To prepare carmine, the powdered scale insect bodies are boiled in ammonia or a sodium carbonate solution, the insoluble matter is removed by filtering, and alum is added to the clear salt solution of carminic acid to precipitate the red aluminium salt, called "carmine lake" or "crimson lake" (the lake here deriving from the word lac, referring to a resinous secretion). Purity of color is ensured by the absence of iron. Stannous chloride, citric acid, borax, or gelatin may be added to regulate the formation of the precipitate. For shades of purple, lime is added to the alum; thus, the traditional crimson color is guaranteed not only by carminic acid but also by choice of its chelating metal salt ion.[5]
Carmine may be prepared from cochineal,[6] by boiling dried insects in water to extract the carminic acid and then treating the clear solution with alum. Other common substances such as cream of tartar, stannous chloride, or potassium hydrogen oxalate can also be used to effect the precipitation, but aluminum is needed for the color. Use of these chemicals causes the coloring and animal matters present in the liquid to be precipitated to give a lake pigment. Aluminum from the alum gives the traditional crimson color to carminic acid precipitates, which are called "carmine lakes" or "crimson lakes". This color is degraded by the presence of iron salts. Addition of lime (calcium) can give carminic acid lakes a purple cast.[5]
Other methods for the production of carmine dye are in use, in which egg white, fish glue, or gelatin is sometimes added before the precipitation.
The quality of carmine is affected by the temperature and the degree of illumination during its preparation, sunlight being requisite for the production of a brilliant hue. It also differs according to the amount of alumina present in it. It is sometimes adulterated with cinnabar, starch and other materials; from these, the carmine can be separated by dissolving it in ammonia. Good carmine should crumble readily between the fingers when dry.