486

DYEING AND CALICO PRINTING.

process which in some respe&s is similar to, but in others different from,China blue. This method consists essentially in the employment of a mixturecontaining white indigo, along with another substance which serves to preventthe too rapid oxidation of the white indigo. The compound is prepared asfollows :—The clear liquid of a strongly-made copperas vat is precipitatedwith acidified solution of protochloride of tin ; the precipitate—indigotate ofhydrated protoxide of tin—is colle&ed on filters made of a woollen fabric,care being taken to protedl the same as much as possible from contact of air.A better preparation is obtained by heating a mixture of indigo, caustic soda,and protochloride of tin, and precipitating it with hydrochloric acid : the pastymass thus obtained is thickened with gum, and frequently some tin-salt is alsoadded, to guard against every possible chance of oxidation before the mixtureis printed on to the cloth, while as soon as it is brought thereon the whiteindigotin ought to be rendered soluble, to make it penetrate the fibre. Thislast condition is readily obtained by passing the cloth at once into milkof lime, which causes the immediate decomposition of the tin compound, thelime combining with the colouring matter, the colour of which changes frompale greyish-white to apple-green : the white indigo becomes indeed solublefor a moment, but the presence of an excess of lime and of the thickener,aided by the affinity of the fibre, prevent the colour from running. Afterbeing withdrawn from the lime tank, the goods are washed in running water,when the oxidation begins at once, and with it also the fixation of the dye-ware, which becomes completed by the immersion of the goods in dilutesulphuric acid. If to the colour, previous to use, a proto-salt of iron is added,the oxide of which is precipitated in the lime tank, and if, instead of a simpleacid bath, to such a bath be added yellow prussiate of potassa, the tintof the indigo-blue, which by itself is always a bright, not deep, blue, isvery considerably deepened : instead of milk of lime a hot carbonate of sodabath (sp. gr. 1*070) is employed, while instead of the sulphuric acid bath asolution of bichromate of potassa is employed. The goods are left for fiveminutes in the soda solution, and next rinsed for the same time in the secondsolution; after which the goods are washed and soaked for an hour in arunning stream. The cloth is next washed in hot soapsuds (70°).

The fast green is obtained by adding to the indigotate of tin mixture anysuitable lead-salt (acetate commonly), which is first converted into oxide oflead in the soda-bath, and next into sulphate, in the sulphuric acid bath.This sulphate, being converted into a chromate by immersion in a solutionof bichromate of potassa, produces, with the blue of the indigo, a fast andsolid green.

Mr. Ward patented, in the year 1857, a method of employing indigowhich has obtained the name of glucose blue: the indigo, in the blue in-soluble state, is mixed with a deoxidising material, preferably grape sugar orglucose, and an alkali, soda or lime. This mixture, technically called thecolour, is applied cold to the cloth, and as soon as the goods leave the printingmachine, and without drying, they are passed into steam for about a minute.The heat of the steam causes the reaction to take place, viz., the reduction ofthe blue indigotin, which, being converted into white indigo, is in that statedissolved by the alkali, and enabled to enter the fibre of the woven tissue,which next requires washing and oxidising to bring up the blue colour. This