It is interesting to notice, here, as previously with the Salt colors, how easy it is to modify and soften the harsh shades of the individual unmixed dyestuffs. And, as before, it is very interesting as well as very useful to dye some bundles even shades of some important compound color, such as brown, for instance, or olive green, or steel grey, and to notice how the color is changed on the fibre by adding a little more red, or yellow, or blue to the bath.
The “eye for color” obtained in this way is of the greatest possible advantage to a dyer, whether amateur or professional; and where, as in this case, the materials are cheap, easy to dye, and possible to utilize, every advantage should be taken of the opportunity.
Permanent Colors on Basketry.—While for most purposes the straw, raffia chips, willows, etc., dyed with Basic colors will be found satisfactory enough, it is best for craftsmen who are making a specialty of very high-grade baskets, to use some of the fast Acid colors, described and listed in the next chapter, for their reds and yellows, and for all mixed shades in which these two colors play an important part. The Acid dyes are applied in a boiling bath, with the addition of a little acetic acid, and, while not fast to washing, and not imparting their colors as readily as the Basic dyes, can be thoroughly depended upon, even in light and delicate shades, against the action of sunlight. Salt dyes can also be used, in a boiling bath with the addition of some salt, but, excepting in some special cases, are not superior to the Acid dyes, although somewhat faster to washing.
Chapter VIII
THE ACID COLORS
The discovery and introduction into commerce of Mauveine and the other Basic dyes, focussed the attention of chemists, all over the world, upon this new and important application of their science. And it was soon noticed that certain organic bodies, of a decidedlyacid character, had the power of dyeing wool and silk. These early dyes were so-called “nitro” compounds, formed by the action of strong nitric acid upon derivatives of coal tar, and in most cases they gave strong and brilliant, but rather fugitive, shades of yellow. The most interesting of these, perhaps, was the compound known as “picric acid,” which at one time was considerably used for dyeing silk yellow. Now it has been abandoned for that purpose but is manufactured on an enormous scale for use as an explosive.
These original acid dyes were of little importance. But in the early seventies chemists began to make use of a reaction—known as “diazotizing”—for making new organic compounds by the coupling of aniline or bodies similar to aniline, with all sorts and kinds of other compounds derived from coal tar. The number of derivatives of this sort proved enormous, and many of them had more or less valuable dyeing properties. And in a very short time new dyestuffs had been discovered, good, bad, and indifferent, numbering not hundreds, but thousands.
A very few of these so-called “Azo” dyes were of the Basic class, like Bismarck Brown, mentioned in the last chapter. Others, discovered ten or fifteen years later, constituted the class of Direct Cotton colors or Salt colors. But the great bulk of these colors belonged to the so-called “Acid” class, forming salts with bases and alkalies, and being liberated from the salts by strong acids.
The number of Acid Azo colors is very large. In the catalogues of commercial coal-tar colors there are some two hundred and fifty of these dyes which have been picked out of the rest as having sufficient value to be carefully described, and to have been placed on the market by the great dye houses. Most of these are red and orange colors, with a few yellows. As a rule they are brilliant and clear, but, with a few exceptions, not particularly fast to light.
When these were introduced it was soon recognized that they were of practically no value for cotton and linen. They are as a rule much more soluble than the Basic dyes of the foregoing chapter, and hence are occasionally used as stains for wood, rattan, and other vegetable materials where considerable penetration is needed, without fastness to washing. But such use is of little importance.
Properties.—Acid dyes are almost exclusively employed for dyeing wool, silk, feathers, and other animal fibres, and for this they are extremely valuable. The introduction of the Acid Azo colors so simplified and improved the dyeing of wool and silk, that every effort was made to increase the range of colors. And when it was found that the Azo colors were weak on the line of blue, purple, and green, efforts were made, which after several years proved successful, to change the various powerful Basic dyes, the Methyl Violets, Fuchsin or Aniline Red, Aniline Blue, Malachite Green, and the rest, into Acid dyes, so that they could all be used in the same dye-baths. This has resulted in a very wide range of colors indeed, for the Acid Azo colors cover fully all the shades of yellow, orange, and especially of red, from scarlets of all sorts and kinds to deep full crimsons. And then the remaining shades are covered by the acidified or sulphonated Basic colors.