GREENS.
Green pigments form an important and numerous class, but many of
those which possess the most brilliant and durable qualities contain
highly poisonous ingredients, and some of the most beautiful are
not permanent. All things considered they are perhaps the least
satisfactory group of colouring matters. The following list comprises
all worth notice.
BARYTA GREEN.--It is said that the manganate of baryta makes
an excellent green pigment, which may with advantage replace for
many purposes those greens which contain arsenic. Several methods of
preparing it have been published:--(_a_) One consists in igniting
together the nitrate of baryta and manganese oxide or dioxide. (_b_)
Another consists in fusing a mixture of pyrolusite or black oxide of
manganese, caustic bartya, and chlorate of potash. (_c_) According
to a third method, mix 2 parts caustic soda and 1 part chlorate of
potash, and gradually add 2 parts very finely powdered manganese; heat
gradually up to dull redness, then allow to cool, powder, and exhaust
with water; filter and cool, and add a solution of nitrate of baryta
to the filtrate; a violet-coloured baryta precipitate forms; this is
carefully washed, dried, and treated with ½-1 part of caustic baryta,
hydrated, and gradually heated up to redness, with constant stirring.
The cooled mass is powdered, and finally washed to remove any excess of
baryta.
By either process a green mass is obtained, but the second method
seems to yield a more beautiful and homogeneous product. In
experimenting with other and more direct methods for preparing a
baryta green of great purity and beauty, Fleicher has made several
observations of its properties. If a green solution of manganate of
potash be precipitated, while boiling, by chloride of barium, a heavy,
granular, but not crystalline, precipitate of manganate of barium is
obtained. This precipitate has a violet colour, approaching blue, can
be washed by decantation at first, and afterwards may be collected on
a filter. On drying the precipitate, its colour grows lighter with
the increase of temperature; and on being heated to a dark red heat,
it looks almost perfectly white, with only a shade of greyish blue.
If, then, it be heated still higher with free access of air, or in an
oxidising flame, it gradually turns green; by carrying the process
farther the colour becomes a beautiful greenish-blue, and finally,
at a very high heat, a dirty greyish-brown mass is formed from the
reduction of the manganic acid to binoxide of manganese. On adding
chloride of barium to a solution of the permanganate of potash, and
boiling, a precipitate is slowly formed of a peach-blossom colour,
while the liquid retains a deep violet colour. By decanting and
bringing the mass, diluted with water, on a filter, the precipitate is
not decomposed, and can be dried at 212° F. without changing colour.
When the dry permanganate of barium is gradually heated, its colour
also grows paler, but does not, like the manganate of baryta, acquire
a green colour at a still higher temperature; for after the colour
has once vanished, an increase of temperature soon converts it into
the greyish-brown mixture of the binoxide of manganese and baryta
or carbonate of baryta. Hence it is impossible to prepare the green
manganate of baryta from the permanganate.
In regard to the colour itself, experiments have shown that the most
beautiful green is that formed by igniting the manganate as described
above. The green prepared by Rosenstiehl’s process--fusing together
caustic baryta, chlorate of potash, and binoxide of manganese--is less
beautiful than the above; while that attained from nitrate of baryta
and binoxide of manganese is far inferior to either of the others.
Perhaps, however, this colour could be improved by preparing it in a
reverberatory furnace with a strong oxidising flame.
The blue-green baryta pigment has different shades, according to its
preparation, some being almost pure blue with only a shade of green,
and resembling the light blue quill feathers of many parrots. The
greener the colour the more it gains in intensity, but it loses in
fineness, although still surpassing the green manganate of baryta.
The production of the blue or bluish-green baryta is due entirely to
the alkaline property of the mass. Whether each definite colour is due
to a definite composition is doubtful, since the temperature, which
must not exceed that of a bright red heat, exerts a greater influence
on the colour. This much is however certain, that both manganic acid as
well as the permanganate of baryta, when mixed with about 20 per cent.
of hydrate of baryta and ignited at a red heat, will always produce
this blue-green colour. It is evident that the blue-green colour is
dependent entirely on its basic character; for on placing this powder
in weak acids, it first turns green and is then gradually decomposed.
The baryta pigment is quite permanent, and may be subjected to the
action of strong sulphuric acid for hours, at the ordinary temperature,
before the colour will be destroyed. Boiling potash solution has no
perceptible effect upon it. The permanence, especially of the blue
shade, is increased by adding a little baryta, which increases its
alkalinity. It is also worthy of remark that the pigment prepared from
the nitrate of baryta is much less permanent, because the nitrous acid
present will after a time exert a reducing action.
The baryta pigments seem especially adapted to fresco painting, because
they appear very bright and lively on stone, and especially on lime,
where many other pigments lose their beauty or are entirely destroyed.
BREMEN GREEN.--This old-fashioned pigment is a basic carbonate
of copper, and has been produced in several ways. At first a basic
chloride or oxychloride was used, its mode of preparation varying
somewhat but without affecting the character of the result, the great
essential being that no subchloride of copper should be present.
Therefore, in some factories, it was the practice to prepare the magma
of basic oxychloride even a year in advance; or, to subject it to
repeated wetting and drying in order to ensure prefect oxidation. The
method has now become obsolete, and is superseded by the following:--
When neutral nitrate of copper is decomposed by an insufficiency of
a potash carbonate solution, the flocculent precipitate of copper
carbonate formed at first is gradually changed into a subnitrate of
copper which is precipitated as a heavy green powder. In practice the
operation is conducted as follows:--Copper scales are calcined in a
reverberatory or muffle furnace, till all the suboxide is converted
into protoxide, or until a sample dissolves in nitric acid without
evolution of red nitrous vapours. The copper nitrate solution is heated
and decomposed by a clear solution of potash carbonate, and when the
effervescence subsides, small doses of potash carbonate solution are
added, till but little undecomposed copper remains in the solution. To
recover this last portion, the clear liquor is decanted, and the green
precipitate is washed several times with small quantities of water. All
the liquors are collected, and the remaining copper is precipitated by
potash solution. The green carbonate of copper is introduced into a new
solution of copper nitrate, in which it is transformed into a basic
salt. The previous liquors are evaporated till they afford crystals of
nitrate of potash, which is a valuable secondary product.
