Mordanting and Fixing Basic Colours.—­Now let us ask ourselves a very important question.  Suppose we have a colour or dyestuff, such as Magenta, which is of a basic character, and not of an acid or phenolic character like the colours Alizarin, Haematein (logwood), or carminic acid (cochineal), and we wish to fix this basic dyestuff on the tissue.  Can we then use “red liquor” (acetate of alumina), acetate of iron, copperas, etc.?  The answer is, No; for such a process would be like trying to combine base with base, instead of base with acid, in order to form a salt.  Combination, and so precipitation, would not take place; no lake would be formed.  We must seek for an acid or acid body to use as mordant for our basic colour, and an acid or acid body that will form an insoluble precipitate or colour-lake with the dyestuff.  An acid much used, and very valuable for this purpose, is tannic acid.  The tannate of rosaniline (colour principle of Magenta) is a tolerably insoluble lake, which can be precipitated by Magenta from a solution of tannate of soda, the Magenta being capable of displacing the soda.  But tannic acid, alone, does not form very fast lakes with Magenta and the other basic dyestuffs, and so a means of rendering these lakes more insoluble is needed.  It is found that tannic acid and tartar emetic (a tartrate of antimony and potash) yield a very insoluble compound, a tannate of antimony.  Perchloride of tin, in a similar manner, yields insoluble tannate of tin with tannic acid.  These insoluble compounds, however, have sufficient acid-affinity left in the combined tannic acid to unite also with the basic aniline colours, forming very fast or insoluble colour lakes.  This principle is extensively used in practice to fix basic aniline colours, especially on cotton.  We should first soak the piece of cotton in a solution of tannic acid, and then pass it into a solution, say, of tartar emetic, when the tannic acid will be firmly fixed, as tannate of antimony, on the cotton.  We then dip the mordanted piece of cotton into the colour bath, containing, for instance, Magenta, and it is dyed a fine red, composed of a tannate of antimony and Magenta.  You now see, no doubt, the necessity of sharply discriminating between two classes of colouring matters, which we may term colour acids and colour bases respectively.  There are but few acids that act like tannic acid in fixing basic aniline dyestuffs, but oleic acid and other fatty acids are of the number.  A curious question might now be asked, namely:  “Could the acid colour Alizarin, if fixed on cotton cloth, combine with a basic aniline colour, e.g. Aniline Violet, and act as a mordant for it, thus fixing it?” The answer is, “Certainly”; and thus an Alizarin Purple would be produced, whilst if Magenta were used in place of Aniline Violet, an Alizarin Red of a crimson tone would result.

Chrome Mordanting of Wool and Fur.—­In studying this subject I would recommend a careful perusal of the chapter on “Mordants” in J.J.  Hummel’s book, entitled The Dyeing of Textile Fabrics, and pages 337 to 340 of Bowman’s work on The Wool-Fibre.