When, however, very fast dyes are being used, as for instance, the Vat colors or, indeed, a great many of the best dyes in all the classes, the action of chlorine is very slow, and slight, and, in order to really destroy the color both the acid and the bleaching powder will often have to be so strong that the chlorine set free will destroy the fibre as well. For the term “fastness to light” implies, as a rule, fastness also to oxidation in general, and dyes like the best modern ones which will let the cloth rot away from under them, when long exposed to the weather without changing color, are very apt also to keep their color, even when the cloth is _burnt_ away from under them by the action of chlorine. Accordingly, this process is distinctly one that needs careful experimentation before it is tried on any important piece of work. There are plenty of dyestuffs among the Salt colors, and also among the Sulphur colors, which discharge well with chlorine. And the calico printer, working, as he generally does to this day, with comparatively fugitive dyes, and weighing accurately both acid and bleaching powder, can generally get good results with it. But there is always the disadvantage, that the least excess of chlorine will attack and tender the cloth, and the better the dyestuff, as a rule, the stronger the oxidizing agent must be to discharge it. (b) _Discharge by Reduction, Hydrosulphite, etc._—The wary craftsman will find the process much less dangerous to the cloth, and not much more difficult, if instead of trying to _oxidize_ the dyestuff, he attempts to discharge it by _reducing_ it; or, in other words, if instead of trying to burn it out, he tries to take the oxygen away from it. It so happens that in a vast majority of cases a dyestuff becomes decolorized by reducing it, just as well as by oxidizing it. There is, however, a difference. When the color is oxidized, it is burnt up and destroyed forever. When it is reduced, however, it is, in many cases, only decolorized and not destroyed; and on standing in the air it is apt to take up oxygen again, and to regain some, at least, of the original color. On the other hand, while any oxidation process is liable to attack the cloth as well as the color, the reducing agents now in use have no effect upon the materials, even when powerful enough to act on the very fastest dyestuffs. As before mentioned, the most satisfactory reducing agent at present known to dyers is hydrosulphite of soda, and this can be incorporated in a paste, and used for discharge stencilling. It is, however, as a rule, more satisfactory to use the more expensive, but more permanent hydrosulphite compound, described, in the last chapter, as acting only when heated. The reducing stencil paste can be easily made by mixing with some “gum dragon” or flour paste, as much as it will hold of a saturated solution of the “Stencil Salt.”