Insolubility may be produced by various methods. Sometimes it depends on the escape of a volatile acid. Thus, if cotton yarn be mordanted with " red liquor " (acetate of alumina), and then hung up to dry, the acetic acid flies off, and the alumina is left behind upon the fibre, in an insoluble state, but still capable of combining with and retaining certain classes of colouring matters.
In other cases, the mordant, when brought into contact with the yarn or cloth, especially in presence of a large quantity of water, is decomposed. An oxide—perhaps under some circumstances an oxychloride, or a basic salt (sub-salt)—is deposited upon the fibre, whilst the bulk of the acid with which it was combined remains in the water. This takes place when yarns or cloths are worked in a solution of tin crystals, or in nitrate of iron.
Sometimes a new compound is formed ; the colouring matter and the mordant, each soluble when taken singly, combine to form an insoluble compound, which is deposited in the pores of the material. This case occurs very frequently in wool dyeing, when the mordant and the colour are applied jointly to the fibre.
Lastly, insolubility may be produced by some special treatment. Thus, if a fibre is worked in stannate or aluminate of soda, and is then passed through a weak solution of sal ammoniac, the first mentioned salts are decomposed, and the stannic acid (peroxide of tin) or alumina is left in an insoluble state in the fibre.
Hence, it may be concluded that, as far as the mineral mordants are concerned, they must be of an unstable constitution, held together by very feeble affinity, which may be easily overcome. In fact, many of the best mordants undergo spontaneous decomposition on keeping for a long time, on dilution with water, or on exposure to light. As a rule, the larger the proportion of acid to the base, the feebler is the mordant. If to a well-made tin spirit be added a proportion of free muriatic acid, the stability of the mixture is increased. The acid refuses to part with the oxide of tin, so the fibre is unable to counteract its opposition. As another instance, alum is not very effica cious as a mordant, because the sulphuric acid retains the alumina with a force which the fibre cannot readily overcome. But if a certain proportion of alkali be added, so as to combine with a
part of the acid, the alum is converted into basic alum, which parts with its alumina more readily, and is a more efficacious mordant. Hence the so-called " balance " or proportion between the acid and the base is a point of the highest importance. If the base be in relative excess, and the mordant consequently too dead, the oxide will be precipitated rapidly and irregularly upon the fibre, and the shade produced will be cloudy, streaky, and dull. If the acid be in excess, the result is worse ; the yarn or cloth is generally corroded (" tendered"), and the mordant is not deposited upon it in sufficient quantity. Hence the colour produced has a hungry, impoverished appearance. There is a very decided difference—which is not easily described—between a light colour fully dyed, and that which ought to have been a full shade, but has turned out meagre. Further, a mordant to be useful must have a decided affinity at once for the material and the dye, so as to enter into permanent union with both. On the other hand, this affinity may be too strong. If the mordant lays hold of the fibre too greedily, the shades produced will not be even. This is often the case in dyeing cochineal scarlets, unless the goods on being first entered into the dye-pan are turned very rapidly, so that the mordant may have less opportunity of attaching itself to one end of the piece of yarn in preference to the other. If, on the other hand, the affinity of the mordant for the dye be excessive, the colour produced, instead of being deposited on the fibre, is to a very considerable extent precipitated to the bottom of the dye-pan as a lake or pigment, whilst the goods are neither thoroughly nor permanently dyed. Such, for instance, is very generally the result if a solution of bismuth is used as a mordant along with any of the red woods. A fine, bright lake is produced, but the fibre is very imperfectly dyed. A combination between tissue, colour, and mordant should be effected regularly and slowly. If this point be not satisfactorily regulated, the shade produced will be flat, irregular, and loose, and will often smear off.