THE MANUFACTURE OF CARMINE.
The colouring matter of cochineal is used in the form of a lake under
the name of carmine, the finest and most expensive colour used in
painting. However simple the preparation of carmine may appear, it
is not easy to obtain a product of great beauty; certain conditions,
still unexplained, play an important part in the process. Until not
long ago the manufacture of fine carmine was regarded as a secret; this
it is no longer, and with proper care any one may produce an entirely
satisfactory product. Many recipes have been given, some of the best
will be mentioned.
Whatever method is used to obtain carmine certain precautions must
be taken, without which it is impossible to obtain a bright colour.
Alkalis and alkaline earths in very small quantity affect the shade of
carmine, so that spring water should never be used in its preparation.
Pure rain water, or, still better, distilled water, should be employed.
The decoction of cochineal is difficult to filter. Paper cannot be
used, because the pores are so rapidly stopped up that new filters
would be continually required. Fine silk is the most serviceable; it
should not be washed with soap—the alkalinity of the small quantity of
soap the fabric would retain would affect the colour. The decoction is
made in a well-tinned copper pan, all other vessels should be of glass
or porcelain, which are most easily cleaned, and great cleanliness
is the prime essential to the success of the process. The greatest
care is required to prevent the contact of the liquid with iron during
the whole process, the smallest trace of this metal would result in a
discoloured product.
The essentials of the manufacture of carmine on the large scale are
that the colouring matter is dissolved in water, and precipitated by
the addition of an aluminium salt, generally alum, absolutely free from
iron. The more slowly the carmine separates the finer is its colour.
It is generally observed that the last portions to precipitate show
the brightest shade. This is because the foreign substances occurring
with the carminic acid in the decoction are thrown down with the first
portions of the precipitate. In the process of Frau Cenette, famous for
the beauty of the product, a solution is made from which the whole of
the carmine is separated in about three weeks. During this long time
the majority of the substances which have been dissolved in the water
together with the carminic acid are decomposed; the liquid acquires an
unpleasant smell, and is covered with mould. In the author’s opinion
so good a product is obtained by this process because the greater part
of the impurities is decomposed: the nearer the product approaches the
pure compound of carminic acid and alumina, the purer and brighter will
be the shade of the carmine.
It is known that light has considerable influence on the beauty of
carmine. During dull winter days it is quite impossible to produce so
fine a product as in summer. Instead of alum a tin solution may be
used to precipitate the colouring matter; the shade of the product is
different to that of alumina carmine.
CENETTE’S METHOD.—1 kilogramme of finely powdered cochineal is boiled
with 75 litres of water for two hours; 90 grammes of saltpetre are
added, the liquid boiled for three minutes, then 120 grammes of salt of
sorrel (acid potassium oxalate) are added and the liquid again boiled
for ten minutes. The liquid is then completely clarified by standing,
drawn off from the residue by a syphon, and brought into shallow glass
dishes which are placed, protected from dust, in a bright light in
a uniformly warm place. During several weeks the carmine separates,
the last portions being always brighter than the first. The addition
of potassium oxalate has the object of assisting the separation of
the carmine, for acid salts separate the carmine from solutions; the
saltpetre may reasonably be omitted.
The majority of the recipes for carmine, which are frequently sold at a
high price, differ but little from the above; acid potassium tartrate
is used instead of the oxalate, but the latter is to be preferred
because of the slight solubility of the tartrate. It is important
not to use too strong decoctions of cochineal, and to add only small
quantities of alum. The clear liquid is placed in shallow glass dishes.
After a few days the nature of the deposit should be examined: if a
considerable quantity of a red precipitate has formed, the liquid is
poured off into other dishes, in which carmine again separates during
the following days, and usually of a brighter colour than the first.
For example, 125 grammes of cochineal are boiled with 5 litres of water
during fifteen minutes, 30 grammes of very finely powdered alum are
added to the boiling liquid, which is again boiled for a few minutes,
allowed to clarify and cool. The greater part of the carmine is then
obtained in a few hours, but the liquid still separates carmine after
several days.
