506

DYEING AND CALICO PRINTING.

chiefly gallo-tannic as well as gallic acids. There is a great field for researchopen as regards the native tannic and gallic acid compounds and substancesallied thereto, which, in many respedts, are only very imperfedtly known.

We have now to explain the use of these substances in dyeing and printing.Nutgalls are used for the preparation of tannin, for the manufacture ofMorocco leather, for the preparation of black writing-ink of good quality, andnext in many dyeing operations. Tannin (tannic acid) prepared as abovedescribed is applied in some instances for fixing certain colouring matters,especially the aniline derivatives, adting as a mordant. The galls known asKnoppern or Piedmont galls are little used in England, though abroad theyare employed for tanning skins for the purpose of Morocco leather, and alsoin dyeing and printing, while an extradt of these galls is made for the latterpurpose. Valonias are often employed on the Continent instead of nutgalls,but in this country, except in some operations of silk dyeing, they are not exten-sively used in dyeing and printing. The tanners, however, use them largely, andwith decided advantage. Sumac is also substituted for galls, and is, although lessrich in tannic acid, advantageous in some cases, on account of the pure coloursit yields. All the astringent matters above named, especially sumac, behave as ifthey contained ayellow colouring matter, imparting a more or less yellowish-drabshade to all fibres, cotton, wool, and silk, but less readily to flax and hemp ifmordanted with alumina. The colour imparted under that condition by sumacis decidedly yellow, but that given by nutgalls is a greyish-drab: if the tem-perature of the dye-bath be raised, or the tissue immediately after dyeingexposed to air, the colour becomes dirty, like iron rust.

Cotton which has been impregnated with oil (see Turkey-red) becomes,when placed in a sumac bath, pale yellow, and turns, on subsequent mor-danting, to bright yellow; with acetate of tin as a mordant sumac yields ayellow, and with zinc salts a brown upon cotton. Analogous shades can beprepared by first steeping the cotton in the sumac decodtion, and aluming thetissues afterwards, but the depth of shade is less.

With the per-salts of iron as a mordant, the astringent materials yield,according to the strength of the mordant, grey, blackish grey, or bluish greytints. The chief use made of sumac consists, as far as its application indyeing and printing is concerned, in being employed, with salts of iron andcopper, as mordants and oxidising agents, sometimes along with logwood, forthe production of greys and blacks. Nutgalls and sumac are used in Turkey-red dyeing, as we have explained while treating of that process. Whenit is desired to convert the greys and olives produced by quercitron intochocolate or cinnamon-brown by means of the subsequent application ofmadder, it is necessary to add to the madder bath an astringent decodtion inorder to prevent the decomposition of the quercitron colour by the madderbath ; sumac is also applied sometimes to the dunging mixture. (SeeMadder, &c.) Stannated tissues (mordanted with tin compounds) destined tobe dyed with corallin and other coal-tar colours, are frequently passedthrough decodtions of galls or myrobalans, in order to obtain the properprecipitation and fixing of these colours upon the tissues, chiefly cotton.Calico mordanted with pyrolignite of iron, fixed by ageing and passing througha solution of an alkaline silicate, is dyed grey by means of sumac, galls,myrobalans, bablah, &c., each of which substances taken separately only