BRIGHTON GREEN.--The following recipe has been published for
making this pigment. Dissolve separately 7 lb. sulphate of copper and 3
lb. sugar of lead, each in 5 pints of water; mix the solutions, stir in
24 lb. of whiting, and when the mass is dry grind to powder.
BRUNSWICK GREEN.--(_a_) Old process.
The Brunswick green of former days was closely allied to Bremen green,
essentially consisting of a basic chloride or oxychloride of copper,
and possessing all the faults incidental to that class of copper salt.
While having fairly good covering power, and capable of being used
either as a water colour or an oil colour, it was tedious and therefore
expensive to prepare, and not thoroughly durable under exposure to
air and sunlight. Nevertheless it was a useful bluish-green pigment.
Following are some of the many methods by which it has been prepared:--
(1) Poor oxidised copper ores are moistened with hydrochloric acid,
and spread out exposed to the air. The metal is thus rendered very
susceptible to the action of chlorine, and is even attacked by
solutions of ammonium chloride and of common salt. The sub-chloride
produced is rapidly transformed into oxy-chloride, and forms a fine
light-green pigment.
(2) Place 2 parts by weight of copper-filings in a vessel capable
of being tightly closed, and over them pour 3 parts by weight of
salammoniac in the form of a saturated aqueous solution. Keep the
mixture in a warm place for some weeks and thoroughly agitate it
occasionally. In due time the newly formed oxychloride is removed from
the vessel, and separated from the non-oxidised copper by washing on a
sieve. This washing must be continued until all traces of alkali have
been destroyed, when the pigment is drained, and very slowly dried at a
low temperature to avoid decomposition.
(3) Copper scrap is covered with a concentrated solution of chloride
of copper and allowed to remain until the chloride has undergone
conversion into basic chloride. The latter is then subjected to the
straining, washing, and drying treatment prescribed in (2).
(4) In a lead-lined vessel, place a quantity of copper filings or
waste, and add to it two-thirds of its weight of common salt, and
one-third of its weight of concentrated sulphuric acid, the latter
being however first diluted by admixture with three times its volume
of water. The mass is left to stand, with occasional stirring till all
the copper has been transformed into oxychloride, when it is strained,
washed, and dried as in (2).
(5) A modification of (4) is to put the copper scrap into a wooden
vessel, and cover it with an equal weight of common salt and an equal
weight of sulphate of potash dissolved in water. After standing and
agitation as before, the oxychloride is formed, and the straining,
washing, and drying are repeated.
(6) A solution of crude carbonate of ammonia is added to a mixed
solution of alum and blue vitriol so long as any reaction takes place.
When it is completed, the precipitate is collected, washed, and dried
as in the other cases.
(7) Lighter shades are produced by the addition of alum, or of sulphate
of baryta.
(_b_) New process.
The modern Brunswick greens, which are made in a variety of shades,
and sometimes known as chrome greens, Prussian greens, Victoria
greens, and by other fancy names, really consist of a white pigment
as a basis--usually sulphate of baryta (barytes), but occasionally
also sulphate of lime (gypsum) and sulphate of lead--coloured green
of varying intensity and depth by addition of a blue pigment in the
shape of Prussian blue, and a yellow in the guise of chrome-yellow.
There are what may be called four distinct standard shades recognised
by colour-makers, viz. “pale,” “medium,” “deep,” and “extra deep”; but
inasmuch as every manufacturer adopts a formula of his own, there may
be appreciable differences among colours of the same nominal standard
if by different makers. Taken as a whole, about three-fourths of their
total weight consists of the foundation white pigment, usually barytes;
about 1 to 6 per cent. is Prussian blue, according to the shade; and 14
to 18 per cent. chrome yellow; but there are brands occasionally met
with which depart considerably from these average figures.
The actual ingredients employed to form these green pigments are
essentially different, according as the wet or the dry method of
combining them be adopted. In selecting the various ingredients the
following points must be borne in mind. The Prussian blue of every
maker is not the same in quality, and while the character of the blue
is not of the foremost importance when dark greens are being made, for
light shades of green, on the other hand, it is essential to select
only the best and brightest brands. In the same way the tint and
quality of the chrome yellow are liable to considerable fluctuation,
and it is almost impossible to ensure two lots having exactly the same
characteristics, consequently the only way in which a certain shade of
green can be ensured is by experimental trial with small quantities for
each batch. Middle chromes can be used for deep greens, but only the
lemon chromes for pale shades. Regarding the barytes which forms the
basis of the pigment, there are no special precautions to be observed;
and the same may be said of the gypsum, should that be adopted as a
substitute for the barytes, except that 1 part by weight of gypsum
takes the place of about 2½ parts of barytes. The latter, however,
is much the more commonly used. For the dry method of compounding
Brunswick greens, the above named ingredients are all that are required.
In the wet method there is this essential difference, that it is
sought to precipitate the blue and yellow colours upon the inert base
by bringing about certain reactions, and therefore while the base
remains the same as in the dry method, the colouring media are totally
distinct, consisting of lead acetate, bichromate of potash, sulphate
of iron, and yellow or red prussiate of potash. The chief condition
to be observed with regard to the lead acetate is that it shall be in
the proportion of slightly more than three to one of the bichromate
of potash; in other words, the bichromate should be a trifle less
than one-third the weight of the lead acetate. As to the iron salt,
if commercial acetate or nitrate of iron could be bought of constant
quality or purity, that would be the most convenient form; but failing
that, recourse is had to freshly made and good quality sulphate of iron
(green copperas). It is found that the best results are secured when
the weight of the sulphate of iron is exactly the same as that of the
prussiate of potash. On the score of economy, the yellow prussiate of
potash (ferrocyanide) is employed, but the red prussiate (ferricyanide
of potassium) gives better and more certain results, and should be
adopted when making a superior paint which will command a higher price.
