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DYEING AND CALICO PRINTING.

crystals are obtained. When this salt is precipitated, in aqueous solution, bysulphate of soda, hyposulphite of soda is formed, which crystallises readily.The hyposulphite has the following advantage as a mordant over the acetateof alumina :—(i), it is more economical, especially when mordants are requiredwhich are either not thickened at all or but slightly ; it yields, when equal instrength to the acetate, more full-bodied colours; (2), it is more rapidly andcompletely fixed than any other alumina mordant; (3), it prevents, to a cer-tain extent, the fixation of iron, because, as long as there remains hyposulphiteof alumina undecomposed in the fibres, any iron which may be contained inthe preparation (mordant) is kept in the state of protoxide, and cannot adherepermanently to the fibre. Only after the complete decomposition of the hypo-sulphite can oxidation of any iron compound present set in, and its fixationtake place. Notwithstanding these advantages, against which has to besetthe slight inconvenience of the evolution of sulphurous acid in the ageingrooms, the hyposulphite is hardly, if ever, now used. The general opinionof English printers is, that the hyposulphite does not present any markedadvantage over the common red liquors.

We now come to another point, viz., the methods of thickening these mor-dants without injuring their properties. We have already stated that one andthe same mordant, thickened to the same degree with different thickeners,does not, on dyeing, yield results of the same value. Gum Senegal especiallyand also some of the gums known as Arabic gums do not adt well for thefixation of an aluminous sub-salt. Moreover, two mordants of the samestrength, but thickened with different substances, give shades which vary inbeauty and intensity. It is, for instance, well known that a mordant thickenedwith starch paste combines more readily with the cloth, and yields deepershades than the same mordant thickened with gum ; but for certain kinds ofcolour gum is preferable, since it gives more transparency to the colours,while a portion, however small, of the starch-paste remains always with themordant, notwithstanding the clearing process. The difference in the inten-sity of a colour may arise from the increase of bulk caused by some thickenerswhich have to be used in larger quantity, in order to obtain an equal con-sistence, and this causes, as it were, a greater or less distance between theparticles of alumina. Any mordant strongly thickened with gum dries toorapidly, and combines only slightly with the fibre, thereby causing a weaknessof the colours. The consistence of a mordant can be readily increased, bymeans of starch or farina, without any such undesirable results ensuing. Astrong and simultaneously acid mordant is not readily thickened by means ofstarch, and the liquid does not then acquire a sufficient consistency, and it istherefore far better to thicken it with gum or torrified starch. A mordantwhich contains an excess of alum (2 parts of alum for one part of acetate oflead), if thickened with starch, attradts water even after the. lapse of a singleday. Hence it may be readily seen that the degree of consistence and thenature of the thickening substance must vary, and are accordingly regulatedby the kind of printing to be done, the specific gravity, and the quantity ofthe acetate contained in the mordant. The thickeners usually applied for redand pink colours are white starch and roasted starch. When starch is usedit is necessary to boil the liquid, which causes the precipitation of a sub-saltof alumina, which, however, remains suspended by the viscosity of the liquid,