Chemists have for a long time debated the question as to just what dyeing really is. The puzzle does not yet seem to be settled. Three theories have been advanced to explain the phenomenon of dyeing:
(1) The chemical theory
(2) The mechanical theory
(3) The solid solution theory
The chemical theory is based on the possibility that a definite chemical compound called a color lake is formed between the fibers of the material being dyed and the coloring matter of the dye. Supporters of this theory say that animal fibers, such as silk and wool, contain animal acids and have the power to react with acid and basic substances, and that they do react and change in color with acid and basic dyestuffs.
In the case of cotton fibers, which are inert chemically, an extra treatment with an acid or basic mordant is necessary in order to secure a change of color with dyes - the acid mordant going with the basic dyestuff, and the basic mordants (such as various metalic oxides) being used with acid dyes. There is very little direct evidence to prove the chemical theory. The fact that alcohol and some other solvents may remove the coloring matter, and leave the fibers unchanged, seems to break down the strength of the arguments for the chemical theory to some extent.
The mechanical theory is supported by possibility that the coloring matter may be composed of fine particles, which are invisible even with a microscope, that lodge between the tiny imaginary molecules that are supposed to be component parts of the fibers themselves.
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.
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. The fibers that are 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.
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