According to another formula 500 grammes of cochineal are boiled with
30 litres of water, 60 grammes of cream of tartar are added, then
30 grammes of alum, boiling is continued for several minutes, and
the liquid then allowed to cool. Carmine may be obtained in a very
short time by means of tin solution. The process is similar to that
just given. The liquid which would be set aside for the spontaneous
deposition of the carmine is, however, returned to the pan, and a
solution of pure stannous chloride added in drops, so long as the
solution is still clearly red. The carmine separates at the bottom of
the pan; the liquid drawn off, even when it appears almost colourless,
produces a further small quantity of carmine in a few days.
According to J. J. Hess, a brighter carmine is obtained when the fat of
the cochineal is previously extracted by ether or benzene.
Proposals have often been made to deepen the colour of prepared
carmine; this must be done with the greatest care, for it is very easy
to produce a less handsome instead of a finer pigment. The carmine is
moistened with distilled water containing about 5 per cent. of ammonia
solution.
Carmine readily dissolves in ammonia. This property may be applied to
test its purity, pure carmine should dissolve without residue in 5 to
6 times its quantity of ammonia. Any considerable residue denotes an
intentional addition of some adulterant. Starch, vermilion, and cheaper
lakes are used for this purpose. The red solution obtained by treating
carmine with the above quantity of ammonia may be used as a red ink.
This solution can also be used to purify carmine; when it is allowed to
stand in an uncorked bottle for some time the ammonia escapes, and the
greater part of the carmine is deposited as a very fine powder.
Carmine solution is made by dissolving carmine in the necessary
quantity of ammonia, adding glycerine equal in quantity to the ammonia,
driving off the latter by heat and diluting the liquid. This solution
is well adapted for colouring confectionery, but cannot be employed in
painting and writing, since the glycerine would prevent it from drying.
Carmine is extensively used in painting, and for many purposes cannot
be replaced by another pigment, _e.g._, the cosmetic known as vegetable
rouge can be made from no other pigment. Carmine may be used in all
methods of colouring; it is quite harmless for confectioners’ purposes.
=Munich, Vienna, Paris, or Florentine Lake.=—This fine deep red lake
differs from carmine in containing a much larger quantity of alumina,
thus possessing the character of an ordinary lake. Occasionally carmine
lake is intentionally mixed with light white substances, such as
magnesia, to obtain paler shades or cheaper pigments.
Whilst only the finest varieties of cochineal can be used to make
fine carmine, the cheaper sorts can be used for Florentine lake. The
cochineal used for carmine is boiled but a short time with water,
so that the residue contains considerable quantities of colouring
matter, which may amount to half of that originally present. The same
materials are used as in the preparation of carmine, and in this case,
too, stress should be laid upon their purity. The weight of the alum
is usually 10 to 15 times that of the cochineal; a little stannous
chloride and cream of tartar are also added to brighten the shade. All
these materials are boiled with the cochineal, soda is added to the
clear solution until effervescence no longer occurs, the separated lake
is then washed. The lake may be made with magnesia instead of alumina,
it is added in the form of magnesium sulphate; the more magnesia is
used the paler is the lake. The proportions in which the materials are
employed vary with each manufacturer. The following quantities have
always given the author a favourable result: Cochineal 10 parts, alum
150 parts, water 250 parts; or cochineal 10 parts, magnesium sulphate 5
parts, alum 0·5 part.
In making Florentine lake a fairly deep colour should be produced.
This may be mixed without difficulty in the wet or the dry state with
a white pigment, thus producing any desired shade, even to the most
delicate rose-red.
=Ammonia-Cochineal.=—This preparation, which is chiefly used by dyers,
is obtained by treating cochineal in a well-closed flask with strong
ammonia, which dissolves the colouring matter. After about a month alum
equal to about 3 per cent. of the amount of cochineal is added without
separating the undissolved residue from the solution, and the whole is
evaporated at a gentle heat in a tinned pan until it becomes a stiff
paste on cooling. The mass solidifies more readily if a small quantity
of starch paste is added. ”Cochineal paste” is cochineal which has been
treated in this manner; it is generally brought into the market in the
form of cakes or small slabs.
In addition to the true cochineal other species of the same insect
contain a red colouring matter and have a restricted use in dyeing. The
most important of these is the Polish cochineal (_Coccus polonicus_),
which lives on the roots of the _scleranthus_. It has not been proved
that these insects contain the same colouring matter as true cochineal,
but it is certain that the colouring matters of Polish and also of
Russian cochineal (_Coccus euvæ ursi_) are far inferior in beauty to
that of true cochineal.