It must be remembered that mordants are not universal in their action ; those adapted for wool are not suitable for vegetable fibres. As a general rule, the preparations employed for woollen and worsted goods are more add than those which serve for dyeing cotton and linen ; the latter require mordants of a very faintly acid, a neutral, or even an alkaline character, such as the aluminates and stannates. On the other hand, alkaline mordants can very rarely be used with safety upon wool. Preparations of lead, iron, and manganese play a very important part in cotton and linen dyeing, but they are of very little value for wool, where lead, indeed, is totally inad missible. The nature of the colouring-matter to be employed is also an essential consideration. Tin has always had the preference for use in conjunction with cochineal, lac, and bright colours produced with the red woods, e. g. bar-wood reds, sapan pinks, &c., but it has given little satisfaction for madder reds, where the compounds of alumina have been found preferable. It has been proved that the acid employed in the composition of a mordant decidedly influences the colouring-matter, and must likewise be regulated in accordance with the fibre. It is known that a chloride of iron, or a nitrate of iron into the composition of which hydrochldic acid has entered, cannot be safely used for dyeing cotton warps of mixed goods, such as coburgs, delaines, merinos, &c. It delivers the iron, not only upon the cotton, but also upon the worsted. OD the other hand, a pure nitrate or nitro-sulphate of iron will mordant the cotton fully, leaving the worsted or woollen warps untouched. Tin mordants which contain oxalic or tartaric acids are used for the brightest cochineal reds upon woollens and worsteds. If sulphuric acid is used instead of the oxalic or tartaric, as was done by Bancroft, the red is of a brown cast. Sulphate of tin produces a redder claret shade, if used upon woollens along with logwood, than does the muriate of tin under similar circumstances.
These facts are of importance as regards the theory of the mineral mordants. It is generally sup posed that hydrated alumina, hydrated oxide of tin, iron, &c., as the case may be, is deposited in the minute pores of the fibres in conjunction with the colouring matter, thus forming a lake. But in view of the facts just mentioned, showing the influence of the acids present, it must be presumed either that the substances precipitated upon the fibre retain a certain proportion of the acid in the state of a sub-salt (basic salt), or that the acids effect a permanent change in the properties of the colouring matter. It must be remembered that acids have a decided affinity for animal matter. If the finger-tips be plunged into sulphuric acid, and immediately afterwards washed in flowing water, the acid taste and the reaction with litmus paper will not be readily removed.
It is obvious that the theory of the action of mineral mordants, briefly given above, cannot be extended to the organic mordants. The action of albumen and its kindred substances in fixing many aniline colours upon vegetable fibres is totally different, and is best characterized by the com mon phrase, " animalizing cotton." Magenta, orchil, &o., are substantive colours upon wool, and, by coating cotton with albumen, it is furnished with a surface which is chemically identical with wool. The magenta is not combined with the cotton, but merely with a substance mechanically adhering to the cotton.
The most recent experiments decidedly refute the theory that dyes are fixed by the acid of a capillary tube in the centre of the fibre, into which the mordants and colours were supposed to penetrate. No such capillary tube exists, for example, in silk.
A brief description of the principal mordants and their application will now be given.