Salt.—For full and indeed for medium shades, it is customary to add to the dye-bath some agent—usually table salt or, when the shade is not very dark, phosphate of soda—which will make the color less soluble in the dye-liquor and will tend to throw it on the fibre. For, after all, there is comparatively little affinity between the cotton fibre and the dyestuff (far less than between silk or wool and the Acid or Basic colors), and when a skein is warmed or even boiled in the dye-bath a large proportion of the color remains in the liquid. The bath is not “exhausted” as the dyers say. Hence, if we try to dye full shades with these colors dissolved in water only, or in soap and water, it can only be done by using large quantities of the dyestuff, most of which will be wasted in the spent dye-liquor.
For dark shades, then, where there is little danger of uneven dyeing, the goods are usually dyed for a short time with the color dissolved in hot water. And then, to deepen the shade, the goods are lifted, and common salt added in considerable quantities, three or four tablespoonfuls to the gallon, and stirred round till it is dissolved. Then the goods are put back and well boiled for half an hour or so, before the dyeing is considered complete. The presence of salt, by increasing the temperature of the boiling bath, also helps to make the dyed goods fast to washing.
Soap cannot be used in the presence of so much salt for fear of its depositing on the fibre in spots and so causing trouble. For medium shades, however, where it is well to use soap in the dye-bath so as to have the color go on the fibre evenly, a little phosphate of soda is often employed instead of salt, one or two tablespoonfuls to the gallon, to diminish the waste of color, without making the soap insoluble.
For the darker shades it is particularly important to thoroughly boil the goods for half an hour or more, before taking them out of the dye-bath. Otherwise the dyestuff will not penetrate the fibre, but will simply stain the surface, and will not only be easily washed off, with very mild soaping, but, when dry, will be apt to crack and rub.
Finishing.—After the materials have been dyed as just described, they should be taken out of the dye-bath, rinsed with water to wash off the excess of dye-liquor, and then shaken out and dried.
When used in this way the best dyes of this class, such as those listed a little further on, will give, on cotton and linen, shades that are very fast to light, and fairly fast to washing. On wool and silk the shades are fast to both light and washing. For purposes of comparison it may be stated here what is generally meant by these terms.
Fastness to Light.—The test for light-fastness is usually made by partially covering a dyed skein with a piece of wood, or heavy piece of blotting paper, and exposing it to direct sunlight, back of a window with southern exposure. At intervals the skein is taken out and the color studied, and it is then easy to see whether any change has taken place in the portion of the goods exposed to the light. If the goods have faded appreciably in the space of one week, the dyestuff is considerednot fast.
If the color changes after two weeks’ exposure, but not after one week, it is to be consideredfairly fast.
If it stands for two weeks but fades in four weeks it is to be calledfast.
And if it resists, without appreciable change, the action of the summer sunlight for full four weeks, it is calledvery fast.