As to the comparative merits of the wet and dry systems of mixing the
ingredients of Brunswick greens, preference must be given to the former
on the score of quality of the pigment produced, but on the other hand
it entails much more trouble and skill, and there never can be the
same degree of control over the conduct of the operation or the shade
of colour developed. The dry method, however, though much more easily
carried out, and enabling the exact shade desired to be obtained to a
nicety by adding a little more of either the blue or the yellow during
the process of manufacture, is seldom adopted, because the quality and
fineness of the tints thus secured are much inferior.
The modus operandi with the wet method is as follows:--The barytes, in
the requisite fine state of subdivision, is very thoroughly stirred up
with water in a capacious vessel fitted with an agitator, the water
being in sufficient quantity to make quite a fluid mass. In convenient
proximity to the barytes tank, and elevated above it, provide three
other tanks of lesser capacity furnished with means of discharging
their contents into the barytes tank. In one of these smaller tanks
dissolve the green copperas in cold water; in another, the sugar of
lead; and in the third the bichromate and prussiate of potash together.
When all the salts are thoroughly dissolved, and while the barytes is
kept in constant agitation, admit first of all the copperas solution,
then the lead acetate solution, and finally the combined bichromate
and prussiate solution, never allowing the stirring to slacken till
after the last drop of these solutions has been introduced. When the
commingling of all the ingredients is judged to be complete, the green
pigment formed is allowed to subside, and the clear supernatant fluid
is siphoned off. The pigment is washed several times by admitting
clean water, agitating and settling, and finally is removed, drained
on a filter, and slowly and carefully dried. Many ways of arranging
the apparatus will suggest themselves, the chief point to keep in mind
being to economise labour as much as possible.
The dry method of mixing is simplicity itself in comparison with the
above, and merely entails putting the component materials--barytes,
chrome-yellow and Prussian blue--through an edge-runner mill
simultaneously, in the proportions adapted for producing the shade
required.
In giving formulæ for compounding these Brunswick greens, it must be
understood that they are not absolute, as every manufacturer adopts
his own particular proportions for a certain shade, but they form
a sufficiently approximate basis from which to work. They are all
computed for 100 lb. of barytes forming the body of the new pigment:--
_Pale_: Wet--1 lb. each copperas and prussiate, 12 lb. lead
acetate, 3¾ lb. bichromate.
Dry--80 lb. chrome yellow, 1¼ lb. Prussian blue.
_Medium_: Wet--1½ lb. each copperas and prussiate, 12½ lb. lead
acetate, 4 lb. bichromate.
Dry--30 lb. chrome yellow, 2¼ lb. Prussian blue.
_Deep_: Wet--2 lb. each copperas and prussiate, 13 lb. lead
acetate, 4¼ lb. bichromate.
Dry--30 lb. chrome yellow, 4½ lb. Prussian blue.
_Extra deep_: Wet--3½ lb. each, copperas and prussiate, 14½ lb.
lead acetate, 4½ lb. bichromate.
Dry--30 lb. chrome yellow, 7 lb. Prussian blue.
The Brunswick greens are in the front rank of green pigments so far as
covering power is concerned, and, when made from reliable materials,
are reasonably durable under the influence of air and light, in which
respect, however, they vary considerably. They can be used as water
colours, but are superior in oil paints. Precautions are necessary
in mixing them with other pigments. By the action of sulphuretted
hydrogen, or sulphur in any form, the colour is darkened to a notable
degree; by the action of acids, the chrome is destroyed and the green
becomes blue; by the action of alkalies, both the blue and the yellow
constituents are affected, and the green gives place to a reddish hue.
The pale and medium shades are yellow greens; the deep and extra deep
are blue greens.
These colours can be distinguished by heating them with caustic soda,
which turns them brownish in tone, owing to the destruction of the
Prussian blue. If the residue be filtered, and to the filtrate some
acid and ferric chloride be added, a blue precipitate will be obtained,
indicative of the presence of Prussian blue. On washing the residue
with water and treating with hydrochloric acid, the brown colour
disappears, and, in most cases, only a white residue of barytes is
left; sometimes the residue may have a faint yellowish colour. The
solution in hydrochloric acid will give the characteristic tests for
iron. The yellow element can be recognised by boiling in hydrochloric
acid, filtering, and allowing the filtrate to cool, when crystals of
lead chloride will deposit; these, separated out and dissolved in
boiling water, will give the characteristic tests for lead, such as
a white precipitate with sulphuric acid, and yellow precipitate with
bichromate of potash. The filtrate will have a green colour, indicative
of chromium.
CHINESE GREEN.--Another name for the vegetable pigment known
in China as Lokao (q.v.)
CHROME GREEN.--This name is often applied to any green in
which chrome enters as an element, but more particularly to the modern
Brunswick greens described on pp. 114-118; and to the green which bears
the name of its first maker, Guignet, and described under the title of
Guignet’s Green, see p. 125.
COBALT GREEN.--This remarkably stable, but somewhat costly,
pigment is also known by the names of Rinmann green and zinc green,
the former after the name of the chemist who first prepared it, and
the latter because it contains a large proportion of zinc. It is in
fact a combination of the oxides of cobalt and zinc, and was originally
produced in the following manner:-½ lb. pure cobalt ore was dissolved
in 4 lb. concentrated nitric acid, and added to a solution of 1 lb.
zinc in 5 lb. nitric acid; the mixture was diluted with water, and
a solution of potash carbonate was added, throwing down a pinkish
precipitate, which was washed on a filter, dried, and calcined at a
high temperature.
