PURPURIN.

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mixed with water. This throws down a mixture of fatty matter and tinctorialsubstance. This precipitate is dried, and exhausted with rectified bisulphideof carbon, which removes the fat. Alizarin remains behind almost pure, andmay be further purified, either by sublimation in a porcelain capsule, by meansof steam, or by the vapour of a mixture of 9 parts water and 1 part alcohol.The alizarin obtained from Turkey-red is so pure that it can be sublimedleaving scarcely any residue. When madder extracts have been employed asthe raw material the sublimed alizarin is contaminated with empyreumaticproducts, which have to be removed by washing with cold alcohol and re-crystallisation from a hot alcoholic solution.

Alizarin yields, with aluminous mordants, red and rose shades, which areextremely fast, resisting the aCtion of light, boiling soap-lyes, and even dilutenitric acid. After this treatment, technically known as clearing ( avivage ), thecolour is a bright full red, or a rose with a slight bluish reflection. With ironmordants alizarin yields solid blacks and violets of great beauty. The tinc-torial power of pure alizarin is about ninety times greater than that of goodmadder. To dye with pure crystalline alizarin it should be first rubbed up ina mortar with some alcohol, and the mixture poured into water to which somechalk has been added. No perceptible action takes place till the temperaturehas been raised to 70° or 8o° C., and the alizarin begins to dissolve. Toexhaust a dye-bath containing alizarin the quantity should not be very great,and excess of water should be avoided ; 15 decigrms. of alizarin to 250 cubiccentimetres of water are sufficient. This colour-bath, into which the mor-danted fabric is now plunged, should be kept at nearly the boiling-point ofwater, by being placed upon the water-bath. The process of dyeing shouldbe continued till the liquid has been nearly evaporated to dryness. This isthe only satisfactory mode of testing the relative value of samples of alizarin.According to the experiments of Schiitzenberger the fabric thus dyed containsa lake of madder, which is red, rose, violet, or Turkey-red, according to themordant applied.

Artificial alizarin is described in the section treating of coal-tar colours.

Purpurin.

As early as 1828 the researches of Robiquet, Persoz, and others, pointed tothe presence in madder of a tinctorial principle distinct from alizarin. It wasfirst prepared by treating Avignon madder with concentrated sulphuric acid,taking care to avoid too great rise of temperature during the operation. Thismaterial, known in commerce as charbon sulphurique , is then well washedwith water to remove acid, and next boiled for a quarter of an hour with anaqueous solution of alum, containing 12 per cent of that salt. The liquid isfiltered while boiling, and to the filtrate 1 part of sulphuric acid is added. Oncooling, a red flocculent matter is thrown down along with crystals of alum.It is collected on a filter, and washed with pure cold water. The quantity ofsulphuric acid required for the preparation of the charbon sulphurique variesWith the kind of madder and the scale of the operation. If a kilo, of Alsacemadder is taken, half that weight of sulphuric acid is sufficient. If only 25 or30 grms. of the same madder are operated upon, 3 or 4 parts of acid arerequired. The effect produced by the acid is the destruction of such mucila-ginous and other matter as would render the subsequent washing of the