340 DYEING tion of several coal-tar colors from alcoholic solutions is another instance of this mode of dyeing. 2. The coloring matter may be ren- dered insoluble by subjecting the material stained with it to the action of warm air, a process called ageing. Such is the case with indigo, quercitron, sumach, and fustic. The oxidizing process may also be accomplished by adding some oxidizing material, as when cloth is dyed black with logwood and chromate of potash, the hsematoxyline of the wood being oxidized and chromic acid reduced to chromic oxide ; or when aniline black is produced by the action of chlorate of potash, ferrocyanide of potassium, or chromate of copper upon ani- line oil. 3. The coloring matter may be pre- cipitated by the use of a mordant, and this is the most important as well as the most com- mon method. A mordant, strictly speaking, is a solution of some substance which, not being itself a dye, has an affinity for the ma- terial to be dyed as well as for the coloring matter of the dyestuff, so that by its means the precipitate will be more firmly fixed upon and within the fibre. Organic coloring mat- ters are generally more firmly fixed than inor- ganic by the use of mordants. A color is often produced upon a fabric by using two metallic salts whose double decomposition produces an insoluble compound, as for instance the forma- tion of Prussian blue by the action of hydro- ferrocyanic acid upon a salt of iron, or the pro- duction of chromate of lead by using chromate of potash and a soluble lead salt. These latter substances when used in this way are consid- ered by some as not being strictly mordants, but they can hardly be placed in any other category with convenience. The following are the more important mordants : alum ; sul- phate, acetate, and hyposulphite of alumina; aluminate and stannate of soda ; chloride of tin ; acetate of iron ; fatty substances, as Gal- lipoli oil in Turkey red dyeing ; albumen, glu- ten, and caseine. Tannin is used in some cases as a mordant for madder and aniline colors, and for some metallic salts. Most dyes of or- ganic origin can only be fixed by the use of mordants, but the original color may be much modified by these agents. With alumina com- pounds madder yields red, pink, and scarlet : with salts of iron, according to the degree of concentration, lilac, purple, and black. Dyed goods are often brightened by passing them through weak acid or alkaline solutions, soap suds, bran bath, or some other dye material. If a piece of cotton stuff is simply stained with a decoction of logwood, the color may be readily washed out with pure water. If, how- ever, it is first dipped in a solution of proto- sulphate of iron and afterward washed, it will be found stained, a portion of the iron having been converted, either by the action of the air or of the fibre, into insoluble oxide. If the cot- ton is now dipped into a decoction of logwood, the coloring matter of the latter will combine with the oxide of iron within and upon the fibre, and form a color which cannot be removed by washing in water. If the cotton is dipped in the sulphate of iron solution and not washed, but passed through an alkaline solution, so that the alkaline base may abstract the acid from the iron salt, a much greater quantity of oxide of iron will be formed within and upon the fibre, and a greater depth of color will be given to it by subsequent immersion in the de- coction of logwood. When a solution of sul- phate of iron and a decoction of logwood are added to each other, the same insoluble color- ing matter will be precipitated in the mixture that was formed by bringing the two solutions together in the fabric. If cotton be now im- mersed in the mixture, it may be feebly stain- ed, but can hardly be said to be dyed. If, however, a piece of woollen be placed in it, it will become strongly dyed. The precipitates which are formed by the combination of the decoctions of dyestuffs, or the pure coloring matters obtained from them with the bases of certain salts, are called lakes. The salts which are principally used for this purpose are those of aluminum and tin, because, their oxides be- ing colorless, the lakes take the tints of the coloring matters. The lakes obtained with the use of iron salts have a different hue from the original coloring matters, being generally duller. In the salts of aluminum, tin, and iron, the base and acid are not held together by a very strong affinity, and consequently their separation by the attraction of the coloring matter for the base is easily effected ; but the acetates are more readily decomposed than the correspond- ing sulphates, and therefore a mixture of alum with acetate of lead is often used as a mordant. Heat is employed in many cases to facilitate chemical action in dyeing, the solution of salt and coloring matter sometimes remaining to- gether in the cold state without producing a precipitate, where boiling will at once cause it to appear. Sometimes the precipitation is effected by the addition of another substance ; thus when a mixture of a solution of alum with certain coloring matters remains clear, a pre- cipitate is formed by the addition of an alkali, which, taking the sulphuric acid from the alum, leaves the alumina to combine with the color- ing matter. The principal use of mordants is therefore to form insoluble lakes ; but the num- ber of substances that can be used as mordants is much less than of those which may be used for making lakes, on account of the action which some of them have upon the fibre, as well as the influence of the fibre upon their action. The tendency of aluminum, tin, and iron salts to become basic, and of their bases to unite with coloring matters, is what renders them so well adapted to the dyer's art. A cloth to which a mordant has been applied re- quires before it is dyed to have a certain amount of the mordant fixed within its fibre, and the remainder removed. Fixing the mor- dant (ageing) is principally accomplished by heat, which promotes the separation of the base