Wagner found that an indispensable condition was to have a protoxide
of cobalt as free as possible from foreign metals, with which object
he practised the following method:--Cobalt oxide is dissolved in three
equivalents of hydrochloric acid, and the solution is evaporated to
dryness; the residue is dissolved in six equivalents of water, and
through the solution is passed a current of sulphuretted hydrogen
gas, so long as any precipitate is formed. This precipitate consists
of sulphides of the foreign metals. The clear solution is siphoned
off, evaporated to dryness, and the residue is dissolved in water. As
required, this solution is treated with carbonate of soda, and the
precipitate, washed, and while still wet, is mixed with zinc white. The
reddish mass produced in this way is dried and calcined. The best tone
is attained by combining 9 to 10 parts of zinc oxide with 1 to 1½ parts
of cobalt protoxide.
Louyet has shown that if the cobaltic solution be precipitated by the
phosphate or the arseniate of potash, the corresponding salt of cobalt
thus produced possesses the property of imparting a green colour to
zinc white at a much lower temperature than is required in the case
of ordinary protoxide of cobalt: moreover, the pigment gains in body,
and the colour gains in purity and brightness. If a small quantity of
arsenious acid is added to the ordinary mixture before calcination, the
calcined mass will assume a remarkably bright green colour; and its
structure being loosened by the disengagement of fumes of arsenious
acid, it will be easy to grind.
According to Barruel and Leclaire’s method, 1 lb. of pure dry sulphate
of cobalt, dissolved in hot water, is mixed with 5 lb. of zinc oxide.
The mixture is dried, and calcined for three hours at a clear red heat
in a muffle; when cooled, it is thrown into water, washed, and dried.
The composition of cobalt green has been shown by Wagner to vary
considerably, as is to be expected from the methods of its preparation.
The proportion of zinc oxide ranges from 71½ to 88 per cent., and the
cobalt protoxide from 11½ to 19 per cent.; in addition, there will be
fluctuating percentages of phosphoric acid, soda, oxide of iron, &c.,
according to the process followed.
With the single exception of its costliness, cobalt green possesses
advantages over most other green pigments. It has a bright colour,
sometimes inclining to a yellowish tint, or, when phosphates are
used in its preparation, leaning to a blue shade. But it is always
permanent, not only under the influence of air and light, but also in
the presence of alkalies and any but concentrated acids; thus it may
safely be compounded with other pigments.
DOUGLAS GREEN.--This pigment, which is fairly permanent, and
possessed of considerable covering power, owes its name to the chemist
who proposes its use, and its colour to the oxide of chromium. The
method by which it is prepared is as follows:--Solutions of barium
chloride and potassium chromate are mixed together. To the barium
chromate thus produced is added one-fifth of its weight of concentrated
sulphuric acid, whereby partial decomposition is brought about,
resulting in a mixture of barium chromate, barium sulphate, and chromic
acid. This mixture is dried, and calcined in a crucible at bright red
heat, the effect of which is that the chromic acid is converted into
green oxide of chromium, and, being scattered throughout the mass,
imbues it with a green colour.
EMERALD GREEN.--This is quite an old-fashioned pigment,
having been in use some 80 years. It is a combination of acetate and
arsenite of copper, and varies in tint from a dark to a pale green,
always with a bluish cast. It possesses good covering power, and can
be used either as an oil-or as a water-colour, but particularly as the
latter, and is much used in paper staining. In composition it varies
considerably, as there are some half-dozen industrial methods of making
it; but in general terms it usually contains over 50 per cent. of
arsenious acid, and about 30 per cent. of oxide of copper, together
with various impurities. Following are some of the processes by which
it is manufactured.
(1) According to the method introduced by Liebig, 1 part of verdigris
is heated in a copper kettle with sufficient distilled vinegar to
effect its solution, and to this is added a solution of 1 part of
arsenious acid in water. The result is a precipitate of a dirty green
colour, which is dissolved in a new quantity of vinegar and boiled for
some time. In this way is obtained a new precipitate, granular and
crystalline, and exhibiting a splendid green colour. When this has been
filtered off, washed, and drained, it is boiled with one-tenth of its
weight of commercial potash, in order to deepen and brighten the colour
and destroy the bluish tint. Should the waste liquor obtained after the
filtration of the pigment from the second boiling in vinegar contain
any remaining copper, arsenious acid is added; and if arsenious acid be
present, copper acetate is added; while if acetic acid survives it may
be used again for dissolving another lot of verdigris.
(2) Form a paste with 1 part verdigris in sufficient boiling water,
pass it through, a sieve to remove lumps, and gradually add it to
a boiling solution of 1 part arsenious acid in 10 parts water, the
mixture being constantly stirred until the precipitate becomes a heavy
granular powder, when it is filtered through calico, and dried very
carefully.
(3) Acetate of copper is mixed with a sufficient quantity of water
heated to 122° F., to make a homogeneous and liquid paste. To 10 parts
of acetate of copper in this condition is added a solution of 8 parts
of arsenious acid in 100 parts of boiling water, the whole being then
kept in a state of ebullition. The addition of a little acetic acid
helps to develop the beauty of the colour. When precipitation is
complete, the clear liquor is drawn off, and forms a convenient solvent
for the next charge of arsenic, the operation being facilitated by
adding a little carbonate of potash, forming an arsenite of potash. The
precipitate constituting the desired green pigment is filtered off and
dried at the lowest effective temperature.
(4) Dissolve 5 lb. of sulphate of copper in water, and add to it a
solution of 1 lb. of lime in 2 gallons of vinegar. Mix 5 lb. of white
arsenic with sufficient water to form a paste. Add the arsenic paste to
the copper and lime mixture, and leave the whole at rest in a moderate
degree of heat. Mutual decomposition slowly ensues, with consequent
formation of the green pigment, which is filtered off, washed, and
dried with the same precaution as before.
When sulphate of copper is used in the production of emerald green,
it is very desirable that it shall be free from sulphate of iron,
which is a common impurity in the commercial article, and greatly
detracts from the purity and brilliance of the pigment. A good method
of eliminating this iron is to add to the sulphate of copper solution
a small quantity of a gelatinous precipitate of carbonate of copper,
produced by decomposing a copper sulphate solution by a soda carbonate
solution, and washing. On adding the gelatinous carbonate of copper,
with agitation, the iron is soon thrown down in flakes of oxide, and
pure sulphate of copper may be filtered off.
