Cochineal consists of the dried wingless females of a species of coccus (C. cacti) which feeds upon several kinds of Opuntia, or cactus. The best quality comes from Tene-riffe, and contains about half its weight of colouring matter. This colouring matter is a glucoside, to which the name of carminic acid has been given. When this substance is treated with weak sulphuric acid, it is resolved into a sugar and another colouring matter called carmine red.
The carmine of commerce is prepared directly from cochineal, and is the most concentrated and purest form of any of the pigments derived from this source. The methods of preparation differ, but in all the colouring matter is extracted from the insects by means of boiling water, with the subsequent addition of small quantities of alum, or nitre, or potassium oxalate, or cream of tartar; occasionally a small quantity of stannous chloride is employed also. The liquor, after a repose of some days or even weeks, deposits a great part of the colouring matter as a deep crimson-red powder, which is then thrown on a filter, washed and dried in the dark. However prepared, it contains fat, albuminoid matter, mineral salts, and other impurities; the finest varieties, however, dissolve perfectly in strong liquor ammoniŠ. The liquor, which has deposited the carmine, gives up the remainder of its colouring matter to freshly - precipitated aluminium hydrate, or, after having been rendered alkaline by potassium carbonate, to a solution of alum. Lakes are thus formed. Lakes are also made by directly precipitating cochineal extract with solutions of potassium carbonate and alum.
A purplish tinge is given to the product by a small quantity of lime; other hues, generally dull, are imparted by the presence of iron, manganese, or copper in the solutions employed. The cochineal lakes always contain a larger quantity of water and of alumina (or other mineral basis) than carmine, and are consequently weaker. Scarlet lake is usually a mere mixture of crimson lake and vermilion, but alizarin lake may replace the former.
Beautiful and rich as are the colours prepared from cochineal, not one of them should ever find a place upon the palette of the artist. They all become brownish, and ultimately almost disappear after a short exposure to sunlight or the more prolonged attack of strong diffused daylight. In six hours of sunshine a strong wash of fine crimson lake on Whatman paper lost 8 per cent. of its original intensity; this was on April 12. The loss during a second period of six hours' exposure was much less, but after the lapse of four months less than 5 per cent. of the original colour remained. In the case of carmine, from one to two years was required for the complete obliteration of every trace of the original crimson from a deep wash of this pigment. All the cochineal pigments become somewhat brownish during the course of fading, but ultimately, when all the red has disappeared, either a greenish-grey or a faint sepia-like brown is the sole residue.
The term 'lake' belongs to all colouring-matters thrown down upon such a basis as alumina; but when purple, crimson, or scarlet is prefixed to the word lake, cochineal colours are always understood. So carmine used alone refers to cochineal carmine, although it is a general term for a group of rich pigments, of which madder carmine and indigo carmine are perfectly distinct examples, derived from madder and indigo respectively.
It is not necessary to say more about the various cochineal pigments, nor to point out their many falsifications, for their value as artists' colours is very small. No artist who cares for his work and hopes for its permanency should employ them.