5i8

DYEING AND CALICO PRINTING.

of ordinary precipitation, the mordant and the colour, each of them soluble,forming an insoluble compound together within the pores of the fibre, muchas they would if mingled in a glass. Sometimes the mordant in a sufficientlydilute state is decomposed on coming in contadt with the fibre, within whichan insoluble oxide or sub-salt is deposited. In other cases insolubility is pro-duced in consequence of the escape of some volatile acid ; thus, when cottonis steeped in a solution of the acetate of alumina and dried, the acetic acid isvolatilised, whilst the alumina with which it was combined remains in aninsoluble state in or upon the fibre.

Hence we learn that such bodies only as are unstable in their constitution—their components being held together by but feeble affinities—can play thepart of mordants. On the other hand, stable compounds do not readily yieldup to the fibre any part of their constituents. Thus common alum is a stablesalt, and, therefore, if used alone it is a feeble mordant. If it is convertedinto basic alum, whose ingredients are held together more loosely, its poweris greatly increased. It must be remarked that the bases of mordants are notof a very decided basic charadter, but hold an intermediate position, and arecapable of playing the part either of acids or bases, according to circumstances.Thus alumina and the oxides of tin can combine either with acids or withalkalies.

Many mordants if preserved for some time undergo spontaneous decom-position, an insoluble sub-salt being deposited, whilst a small portion of thebase remains in solution in an excess of acid. This change is accelerated bydilution, by the application of heat, by exposure to light, and in some casesby the adtion of very low temperatures. “ Nitrate of tin ” is very unstable atan ordinary summer heat, whilst the nitrates of iron are often converted intoa paste-like mass if exposed to intense frost.

There is a variety of opinion as to what is deposited upon the fibre by anymordant. Some consider that it is a hydrated oxide of aluminium, tin, orother metal. Others hold that the body thrown upon the fibre is a basic sub-salt, retaining a certain quantity of the acid in which the tin, &c., may havebeen originally dissolved. This latter view is strengthened by the fadt thatthe resultant shade is modified by the acid originally present. A sulphate oftin, or a chloride of tin, containing an admixture of sulphuric acid, gives withlogwood a redder shade upon wool than is'produced if a pure chloride of tinbe applied, and this difference no amount of washing will remove.

One and the same mordant, however fully it possesses the attributes wehave just described, does not suit equally well for all cases. What isefficacious upon an animal fibre may be useless or even hurtful if applied tovegetable tissues. Certain dye-wares, moreover, are suited to some particularmordants, and give unsatisfadtory results with others. Thus with madderalumina is pre-eminently the mordant to be used, whilst with cochineal and lacdye tin has an indisputable superiority. Some acids deliver the metallic basemore readily to one class of fibres than to another. Thus if a piece of anytissue, consisting of a mixture of cotton and woollen, he steeped in dilutenitrate of iron, a buff colour will be produced upon the cotton, whilst the woolremains pradtically untouched. If, on the other hand, a similar piece of clothis soaked in dilute chloride of iron, the result is reversed, the iron being mainlydeposited upon the wool.