(5) Braconnot proceeds as follows:--A solution of 3 lb. of sulphate
of copper is made in a small quantity of hot water; and a second
solution of 3 lb. of arsenious acid and 4 lb. of commercial carbonate
of potash in boiling water. When the evolution of carbonic acid gas
has ceased, the two liquors are mixed together while being kept
continuously stirred; the result is an abundant precipitate of a dirty
yellowish-green colour. On adding a slight excess of acetic acid, a
fine crystalline green is developed; this is washed with boiling water
on a filter, and dried very slowly and carefully.
(6) A rough and ready process is to mix white arsenic with water, and
then stir in an equal weight of verdigris, allowing the mixture to be
at rest for a time in a moderately warm temperature till the pigment is
completely precipitated, when it is washed on a filter, and dried very
gradually.
(7) A method due to Köchlin is described in the following terms:--An
aqueous solution of sulphate of copper is made by adding 100 grammes
of the salt to 500 cc. of water. To this, when solution is complete,
is added 187½ cc. of a solution of arsenite of soda, which is of the
strength represented by 500 grammes of arsenite in 1 litre of water.
The result is that a precipitate of arsenite of copper is thrown down.
This precipitate is treated with 62 cc. of acetic acid at 11° to 12°
Tw., or half that quantity of pure formic acid, for one hour, at a
temperature ranging from 104° to 122° F. The pigment thus produced
is of good colour, but its superiority would not seem to justify the
use of such an expensive article as pure formic acid, nor the minute
adjustment of the proportions of the ingredients, in an operation to be
conducted on a commercial scale.
(8) Another complicated process has been invented by Prof. Galloway,
which, under skilled supervision, and when the correct proportions of
the several ingredients have been ascertained by careful experiment,
may give good results, but several precautions have to be observed
which cannot be entrusted to ordinary factory hands. The principle
of the process is that when a quantity of sulphate of copper is
dissolved in water, sufficient carbonate of soda is added to throw down
one-fourth of that copper sulphate as carbonate of copper, and then so
much acetic acid is introduced as will convert that copper carbonate
into acetate. In order to convert the balance of the copper sulphate
into arsenite, a solution of arsenic in boiling carbonate of soda is
made and added to the copper acetate solution, both solutions being at
a boiling temperature.
Emerald green is a pigment which possesses considerable stability in
dry pure air, but in damp atmospheres it becomes brown; in the presence
of acid or ammoniacal vapours it turns blue, and under the influence
of sulphuretted hydrogen it blackens; moreover, strong alkalies
destroy it. Consequently it cannot be used in many situations, nor
in association with such pigments as contain sulphur compounds. In
decorative painting it is difficult to apply on large flat surfaces,
and necessitates stippling in order to get it to lie well; but when
stippled on a ground of proper green it develops an exceedingly
beautiful bloom-like appearance.
Its peculiar shade distinguishes it from all other green pigments,
none of which approaches it in the paleness and brightness of its
colour. It can be distinguished by the fact that it is soluble in acids
and ammonia, to a blue solution which does not change on boiling. In
caustic soda it also dissolves with a blue colour: on boiling, a red
precipitate of cuprous oxide falls down. No other green pigment answers
to all these tests.
There are a good many imitation emerald greens on the market, some
of which are offered as genuine emerald greens, others as “emerald
tint” green, which is much more honest. The composition of these
greens necessarily varies greatly, some are prepared from coal tar
greens, others by careful admixture of various green, blue, and yellow
pigments. If the tint of these substitutes is right and they are sold
for what they are, there is no reason why they should not be used
in place of the real article, over which they have the advantage of
not being poisonous, which is a great disadvantage of the genuine
emerald green. Although one authority disputes this point, certainly
the poisonous action of emerald green varies very considerably with
different individuals. The genuine emerald green may be distinguished
from the spurious by being perfectly soluble in acids and alkalies,
which the imitations are not; the character of the latter must be
inferred by the application of a few special tests, the nature of
which will be readily deduced from what is said as to the properties
of other green pigments. Emerald green should be assayed for purity
and tint; this is important, as pure emerald green has a tint of its
own, which is difficult to imitate, and sometimes really pure emerald
green offered for sale is of a defective tint, due to some fault in the
process of manufacture. Such samples should be rejected.
GUIGNET’S GREEN.--The greens of this class, which owe their
colour to chromium oxide, are also known as “chrome greens,” a name
which they share with a totally different group into whose composition
chrome yellow enters as a constituent, and which have been already
described under the synonym “Brunswick greens,” on pp. 114-118.
Though one of the simplest of chemical products, a great many ways of
preparing chromium oxides have been proposed. One of the earliest for
industrial application was that of Guignet, who has given his name to
the pigment, and this may fitly commence the long list.
(1) The first method adopted by Guignet consisted in mixing bichromate
of potash with three times its weight of boracic acid and moistening
the mass with just sufficient water to form it into a thick paste.
This paste is put on the hearth of a reverberatory furnace, which is
carefully heated to a point never exceeding a dark red heat; if this
precaution is neglected, the mass, instead of becoming porous, will
fuse entirely, and the anhydrous oxide will be produced, which has a
pale-green colour. The heated paste, while still red hot, is thrown
into cold water and washed with boiling water, in order to remove
borate of potash in solution; and this solution, when boiled down
and treated with hydrochloric acid, can be made to yield up most of
the boracic acid it contains. The filtered and washed residue is the
hydrated oxide of chromium.
(2) A modification of (1), followed by Guignet, was to replace the
bichromate of potash by chromate of soda, prepared by dissolving in
boiling water 61 parts of neutral chromate of potash and 53 parts of
nitrate of soda. For the neutral chromate of potash, also, may be
substituted a mixture of 92 parts of bichromate of potash and 89 parts
of crystallised carbonate of soda, the nitrate of soda remaining as
before. On cooling, in either case, the solution deposits much nitrate
of potash, which is commercially valuable. The chromate of soda present
in the mother liquors is obtained by evaporating to dryness. The
pigment produced by the chromate of soda process is lighter in colour
than that obtained with bichromate of potash. It may be still further
paled by adding a little alumina, baryta, or other white pigment to the
bichromate and boracic acid mixture before calcining.
