of fustic, adding a small quantity of
stannous chloride and precipitating with caustic soda. To 100 parts of
copper sulphate 10 to 14 parts of stannous chloride are used. According
as the copper salt or fustic extract predominates, the colour of the
precipitate inclines to blue or yellow.
Elsner’s green is also sold under the name of ”non-poisonous green”.
This description is incorrect; the pigment is indeed free from arsenic,
but is poisonous on account of the copper it contains.
=Casselmann’s Green= approaches emerald green in brightness. It
consists of a compound of copper sulphate with copper hydroxide
and water, CuSO₄.3Cu(OH)₂.½H₂O. This pigment is obtained by mixing
solutions of 4 equivalents of copper sulphate and 3 equivalents of
sodium acetate at a certain temperature. The author’s experiments
have shown that the best results are obtained when the solutions are
mixed at a temperature of about 100° C. With this object the solutions
are placed in vessels standing in a pan of boiling water; when their
temperatures have risen to about 100° C. they are quickly mixed, the
mixture stirred, and the precipitate allowed to settle. When the
precipitate is cautiously treated with very weak caustic soda solution
a rather deeper colour is obtained. This treatment should not be
continued too long, or the precipitate may acquire an ugly bluish shade.
=Lime Green= is a mixture of copper arsenite and calcium sulphate.
It is thus a pigment which ought no longer to be used, on account of
its poisonous properties. It is made by boiling milk of lime with
excess of arsenious acid so long as the latter is dissolved. To this
solution of calcium arsenite, copper sulphate solution is added so long
as a precipitate is formed. A mixture of copper arsenite and calcium
sulphate is precipitated.
=Patent Green= is similar in composition to lime green. A solution of
calcium acetate is made by adding quicklime or pure powdered limestone
to acetic acid; copper sulphate solution is then added, when gypsum is
precipitated and copper acetate remains in solution. A hot solution of
arsenious acid is next added, and the precipitated copper salt mixed
with the gypsum at the bottom of the vessel.
=Copper Borate.=—A pale green precipitate of copper borate is obtained
by adding a solution of 3 parts of borax to a solution of 2 parts of
copper sulphate. It must be dried at a very moderate temperature, or it
will decompose. When perfectly dry the precipitate can be heated to a
red heat without decomposition; according to the temperature employed
different shades are obtained. It is most convenient to take tests out
of the crucible from time to time during the ignition, and to quickly
cool the crucible when the desired shade is obtained. When levigated
this pigment may be used in oil, or as a porcelain colour.
=Copper Silicate (Egyptian Blue).=—When a solution of water-glass is
added to a solution of copper sulphate a pale green precipitate of
copper silicate is obtained, which can be heated to redness without
alteration. When 70 parts of white quartz sand, 15 parts of copper
oxide, 25 parts of chalk and 6 parts of soda are fused together a glass
is produced which, after pouring into water, grinding and levigating,
exhibits a pretty blue and very permanent colour. It appears from the
examination of the colours of the Egyptian mural paintings that this
pigment was already known to the ancient Egyptians.