Cochineal and Carmine.

The purple of the ancients was obtained from the shellfish purpura; it was so costly that it was regarded as an attribute of royalty. According to history the Tyrians discovered this colour; at any rate they were able to use it for dyeing. In the writings of the ancients Tyrian purple garments were regarded as the greatest luxury. The art of obtaining a purple from this source was lost; more recently it has been re-discovered, but the colour is found in no way to correspond with what we regard as a bright shade.

When the Spaniards conquered Mexico they found an insect cultivated which produced a splendid red colouring matter. This insect, the cochineal, is parasitic upon certain cacti, especially Cactus coccinellifer and Cactus opuntia. The females only are used as colouring material; the males are very small, and much fewer in number. The cultivation of the proper species of cactus and of the cochineal has spread to most tropical countries. The females, which are attached in enormous numbers, are brushed off and killed by heating upon hot plates. They then appear as grains of the size of millet seeds, with a wrinkled surface covered with a silvery grey dust. This variety of cochineal is the best. Black cochineal also comes into commerce; it has a brownish black colour. It is produced by killing the insects in boiling water, by which the grey dust which covers the living insect is removed. Cochineal produces when ground an ugly reddish brown powder.

The animal nature of cochineal cannot be recognised by the naked eye, it is seen on examination under the microscope; this has given opportunity for the most incredible adulterations of the costly substance. Cases have been known in which a paste chiefly composed of flour has been pressed into the form of cochineal, the grains coloured by some cheap red colouring matter, powdered with the dust from boxes in which cochineal has been packed, and placed upon the market as cochineal. The expert would not be deceived by this gross fraud, but another method of adulteration is far more difficult to recognise; genuine cochineal is brought into commerce from which the greater part of the colouring matter has been extracted, and which has been again powdered with the grey cochineal dust. Such extracted cochineal does not produce the same brownish-red powder as the fresh substance.

In the cochineal insect there is a very large quantity of colouring matter: it may reach 50 per cent. of the weight of the dry insect. The colouring matter, which is known as carmine (the same name is also applied to its lake), is soluble in water with a fine red colour. When cochineal is extracted with water the operation must be often repeated, and each time fresh quantities of colouring matter are dissolved. The cochineal may be exhausted by repeated boiling with water, but a large quantity of liquid containing little colouring matter is then produced. In order to obtain a strong solution of the colouring matter the cochineal must be powdered; this is difficult on account of the softness of the material, which does not give a fine powder, but a pasty mass. The object is best attained by grinding the cochineal through a mill similar to a coffee mill, but since the grooves of the steel cone, which effects the crushing, may be easily clogged, the mill must be arranged so that the cone can be taken out and cleaned.

The colouring principle of cochineal is an acid, carminic acid, which was obtained by its discoverer, Warren de la Rue, in the following manner: 1 part of powdered cochineal is boiled with 40 parts of water for 20 minutes; after standing, the liquid is poured off from the sediment, and mixed with a solution of 6 parts of lead acetate, acidified by 1 part of acetic acid. The precipitate, which consists of impure lead carminate, is filtered from the colourless liquid and carefully washed; whilst still wet it is suspended in water, through which sulphuretted hydrogen is passed; the lead carminate is decomposed into lead sulphide and carminic acid, which dissolves in the water. The colouring matter is not yet quite pure; the treatment with lead acetate and sulphuretted hydrogen must be repeated, the solution then obtained is evaporated at a low temperature, the residue dissolved in boiling alcohol, phosphoric acid added to decompose traces of lead carminate still present, then ether, finally the clear liquid is separated from the precipitate and evaporated. By this process carminic acid is obtained in a state of complete purity; unfortunately the process is too complicated for practical application. Pure carminic acid is a purple red mass, which transmits red light at the edges and forms a pure scarlet red powder when ground. The aqueous solution gives, with alum and ammonia, a characteristic precipitate, the colour of which is the purest carmine-red; lead, zinc and copper salts produce purple red precipitates. The composition of carminic acid is expressed by the formula C₁₇H₁₈O₁₀.