(3) Equal quantities of potash bichromate and potato starch are
thoroughly mixed and then calcined in a crucible at a high temperature.
The product is washed with boiling water, to remove the potash
carbonate formed, and any remaining undecomposed bichromate. The
precipitated chromium oxide is filtered, dried, and again calcined to
drive off the water. The final result is a handsome pigment which flows
well from the brush.
(4) On heating in a crucible a mixture of 3 parts of neutral chromate
of potash with 2 parts of salammoniac, the two salts are decomposed,
the result being formation of chromium oxide mixed with potassium
chloride, which latter is removed by several washings with hot water.
The brilliancy of the chromium oxide is enhanced by calcination at a
dull red heat.
(5) Fuse together 3 parts of boracic acid and 1 part of potash
bichromate at a dull red heat on the hearth of a reverberatory furnace.
Thus is formed a borate of chromium and potash, with evolution of
oxygen. The mass is repeatedly washed with boiling water, which causes
decomposition, and consequent separation of hydrated oxide of chromium,
and a soluble borate of potash. The chromium oxide is washed, and
ground very fine.
(6) When a solution of potash bichromate is poured into a neutral
solution of mercury proto-nitrate, it forms an orange-coloured
precipitate, which is washed and gently dried, then powdered, and
heated in a stoneware retort provided with an arm dipping into cold
water, by which the mercury is distilled and condensed. The residue in
the retort is a highly comminuted chromium oxide, of a fine dark-green
colour.
(7) On calcining potash bichromate in a crucible at a very high
temperature, it is decomposed, and results in chromium oxide and
potash, the latter of which can be washed out. The chromium oxide thus
obtained is very dense and of a dark-green colour resembling (6).
(8) Equal quantities of flowers of sulphur and bichromate of potash
are thoroughly mixed, and heated to redness in a crucible, producing a
mixture of oxide of chromium with sulphide and sulphate of potash. The
latter are dissolved out by washing repeatedly with hot water, leaving
the chromium oxide as a finely comminuted dense powder of an intense
green colour.
(9) A modification of (8) consists in adding small successive
quantities of flowers of sulphur to a boiling concentrated solution of
potash bichromate. From this results a gelatinous oxide of chromium,
which is washed with boiling water, dried, and calcined in a crucible
at a red heat.
(10) Hydrochloric acid decomposes bichromate of potash, forming a
soluble chloride of potash which can be removed by washing, and a
residue of chromium oxide, which is washed on a filter and dried.
There remain for description two or three processes in which phosphoric
acid plays a part, but the greens made by these methods do not possess
the freshness of the others, and it is difficult to see what advantages
can attend this modification.
(11) According to Arnaudon, 149 parts of bichromate of potash are
thoroughly incorporated with 128 parts of crystallised neutral
phosphate of ammonia, and the mixture is heated in thin layers to a
temperature between 338° and 356° F., which brings about intumescence,
change of colour, and disengagement of water and ammonia; the heating
is continued for half an hour, but must not be allowed to exceed
392° F. When the development of the green colour is complete, the
product is washed with hot water to remove soluble salts, and the
residue constitutes an impalpable powder of chromium oxide, forming a
leaf-green pigment.
(12) Dissolve 10 lb. of bichromate of potash and 18 lb. of phosphate
of soda in boiling water, and add to the boiling mixture 10 lb. of
thio-sulphate of soda solution and a little hydrochloric acid. A
precipitate of phosphate of chromium is gradually thrown down as the
boiling is maintained.
For general utility no class of pigments can exceed the several
forms of Guignet’s green. It is capable of affording a great variety
of tints, all absolutely permanent under reasonable conditions. No
ordinary agent will decompose them, and they will stand almost any
test to which they may be subjected without losing colour. They are
quite insoluble in acids and alkalies, and are not affected even by the
extreme heat of the glass furnace. They possess good covering power,
do not suffer in brightness or purity under artificial light, and are
equally useful as oil or water colours, besides being admirably adapted
for fresco and silicious painting, and employed in making green glass
and in calico printing. They can be mixed with any other pigment.
Adulteration with Brunswick or Prussian greens is often practised, but
may be discovered by a portion being dissolved on boiling with caustic
soda, the solution giving a precipitate of chrome yellow on adding
acetic acid, and (a separate portion, of course) Prussian blue with
hydrochloric acid and perchloride of iron.
LOKAO.--This pigment, which is also known as “Chinese green,”
was first met with as a sediment left after dyeing cotton cloths
with the barks of one or more species of buckthorn, notably _Rhamnus
chlorophorus_ and _R. utilis_, and passing in China under the general
name of Lo-Kao. This sediment is spread on blotting paper and thus
dried, forming thin cakes. Latterly, the juice afforded by the berries
of the same trees is extracted by pressure, absorbed by alum, and dried
in the same form of little cakes. When first introduced into England
it was highly valued as affording a pure green, even in artificial
light. Its price on the London market in 1861 was 7_s._ 6_d._ an ounce.
So long ago as 1853 it was imported into France and used for dyeing
silk. The colouring principle appears to consist of a glucose (lokaose)
and an acid (lokaonic acid). In 1864, Chauvin obtained an identical
colouring matter from _Rhamnus catharticus_, or the common buckthorn,
a shrub which grows wild in most parts of Europe, and found a ready
market for the pigment at 37_s._ a pound. This was simply the article
known as sap green (see p. 132.)
MALACHITE.--This is one of the names applied to mountain green
(q.v.).
MANGANESE GREEN.--Several formulæ have been published for
making a green pigment from manganese, as follows:--
(1) An intimate mixture of 80 parts of nitrate of barium, 14 parts
of oxide of manganese and 6 parts of sulphate of barium, is placed
in a crucible and heated to bright redness until the green colour
is thoroughly developed. The fused green mass is poured out of the
crucible, cooled, and ground wet to a fine condition.
