After-treatment.—The professional dyer, who is occasionally called upon to produce fast colors with these dyes, and even with the inferior members of this class, has found various methods of after-treatment, by which the colors are rendered more permanent.
A favorite process, where the dyer is enough of a chemist to carry it out, consists of making an entirely new dyestuff in the fibre, generally of an entirely different shade, and with much greater power of resistance to washing and to light, by treating the dyed goods first with a mixture of sodium nitrite and of sulphuric acid, and, after this, passing them through a solution of some organic chemical such as carbolic acid, alpha- or beta-naphthol, or others known as developers.
This process, known as “diazotizing and developing,” is considerably used in the trade, especially for various shades of black, but is too complicated and delicate for craftsmen in general.
A simpler process is to warm the dyed goods for five or ten minutes in a weak solution of the orange-colored salt, bichromate of potash, acidified with a little acetic acid—or of the not uncommon chemical, sulphate of copper, long known to chemists as blue vitriol.
When the best dyes are used, like those in the preceding list, it is not often necessary to use either of these reagents. But when, as sometimes happens, one is obliged to use dyes of this general class, bought at the country store without a chance of knowing how fast they are, it is well to know about it. For a piece of goods the size of an ordinary linen skirt, the after-treating bath would be made as follows: In two and a half gallons of hot water, dissolve two tablespoonfuls of sulphate of copper, one tablespoonful of bichromate of potash, and two teaspoonfuls of ordinary acetic acid (equivalent, say, to three or four teaspoonfuls of strong vinegar). The goods, after dyeing and rinsing, but before drying, should be soaked in this bath and heated for ten minutes until not far from the boiling point. They should then be taken out, rinsed carefully, and dried. This after-treatment does not benefit every single color of this class, but it helps greatly the fastness to light and to washing of almost all of them. The chief objection to it, besides the time and expense, is that the shade of the finished goods is often considerably changed by the process.
Properties and Uses of the Salt Colors.—Generally speaking, the shades produced by the individual members of this group cover all the colors of the rainbow and include several good greys. It is hard, however, to get a full deep black on cotton or linen with these dyes, without using the “diazotizing and developing” process of after-treatment. The dyes go on the fibre in a soluble form, and unless a developing process like this is used they combine directly with the fibre, and do not form a coating or layer upon it, as do some of the “developed” dyestuffs. Accordingly, no matter how fully or how deeply we dye a piece of yarn or cloth with a black dye of this class, the finished goods will showgrey, a very dark grey, to be sure, but still grey, and not a flat, heavy, true black. The color of most of the salt blacks is greatly improved, however—as well as their fastness to light and washing—by soaking the dyed goods, after rinsing, in a solution containing four or five spoonfuls of formaldehyde to the gallon.
This same property, however, of combining directly with the fibre, makes the colors brighter and more brilliant than many of the other classes, especially in the lighter shades. Accordingly for bright, pretty shades of pinks, blues, yellows, and of mixed shades, fast to light, but not very fast to washing, very easily and simply applied, these colors are extremely valuable. For instance, in dyeing large quantities of bright colors on calico or cheesecloth, for some special occasion, as a pageant or spectacle, these are the colors to use.
Another great advantage they possess is that they dye true; that is, they do not alter their color when exposed to the air, and the color of the finished goods can be fairly estimated from the color of the dye-bath.
Accordingly, the student is strongly urged to practise the art of dyeing with these colors. They are cheap and can be readily obtained, although not always of the very best quality, under the name of Diamond Dyes for cotton, ezy dyes, etc., from druggists and grocerymen all over the country.
They can be easily applied to cheesecloth, muslin, and other inexpensive materials, and if care is taken to soak and boil the goods thoroughly, to linens and heavy cottons. In case of necessity they can be used on wool and silk, but, as a rule, their use is limited to vegetable fibres. They are particularly valuable to amateur dyers and to beginners in the art, because they have great “levelling” power; that is, it is easy to dye evenly with them.