GREEN LAKES.
The yellow colouring matters produce green compounds with copper salts.
These are important because they can be produced without great cost,
and are not as poisonous as the arsenic pigments, although copper
compounds are far more poisonous substances than should be used for
certain purposes. Their employment might be attended with danger if
used to colour children’s toys and pictures.
These green copper lakes are simply made by adding copper sulphate
solution, free from iron, to a hot decoction of yellow berries or
weld until the liquid is emerald green, and then adding caustic soda
solution in small quantities to precipitate the lake. The temperature
of the liquid should not be more than 50° to 60° C. when the caustic
soda is added. It has been maintained that these lakes turn out well
only when the precipitation is so conducted that the residual liquid is
quite colourless. The author has, however, found it more satisfactory
to discontinue the addition of caustic soda when the liquid is somewhat
coloured, since when the precipitation is complete other substances
besides colouring matter may be precipitated.
=Chlorophyll.=—All the higher plants contain the same green colouring
matter—chlorophyll. It has a very fine shade, is fairly fast, and the
raw material from which it is produced is to be obtained at nominal
cost. Special attention should, therefore, be paid to this colouring
matter.
Chlorophyll is very easily obtained. Grass of a good green, green
leaves, or any green portion of a plant is allowed to stand with weak
caustic soda solution in a large vessel for twenty-four to thirty
hours. The liquid is poured off, brought to the boil for a moment, at
once filtered and neutralised with hydrochloric acid. The chlorophyll,
which has been dissolved by the caustic soda, is then thrown down as a
grass green precipitate; after washing and drying it can be used as a
pigment. A chlorophyll lake can be obtained by dissolving the colouring
matter in caustic soda and adding alum solution. The green precipitate
is a compound of chlorophyll and alumina.
Tschirch gives the following method for obtaining chlorophyll: Green
leaves (grass), not containing tannin, are extracted with boiling
alcohol, the extract cooled, filtered and evaporated until a sticky
mass remains. This is washed with hot water until the washings are
colourless, and the residue extracted with cold alcohol. The solution
is evaporated to half its volume, the separated crystals are dissolved
in alcohol, and the solution heated on the water bath with zinc dust.
A deep emerald green solution with a red fluorescence is obtained; it
can be preserved unaltered for a long time in blue glass bottles. It
retains its colour in diffused daylight also for a lengthy period. A
beautiful green chlorophyll lake is obtained by boiling this solution
with the solution of an aluminium salt and precipitating with soda.
Unfortunately, the lake is not fast. The chlorophyll solution obtained
by this process is quite innocuous, and is well adapted for colouring
liqueurs and confectionery.
=Sap Green.=—This lake, also known as bladder green, is obtained from
unripe yellow (Persian) berries. It can be used as a water colour, but
not in oil painting. This pigment differs from the ordinary yellow
berry lake in colour and chemical composition; the yellow lake is a
compound of xanthorhamnin with the metallic oxide, whilst sap green
contains an uncrystallisable bitter substance, rhamnocathartin.
The lake is obtained from yellow berries which are not quite ripe.
They are broken up and left in a warm place. The mass soon ferments;
after about ten days it is pressed. Four parts of the liquid are then
mixed with 0·5 part of alum and 0·5 part of potassium carbonate. The
salts are dissolved in boiling water, and the solutions added to the
hot sap. The mixture is then evaporated to the consistency of a thick
syrup, which is generally packed in animal bladders, hence the name of
”bladder green”. Some care is necessary in the evaporation to prevent
the burning of the soft mass on the bottom of the vessel. If the
evaporation is carried somewhat further the mass solidifies on cooling;
it is then black, and only transmits green light at the edges. The use
of potash in preparing sap green is attended with the disadvantage that
the hygroscopic nature of the salt prevents the colour from drying. If
magnesia is used instead of potash the lake dries far more rapidly,
but has much less covering power. Sap green is principally used for
colouring paper and leather.
Orange-coloured lakes also can be obtained from yellow berries by
precipitating the decoction with stannic chloride. These lakes are not
directly employed, but are produced immediately upon the fabric in
dyeing.
=Chinese Green, Lokao.=—Under this description a green lake has
recently been imported from China. It is a valuable pigment. Chinese
green is sold in flat cakes, which are blue, with a green or violet
lustre. The powdered lake is pure green and partially soluble in
water; it contains colouring matter, water and inorganic matter
consisting chiefly of clay and lime.
Chinese green is made in China in a peculiar manner from the twigs
of certain species of _Rhamnus_. According to report, the bark is
boiled with water and a cotton cloth immersed in the decoction. The
fibre fixes a colourless substance which becomes green on exposure to
air. The cotton is repeatedly dipped into the decoction until it has
absorbed a large quantity of colouring matter. It is then washed with
cold water and boiled with water upon which cotton yarn is laid. The
colouring matter suspended in the liquid adheres to the yarn, which
is washed with a little cold water, and the colouring matter then
collected upon paper and dried.
If Chinese green is really made by this process, it is not a lake but
a vegetable colouring matter, to which clay has been added to increase
its weight or to make it plastic. Very fine lakes can be obtained from
Chinese green by dissolving it in a solution of alum, and adding soda
solution. When Chinese green is dissolved in acetic acid and ammonium
chloride and a zinc salt added, a blue lake is produced on the further
addition of sodium acetate. A bluish violet lake is obtained by
treating Chinese green with a strong reducing agent and adding calcium
acetate.
=Charvin’s Green.=—The very high price at which genuine Chinese green
was sold whilst it was still a novelty, gave rise to attempts at
imitation. Charvin, of Lyons, succeeded in producing a pigment from
_Rhamnus_ which has the characteristic property of Chinese green of
retaining its colour in artificial light. He plunged the bark of common
buckthorn into boiling water, boiled for a few minutes, and allowed
the whole to stand for twenty-four hours. Lime water was added to the
brown liquid, which was then exposed to the action of the air in
shallow vessels, when it gradually turned green and deposited a green
precipitate. When this appeared, the liquid was brought into glass
vessels and potassium carbonate solution added so long as a precipitate
resulted, which after drying had all the properties of genuine Chinese
green.
Although for some time Chinese green was a very fashionable colour, its
use is now almost discontinued, although it can be used with advantage
as an artists’ pigment, and its preparation by Charvin’s method is
neither difficult nor specially costly.