(2) To 3 or 4 parts of caustic baryta moistened with water are added
2 parts of nitrate of barium and 2 parts of oxide of manganese; the
whole mass is most intimately mixed, then put into a crucible in a
furnace, and subjected to a dull red heat so long as may be necessary
for securing complete decomposition. When the green colour is
satisfactorily produced, the mass in a state of fusion is poured out,
cooled, pulverised, digested in boiling water, then washed with cold
water, and finally dried in an atmosphere which is free from carbonic
acid.
(3) The oxide of manganese may be replaced by the nitrate, when the
quantities are 46 parts nitrate of barium, 30 parts of sulphate of
barium, and 24 parts of nitrate of manganese; the fusion, grinding and
washing are repeated as before.
According to some recipes the powdery pigment, consisting essentially
of manganate of barium, is mixed with a little dextrine to make sure of
its stability, but it is not clear whether this is really essential.
MINERAL GREEN.--This is only another name for the green made
from copper carbonate, and described under mountain green, see p. 131.
MITIS GREEN.--This pigment is an arseniate of copper, and
bears a very close relationship to the emerald green made according
to Braconnot’s formula, and described in the fifth paragraph of that
section, see p. 123. Mitis green is prepared by dissolving arseniate of
potash in five times the quantity of hot water and adding a solution of
an equal weight of sulphate of copper, keeping the whole in constant
agitation. A pulverulent precipitate is formed, possessing a grass
green colour. This is washed and dried. The tint can be varied by
altering the proportions of the arseniate and sulphate. The arseniate
of potash is made by boiling arsenious acid in concentrated nitric
acid, filtering, and saturating with carbonate of potash. The arseniate
is allowed to crystallise out of the liquor.
MOUNTAIN GREEN.--This pigment is also known by the names of
malachite and mineral green.
(1) In its native form the mineral malachite or green carbonate of
copper is very widely distributed in Europe, Asia, America, and
Australia, but on a commercial scale it is chiefly produced in the Ural
mountains of Siberia and in the Banat of Hungary. It only needs to be
picked clean from adhering rock and to be ground to a very fine powder
in order to render it ready for use. It is much superior to any of the
artificial substitutes referred to below, but its cost confines its
application to artistic work.
(2) Sometimes a little orpiment or chrome yellow is ground up with the
malachite.
(3) A very simple formula for making the artificial pigment is to add
solution of carbonate of soda or potash to a hot mixed solution of alum
and bluestone (sulphate of copper).
(4) Other recipes for making mountain greens have been published which
bear no relation to the composition of the original article, _e. g._
by mixing a solution containing potash and arsenic with a solution of
bluestone; or, as a much more complicated example, treating a solution
of bluestone first with slaked lime, then with a solution of arsenic
and soda obtained by boiling in water, and finally with tartaric acid.
The advantages attendant on so much trouble in producing what is at
best an unstable pigment are not very apparent.
PARIS GREEN.--This is another name, used especially in
America, for the emerald greens described on p. 121.
PRUSSIAN GREEN.--A name often applied to class _b_ of the
Brunswick greens (see p. 114), or in other words those which are
prepared from Prussian blue.
RINMANN GREEN.--The first cobalt green (see p. 119), put on
the market was made by Rinmann, and hence it is still often called by
his name.
SAP GREEN.--This vegetable pigment or lake is closely allied
to the Chinese green or lokao, described on p. 129.
It consists of the solidified juice extracted from the berries of the
common buckthorn shrub (_Rhamnus catharticus_), which is obtained
either by allowing the berries to undergo slight fermentation for about
a week in wooden tubs, then pressing and straining; or by boiling the
berries, and straining off the juice. In either case the clean juice is
boiled down to a syrupy consistence, and a little alum (about ½ oz. to
the pint of thickened juice) is added, the liquor being then evaporated
to dryness, or very nearly to that point, the drying being left to
complete itself after the pigment has been ran into bladders.
The quality of this green is liable to serious fluctuation, owing to
the neglect or ignorance of certain simple precautions. Thus, for
a true green the berries should be selected before they have quite
reached maturity. The more nearly ripe the berries are, the more yellow
will be the tint of the green afforded by them. The boiling of the
berries, if followed, and the evaporation of the juice, must be done
at a low temperature, and the final stages of the evaporation cannot
safely be done with direct fire heat, but should be effected in a water
bath. The only substance incorporated with the juice should be potash
alum. Sometimes it is replaced by carbonate of magnesia (which destroys
the transparency of the pigment); or by carbonate of potash (which
introduces a stickiness or viscosity).
Sap green possesses too little body and is too translucent for use as
an oil paint; but being non-poisonous, and in fact perfectly harmless,
it finds many useful applications outside of water colour and pastel
painting, viz. in colouring alimentary substances such as drinks and
sweets. Its true colour is a leaf green, glossy and translucent. In
durability it is not remarkable.
SCHEELE’S GREEN.--For more than a century has Scheele’s green
been a familiar pigment, but the reputation it enjoyed in its early
days has long since departed, and it is now to be classed among the
inferior green colouring matters. It consists essentially of a basic
arsenite of copper, and contains from 8 to more than 40 per cent. of
arsenic, according to the mode of preparation, of which there are
several, as follows:--
(1) A mixture of 2 parts of commercial carbonate of potash and 1 part
of powdered arsenious acid (white arsenic), are dissolved in 35 parts
of boiling water; the solution is filtered clear, and then added
gradually and while still warm to a filtered solution of 2 parts of
sulphate of copper until no further precipitate goes down. This latter
is collected, washed with warm water on a filter, and slowly dried
without excess of heat.
(2) The preceding formula is modified by making one solution of the
arsenic and the sulphate of copper, and precipitating by adding the
carbonate of potash solution till the colour is fully developed,
agitation being constantly maintained.
