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. These latter, by the way, though very brilliant and strong and rich, are no faster to light than the original Basic colors from which they are derived. Of late years the Acid colors have held their own, and still monopolize the commercial, as well as the special, dyeing of wool and silk excepting under unusual circumstances, when considerable fastness to washing is required. With these dyes, as in the case of the Basic dyes, the fastness to washing is of little or no consequence to the craftsman. Nobody expects to scrub hand-dyed leather; and woollen and silk goods, unless specially prepared, are not supposed to be turned over to the tender mercies of the family laundress. However, it may be well to emphasize here the fact that these dyes are as a rule “stripped” quite readily by boiling in a neutral soap bath. And when the craftsman wishes to dye wool or silk fast to washing, he must either use the Salt dyes, in a boiling bath, or must dye, with special precautions against tendering, with either the Sulphur or the Vat Dyes.