the two principal forms of cochineal dye are cochineal extract, a colouring made from the raw dried and pulverised bodies of insects, and carmine, a more purified colouring made from the cochineal. to prepare carmine, the powdered 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. purity of colour 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.