(3) Another variation is to mix the arsenic with soda crystals in
boiling water, and to pour the arsenite of soda solution thus formed
into the bluestone solution, the boiling being kept up for a few
minutes.
Scheele’s green has a pale yellowish cast, and mixes well with either
water or oil, but it lacks brightness, durability, and covering power,
in addition to being highly poisonous, and though once much employed in
staining wall papers, is now generally discarded.
SCHWEINFURTH GREEN.--This is an old-fashioned name for emerald
green, which has been described on pp. 121-125.
TERRE VERTE.--Rendered into English, the name _terre verte_
means “green earth.” It is applied to a number of green-coloured earths
found widely distributed in rocks of various ages, but especially in
those of a basaltic or porphyritic character. In commercial quantity it
occurs notably in Cyprus and near Verona in Italy; the latter locality
is so important that the pigment is often known as “Verona earth.”
Notwithstanding minor points of dissimilarity in samples from different
sources, there is a great family likeness among them, sufficient to
indicate that the essential constituent is a silicate of iron and
magnesia. The other ingredients vary with the locality producing the
mineral. The same may be said of the physical characteristics, some
specimens being soft and earthy, while others are hard and glassy.
All possess the peculiar soapy touch of the magnesian earths, and a
clay-like odour. Analysis of a Verona earth gave:--
Per cent.
Silica 51·21
Iron protoxide 20·72
Magnesia 6·16
Water 4·49
Alumina 7·25
Soda 6·21
Manganese protoxide trace.
While a Cyprus earth showed:--
Per cent.
Silica 51·5
Iron protoxide 20·5
Magnesia 1·5
Water 8·0
Potash 18·0
The presence of copper would point suspiciously to adulteration, and in
any case should suffice to condemn the sample for use.
Naturally there is considerable variety of tint among the many kinds
of terre verte, but they all belong to the pale greyish class, and are
more or less translucent, consequently their covering power is small.
Their value lies in their durability, and the resistance they offer to
the injurious effects of strong light and impure atmosphere. They can
be employed either as oil or water colours. The only preparation to
which the natural pigments are submitted is fine grinding and washing.
TITANIUM GREEN.--An excellent dark green pigment, though
rather costly, can be prepared from rutile or any titaniferous iron ore
by the following method:--
The ore is dressed clean, and fused with twelve times its weight of
acid sulphate of potash in a crucible. When cool, it is reduced to
fine powder, and digested at 120° F. in dilute hydrochloric acid (half
water) until solution is complete. The hot solution is filtered off
from the residue and carefully evaporated down to a syrupy consistence,
when the nearly pure titanic acid is allowed to cool in the dish and
thrown on a filter. When sufficiently drained, it is boiled in a large
volume of water containing a little ammonia, and the precipitated
titanic acid is filtered and washed.
If an ore is used containing carbonate of lime, it must first be
treated with dilute hydrochloric acid before the sulphate of potash is
applied.
The titanic acid on the filter is next mixed with a concentrated
solution of sal ammoniac, and again filtered. Then it is digested
in dilute hydrochloric acid at 120° to 140° F. till the solution is
complete. On adding ferrocyanide of potassium to the acid liquor,
and bringing quickly to a boil, a precipitate of ferro-cyanide of
titanium is thrown down. This is very carefully and slowly dried, at a
temperature never exceeding 200° F.
VERDIGRIS.--The chemical examination of verdigris shows it to
be a basic hydrated acetate of copper, containing variable proportions
of the bibasic and tribasic acetates.
Commercially it is prepared in districts where acetic or pyroligneous
acid can be had at small cost. Thin pieces of scrap copper are
subjected to the action of fermenting grape skins in mass, or cider
refuse, for a fortnight or three weeks; or to the influence of
pyroligneous acid for four or five days. By this means the copper
surfaces are attacked by the acetic acid being generated or liberated,
and become coated with acetate of copper. At intervals the pieces
are removed, and surfaces are cleaned of the accumulated acetate or
verdigris and this is repeated till the metallic copper has thus been
completely converted. The collected verdigris is washed, and carefully
dried at a very low temperature.
Its composition is subject to many irregularities, and the colour
varies from green to bluish green according to the proportion of
sesquibasic acetate present. It is one of the least permanent pigments,
especially in the presence of water, and is exceedingly poisonous.
At one time it was largely used as a pigment, but is now gradually
going, if indeed it has not already gone, out of use. It can be
distinguished by its solubility in acids and ammonia, the latter giving
a deep azure blue solution. On being heated, it turns black, owing to
its parting with acetic acid and leaving the black oxide of copper
behind. This should be entirely soluble in nitric acid, the solution
giving the characteristic tests for copper. The solution should give
no precipitate with chloride of barium or nitrate of silver, and the
original pigment should be freely soluble in any acid and in ammonia
without effervescence.
VERDITER.--Green verditer is another of the copper greens
which has practically disappeared from the modern painter’s list of
pigments. It is a yellow tinted very fugitive colour, consisting of
a basic carbonate of copper, and is manufactured by treating copper
solutions with carbonate of soda, or of potash.
VERONA EARTH.--One kind of terre verte (see p. 134), is known
by this name because it is produced in the neighbourhood of Verona.
VICTORIA GREEN.--This is a fancy name for the Brunswick greens
compounded from Prussian blue, and already described on p. 114.
VIENNA GREEN.--The aceto-arsenite of copper described under
the heading of emerald green (see pp. 121-125), is sometimes called by
this name.
ZINC GREEN.--The pigments described under cobalt green (see
p. 119), as often pass by the name of zinc greens, and in fact they
contain much more zinc than cobalt.
A handsome but not permanent green may be made by combining zinc
with iron instead of cobalt, in the form of a double cyanide. The
process is as follows:--Finely powdered Prussian blue is stirred into
a concentrated solution of chloride of zinc, and put by to allow
the decomposition to take place. After some time, the precipitated
ferro-zinc cyanide is thoroughly washed, and dried out of reach of the
light.