But, apart from the actual loss in valuable dyestuff, there is a much more serious drawback to this method of indigo dyeing, namely, the waste of time and energy involved. There is always a considerable delay in getting a fermentation vat fairly started, even where all the conditions are favorable; and when it is running smoothly, the reducing process is a very slow one. Furthermore, the indigo, not being dissolved in the liquid but only suspended in it, has a constant tendency to sink to the bottom in the form of a blue mud, and thus escape the chemical action of the fermentation gases entirely.
A short time ago a teacher of handicraft dyeing was expatiating, in my presence, upon the impropriety of using any of the new chemical processes for dyeing, and insisted that the only way to dye indigo was to set up a vat, and feed it, and work with it as our ancestors used to. It was suggested to her that it would be at least two or three days before cloth could be dyed in such a vat. “Eight or ten days at the earliest,” was the reply. And when it was hinted that the vat would have to be frequently stirred during all that time, she proudly answered, “Stirred regularly and thoroughly every single half-hour, night and day, during the whole period.”
“H—m,” remarked a bystander, “that’s a little worse than sitting up with a baby sick of the croup.”
Somehow the great advantage of this particular process over the modern ones, by which a proper bath can be prepared in perhaps five minutes, failed to impress itself on some of her listeners.
Modern Chemical Vats.—As soon as it was understood just what chemical action was going on in the vats, and the object of it, chemists began to find out methods for reducing the indigo without the necessity of a long, tedious, and even nasty fermentation process. They first introduced the “copperas-lime” vat, where the reduction was done by the use of ferrous sulphate (green vitriol or copperas), and slaked lime was the alkali used to keep the indigo white dissolved.
Later they introduced zinc dust, a very powerful reducing agent, in place of the copperas, avoiding in this way the large amount of precipitated iron oxide which always forms in the copperas vat, and leads to the loss of dye, and muddiness and dulness of color, necessitating a special clearing bath of dilute mineral acid.
At present the most satisfactory method is to use the chemical known assodium hydrosulphite, as a reducing agent, in a bath made strongly alkaline with caustic soda. Hydrosulphite is not expensive; it acts very rapidly, leaving no sediment; it causes no loss or waste of the indigo; and it does its work perfectly. Hence, with its introduction, the dyeing of indigo has become extremely simple.
To still further shorten and simplify the process, the large manufacturers not only furnish indigo already ground up to a fine paste with water, but also supply it already reduced by hydrosulphite or some other reducing agent, so that it is almost ready to dye with as it is, and will dissolve almost instantaneously in an alkaline bath with the addition of just a little more reducing agent. Such products are the Indigo Vat III (Metz), and the Indigo Solution 20% (Badische). By using either of these, the preparation of a vat large enough to dye 3 or 3½ pounds of cotton is the task of but a few moments. These special preparations, however, are more expensive than the regular 20% pastes, and the hydrosulphite vat is so easy to prepare that the saving of time is hardly worth the extra cost.
DYEING DIRECTIONS
For dyeing by the Vat method the dye-pot is two-thirds filled with warm water, at about 120° F. (when the finger can hardly bear the heat), and one or two tablespoonfuls of caustic soda are added—enough to make the bath decidedly alkaline. The dyestuff, preferably first mixed up with some hot water, to thin the paste, is stirred into the liquid, and then to this is added sodium hydrosulphite, in powder, or preferably dissolved in water, until the color of the bath changes from blue, first to green, and then to greenish yellow, with a bluish-green coppery scum. If the bath is bright yellow, too much hydrosulphite has been used, and some more indigo should be added; or, if this is not desirable for fear of getting too dark shades, the bath should be exposed to the air and stirred frequently until the color is right. If the bath, on scraping aside the scum, looks blue, or even markedly green, it needs a little more hydrosulphite. If, after reduction, the bath looks yellow but turbid, it probably needs more alkali.