Berthollet says,—"Whenever, therefore, a water is limpid, when its flow is constant, when it has no sensible taste, and dissolves soap well, it may be regarded as very proper for dyeing." He also goes on to say that for correcting water that is bad, sour water is principally used, that is, water in which bran has been fermented.
CHAPTER III.
MORDANTS
Definition of mordant. The principal mordants. The mordanting of silk and wool. Of linen and cotton. Astringents for cotton. Alum. Various examples of using alum for wool, silk, cotton and linen. Iron. Examples of iron mordants. Tin. Examples of tin mordants. Chrome. Examples of chrome mordants. Copper. Examples of copper mordants. General observations. Tannin and the galling of cotton and linen. Examples of various galling processes.
Mordants.—Dyes are divided into two classes. First, the substantive dyes, which give their colour directly to the material with which they are boiled: and second, the adjective dyes, as they are sometimes called. These latter include the greater number of dyes and require the use of a mordant to bring out their colour.
There are thus two processes concerned with the dyeing of most colours; the first is mordanting and the second is the colouring or actual dyeing. The mordanting prepares the stuff to receive the dye—(mordere, to bite.) The early French dyers thought that a mordant had the effect of opening the pores of the fibres, so that the dye could more easily enter; but according to Hummel and later dyers the action of the mordant is purely chemical; and he gives a definition of a mordant as "that body, whatever it may be, which is fixed on the fibre in combination with any given colouring matter." The mordant is first precipitated on to the fibre and combines with the colouring matter in the subsequent dye bath. But, whether the action is chemical or merely physical, the fact remains that all adjective dyes need this preparation of the fibre before they will fix themselves on it. The use of a mordant, though not a necessity, is sometimes an advantage when using substantive dyes.
In early days the leaves and roots of certain plants were used. This is the case even now in India and other parts where primitive dyeing methods are still carried on. Alum has been known for centuries in Europe. Iron and tin filings have also been used. Alum and copperas have been known in the Highlands for long ages. Stale urine is also much used in Scotland and Ireland, but perhaps more as a clearing agent than as an actual mordant.
Silk and wool require very much the same preparation except that in the case of silk high temperatures should be avoided. Wool is generally boiled in a weak solution of whatever mordant is used. With silk, as a rule, it is better to use a cold solution, or a solution at a temperature below boiling point. Cotton and linen are more difficult to dye than wool or silk. Their fibre is not so porous and will not hold the dye stuff without a more complicated preparation. The usual method of preparing linen or cotton is to boil it first with some astringent. The use of astringents in dyeing depends upon the tannic acid they contain. In combination with ordinary mordants, tannic acid aids the attraction of the colouring matter to the fibre and adds brilliancy to the colours. The astringents mostly used are tannic acid, gall nuts, sumach and myrobalams. Cotton has a natural attraction for tannic acid, so that when once steeped in its solution it is not easily removed by washing.
ALUM. (Aluminium sulphate.)—This is the most generally used of all the mordants, and has been known as such from early times in many parts of the world. For most colours a certain proportion of cream of tartar should be added to the alum bath as it helps to brighten the ultimate colour. The usual amount of alum used is a quarter of a pound to every pound of wool. As a rule, less mordant is needed for light colours than for dark. An excess of alum is apt to make the wool sticky.
"For dyeing worsted and stuffs yellow, you make use of the usual preparation, viz., of tartar and alum. You allow four ounces of alum to every pound of wool, or twenty-five pounds to every hundred. With regard to the tartar, one ounce to every pound is sufficient for yellow, though it requires two for red."—Hellot.
The usual length of time for boiling with alum is from ½ an hour to 1 hour; but some dyers give as much as 2½ hours.