The proper amount, to be determined only by experience, is first thinned with a little hot water, and then stirred into the dye-pot, two-thirds full of hot water, about 140° F. (This is well below a boil, and yet hot enough to slightly scald the tips of the fingers.) To this is added caustic soda, in the proportion of two to three spoonfuls to each one of the color, the amount of soda being proportionately greater for light shades than where large amounts of color are used.
After this has been dissolved the dyestuff is reduced by adding slowly, with constant stirring, spoonful after spoonful of the powdered sodium hydrosulphite until the bath clears and generally the color changes. In most cases, as with indigo, the completion of the reducing change can be told by a marked alteration in the shade of the bath.
Thus, in general, the blue dyes, like indigo, turn yellow or orange when the proper amount of hydrosulphite is added. For the other colors there is no general rule. Thus Indanthrene Yellow (Badische), when reduced, is blue—while the Helindone Yellow (Metz) is blood red. Helindone Scarlets (Metz), when reduced, appear green, while the Thio Indigo Red and Scarlet have about the same color, when reduced, that they have when oxidized.
The best way to tell whether the bath is in proper condition is to dip a piece of white blotting paper into it, and notice, on taking it out, whether the color is in specks or is dissolved. On standing in the air for a few minutes the color should become oxidized, and firmly fixed to the paper. As a rule these Vat colors should be reduced warm, because, in many cases at least, the reduced color does not dissolve in a bath of cold alkali. In most cases, however, after having been reduced at a temperature of about 140° F, the bath may be allowed to cool considerably, before it loses its dyeing value. This enables these colors to be used for Batik, or other processes where the temperature must be kept below 80° or 90° F. The dyestuffs which can be thus used will be found marked with an asterisk in the list of selected dyestuffs above.
The well-wetted materials are placed in the reduced dye-bath, and stirred and worked for five or ten minutes, or longer, according to the depth of shade experienced. For full shades, however, as in the case of indigo, it is much better to build up the color by successive dippings than to try to put it all on in one bath. For heavy goods the addition of a little Turkey red oil, about half a tablespoonful to the gallon, is an advantage, though not absolutely necessary. When thoroughly impregnated with the dye-liquor, the goods are taken out, wrung carefully, two or three times, to remove the waste liquor as evenly as possible, and then shaken out and exposed to the air for fifteen or twenty minutes. They are then boiled in a soap bath for about twenty minutes, and then well rinsed, and dried. This hot soap bath, as before mentioned, is of great importance in most of these colors, not only for getting rid of loosely fixed dyestuff, but for oxidizing and fixing the color itself.
For dark shades it is well, as in the case of the Sulphur colors, to add salt—three or four tablespoonfuls per gallon of dye-liquor—to the bath, taking care to have it well dissolved before the goods are entered. This is always done when dyeing with Helindone Yellow 3GN, Metz.
The shades of these new Vat colors are extremely bright and clear, and, by combining these properly, any desired effects may be produced. The splendid series of reds and scarlets for the first time allow the characteristic shades of Turkey red to be obtained, in one bath, and of at least equal, if not of superior fastness to the original. One peculiarity of these colors is their extreme fastness, not only to light and washing, acids and alkalies, but also to various oxidizing agents, such as chloride of lime or bleaching powder. Accordingly goods properly dyed and finished with these dyestuffs can be entrusted with safety, so far as the color goes, to agencies which would speedily ruin fabrics dyed in any other manner.
Chapter VII
THE BASIC COLORS
In an earlier chapter it was mentioned that the modern dyestuffs originated with the discovery by Perkin, in 1856, of the violet coloring matter known as Mauveine. This dye was made by the oxidation of the then rare chemical, aniline. Following this discovery, other chemists, especially in France and Germany, soon obtained from the same chemical or from substances very closely resembling it, a considerable quantity of powerful and brilliant dyestuffs of the same general character.
The original Mauveine was before long superseded, first by Hofmann’s Violet, and then by a very important series of violet and purple dyes known as Methyl Violet, with shades ranging from 6 or 7B for the deep, full purples, to the 6 or 7R for the very red shades. These violet colors have never been surpassed, or even equalled by any other dyes for brilliancy and richness, although, in common with almost all the other dyes of this class, they are not fast to sunlight.