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Coloring Finishing And Painting Wood
However, it is known that a given dye produces varying tones and nuances on different fibers, which in some cases are fast to light and in others are fugitive.
Adherents of this theory sometimes say that the size of the molecules of the dye and different fibers may vary, allowing the coloring-matter to find a lodging-place in some materials and not in others to such good advantage.
To support this theory it is pointed out that, in general, hot dyeing is more effective than cold dyeing. This may be due to greater expansion in the size of the molecules of one material than of the other, thus permitting them to unite to better advantage and to form a different and new color. The mechanical theory is not thought by most chemists to be a satisfactory explanation of dyeing reactions on wool and silk, but some people still think that it really shows what takes place with cotton and possibly other vegetable fibers during the dyeing process. If dyeing were merely mechanical, it seems probable that repeated washings in boiling water would remove all the color. This cannot be done in many cases where permanent dyes have been used.
(3) The solid-solution theory, advanced by Witt, is more difficult to understand than either of the others, but it is now believed by many color chemists to be the most probable explanation. According to this theory, one solid substance, such as coloring-matter, may be dissolved by another solid, such as fiber, just as molten glass may be colored by absorbing certain pigments.2 The fibers being dyed act as solvents, which absorb certain coloring-matter from the dye solution. In some cases, mordants, which are of three classes, may enter into the dyeing operation.
72. Mordants.—Mordants change and increase the affinity of some fibers for dyes. Their use is necessary with certain dyes especially the old fashioned vegetable colors, some of which required complicated treatments with mordants in order to make the dye "bite." Vegetable fibers will combine with certain tannin mordants, such as tannic acid, cutch, and sumac; while wool will form color-lakes with various dyes, if first treated with metallic oxides, such as those of aluminum, iron, and chromium, which prepare the fibers to react properly with the dye and absorb it. Oil mordants, such as Turkey-red oil, are sometimes used in fixing basic dyes on cotton.
Mordants are sometimes classified under three heads: tannin mordants, metallic mordants, and oil mordants. Other names are used also, but these seem to be the most distinct types.3 ANIMAL DYES Previous to the year 1856 certain important animal dyes, such as cochineal and Tyrian purple, were in common use. The kermes insects were also the source of a valuable red or reddish purple dye known and used even in very ancient times.
73. Cochineal.—An insect commonly found in Mexico and Central America, produces the raw material for crimson, scarlet, and purple lakes, the color usually being called carmine or carmine lake.
