266

DYEING AND CALICO PRINTING.

off freely, and the bag gradually but strongly pressed; the clear fluid collectedin a wooden tub; the contents of the bag are again placed in the tub fromwhich they were first removed, and again treated with ten times their weightof sulphurous acid solution, and the yield added to the first obtained liquid.This operation is once more repeated, but the liquid is employed as anaddition to the sulphurous acid solution required for a fresh portion of madder.The liquids run off should be carefully kept from contadt of air, and pouredinto tubs closed with tight-fitting wooden lids. The madder, which has beenexhausted with cold or tepid sulphurous acid, is placed in a vessel providedwith a false bottom, pierced with small holes, over which is laid a stoutpiece of coarse flannel. The material is thoroughly washed out first withwarm and then with boiling water. It now constitutes a weak fleur degarance, which yields on dyeing very pure tints, and leaves the whitesthoroughly unsoiled. By adding from 3 to 5 per cent of sulphuric or hydro-chloric acid to the sulphurous acid solution, and heating to 50° or 60°, a pre-cipitate of raw purpurin is obtained, which is collected, washed with coldwater to remove all the acid, and next dried. The remaining liquor, from whichthe purpurin has been removed, is boiled, when carbonic acid is disengaged,and a blackish-green pulverulent matter is thrown down, which is alizarin,stained with a very dark green substance, a produdt of decomposition ofchlorogenin. The liquid should be kept at a boil for fully two hours in orderto ensure the complete precipitation of all the raw or so-called green alizarin.The green alizarin is completely thrown down, and settles in from twenty-four to thirty-six hours. Since it is in a very fine powder it clings to thesides of the vessel, and only falls to the bottom on agitation. The supernatantliquid is run off, and the alizarin thrown upon a stout canvass filter, andthoroughly washed with cold water. When the boiling process has beencarried on sufficiently long the dark yellow mother-liquor does not contain anymore utilisable colouring matter. The alizarin is washed until all the acid isthoroughly removed. The crude material thus obtained is a most excellentdye stuff, readily manageable in the dye-beck, and yielding full bodied, verybright, yet deep colours, thoroughly fast and resisting cleansing and boilingsoap lyes. The whites are scarcely soiled. Sometimes a small addition ofchalk (o'i to 0*2) is advantageous in the dye-bath.

Several processes have been proposed to obtain pure alizarin from this,but none are readily pradticable on the large scale, owing to the expense of thesolvents, and from the difficulty of separating the impurities from largequantities of alizarin. The following process is the best and quickest:—1 part of the raw alizarin is boiled with from 15 to 20 parts of light petroleumoil, boiling at about 150°. After about fifteen minutes’ boiling the vessel istaken from the fire, and after standing for a few minutes, the clear liquid isdecanted from the sediment. As soon as the decanted liquid cools it depositsa quantity of pure alizarin. When the temperature of the decanted fluidhas fallen to 100°, there is added with careful stirring about 10 to 15 per centof a solution of caustic soda, containing from 5 to 8 per cent of solid hydrate ofsoda. The soda takes up all the alizarin, yielding a purplish-coloured solution.The alkaline liquid is separated by decantation from the petroleum oil, andsupersaturated with dilute sulphuric acid, which causes the precipitation ofthe alizarin in the shape of a crystalline magma, which is w’ashed and dried.