226.  Wool and Cotton Dyeing.  If a piece of wool is soaked in a solution of a coal-tar dye, such as magenta, the fiber of the cloth draws some of the dye out of the solution and absorbs it, becoming in consequence beautifully colored.  The coloring matter becomes “part and parcel,” as it were, of the wool fiber, because repeated washing of the fabric fails to remove the newly acquired color; the magenta coloring matter unites chemically with the fiber of the wool, and forms with it a compound insoluble in water, and hence fast to washing.

But if cotton is used instead of wool, the acquired color is very faint, and washes off readily.  This is because cotton fibers possess no chemical substance capable of uniting with the coloring matter to form a compound insoluble in water.

If magenta is replaced by other artificial dyes,—­for example, scarlets,—­the result is similar; in general, wool material absorbs dye readily, and uniting with it is permanently dyed.  Cotton material, on the other hand, does not combine chemically with coloring matter and therefore is only faintly tinged with color, and loses this when washed.  When silk and linen are tested, it is found that the former behaves in a general way as did wool, while the linen has more similarity to the cotton.  That vegetable fibers, such as cotton and linen, should act differently toward coloring matter from animal fibers, such as silk and wool, is not surprising when we consider that the chemical nature of the two groups is very different; vegetable fibers contain only oxygen, carbon, and hydrogen, while animal fibers always contain nitrogen in addition, and in many cases sulphur as well.

227.  The Selection of Dyes.  When silk and wool, cotton and linen, are tested in various dye solutions, it is found that the former have, in general, a great affinity for coloring matter and acquire a permanent color, but that cotton and linen, on the other hand, have little affinity for dyestuffs.  The color acquired by vegetable fibers is, therefore, usually faint.

There are, of course, many exceptions to the general statement that animal fibers dye readily and vegetable fibers poorly, because certain dyes fail utterly with woolen and silk material and yet are fairly satisfactory when applied to cotton and linen fabrics.  Then, too, a dye which will color silk may not have any effect on wool in spite of the fact that wool, like silk, is an animal fiber; and certain dyestuffs to which cotton responds most beautifully are absolutely without effect on linen.

The nature of the material to be dyed determines the coloring matter to be used; in dyeing establishments a careful examination is made of all textiles received for dyeing, and the particular dyestuffs are then applied which long experience has shown to be best suited to the material in question.  Where “mixed goods,” such as silk and wool, or cotton and wool, are concerned, the problem is a difficult one, and the countless varieties of gorgeously colored mixed materials give evidence of high perfection in the art of dyeing and weaving.