ALIZARIN, the principal colouring matter of madder, may be obtained by subliming on paper an alcoholic extract of madder, or by exhausting the root with water, precipitating with sulphuric acid, dissolving the moist precipitate in a solution of chloride of alumina, and separating the impure alizarin by the addition of hydrochloric acid. The impure alizarin is dissolved in alcohol, and separated as a lake on treating with hydrate of alumina, which is now boiled with carbonate of soda to separate another colouring matter called purpurin, and is finally treated with hydrochloric acid, which dissolves the alumina and leaves the pure substance.

Alizarin in the anhydrous state forms red prisms, and in the hydrated condition crystals like mosaic gold. It dissolves sparingly in water even at the boiling point, but is soluble in alcohol or ether. Mineral acids do not decompose the colouring matter at ordinary temperatures. Caustic alkalis or alkaline carbonates dissolve alizarin.

forming deep purple solutions, from which acids precipitate in orange-coloured flakes. Alizarin has the atomic composition C j4 H 8 O 4 , and has recently been made synthetically from the hydrocarbon C 14 H 10 called anthracene, which occurs among the products of the destructive distillation of coal. This is the first example of the artificial formation of a natural colouring matter. For further details see Chemistry.