MORDANTS.

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IV. Stannic Salts, or Per-Salts of Tin.

These compounds, very extensively used in cotton- and silk-dyeing and inprinting, are composed of the peroxide (binoxide or tetroxide) of tin, unitedwith various acids, and of the stannic chloride or perchloride of tin. Theyare produced in a variety of ways, sometimes dire&ly from the metal, and atother times by the oxidation of the proto-salts of tin, whether by means offree chlorine or of nitric acid.

The making of these preparations is a matter requiring much care, especiallyin all those processes where a mixture of nitric and hydrochloric acids isallowed to adfc upon the metal or upon proto-salts. According to the temper-ature employed and the speed at which the operation has been conducted, twolots, composed of the same materials, may contain very different proportionsof protoxide and peroxide. The result may be in one case a perfect peroxide,whilst in another the proto-salt may largely predominate. Such varying com-pounds yield, of course with the same dye-wares, different shades of colour,to the great discomfiture of the dyer, who, using what he supposes to be iden-tical materials, finds ever-fludluating results. The condition or state ofoxidation of a sample may be very easily detected. If to a little of the spiritplaced in a test-glass we add a few drops of a solution of mercuric chloride(corrosive sublimate) in water, then, if any of the tin was present in the stateof a proto-salt, a white turbidity is formed, which speedily blackens. If thetin be a pure per-salt there will be no apparent adtion.

But whilst the process should be brisk enough to ensure the formation of aperfedt per-salt, as the only way to secure constant results, too rapid an adtion,and a consequently excessive temperature, can occasion serious mischief.The tin, in great part or entirely, instead of becoming dissolved, maybe depo-sited at the bottom of the vessel as an insoluble peroxide. This result, knownas il firing,” is common in careless or inexperienced hands.

But without precipitating the tin in this insoluble state, its affinity for thefibre may be destroyed by a small excess in temperature. A spirit thusspoiled, if compared with one of the same class and made from the samematerials, but in proper condition, shows no tangible difference. They, too, agreein specific gravity, colour, smell, in degree of acidity, and in the amount oftin contained; yet the one attaches itself readily to She fibre, which then, incontact with the colouring matter, dyes up a full shade ; the other, similarlytreated, yields merely an irregular and impoverished stain. It would seem, inconsequence, that the per-salts of tin may exist in at least three differentstates, in one only of which they have a marked affinity for animal andvegetable fibres, and can be, by them, withdrawn from solution.

The acids present along with the per-salts of tin are chiefly the nitric andhydrochloric ; the sulphuric is rarely used. Oxalic acid is also less frequentlyad^ed than with the protosalts of tin. Alkaline chlorides and nitrates are not^infrequently present, and have modifying effedts, especially the chloride ofammonium (sal-ammoniac), which forms with the stannic chloride a definiteCompound, to which we shall return below. The nitric acid used in preparingthese compounds, except where otherwise specified, is the “ double aqua-fortis ” of commerce, of about 64° to 65° Tw. The tin is generally used notfeathered or granulated, but in the rod.