The principle on which these processes are based is as follows: The aniline, usually in the form of aniline salt (aniline hydrochloride), is mixed with an oxidizing agent like chlorate of soda, and also with a small amount of a third substance which, on steaming, acts as a carrier of oxygen between the aniline and the chlorate. This substance, often called a catalytic agent, because at the end of the operation it remains unchanged, although it has accomplished a large amount of work, may be one of a number of compounds as, for instance, a salt of the metal vanadium, prussiate of potash, a salt of copper, etc., each one having special advantages and disadvantages of its own. Now, almost any printing paste properly composed so as to give a good clear Aniline Black on steaming, (the formulæ can be obtained from any good book on calico printing, or from any competent dyeing chemist), will generally work fairly well as a stencil paste—as long as it is fresh. But even when kept from the air as far as possible, in a tight tube, it decomposes on standing and becomes very unsatisfactory. Besides this, there is always a difficulty with these regular pastes on account of the irregular and uncertain steaming process that can be used by the average craftsman. In a calico print works, the temperature of the steam chest, the proportion of steam in it, and the length of passage of the cloth through it, are all accurately determined, and kept at the exact points necessary for the best results with any given formula. But with irregular steaming, unless very great care is taken with the formula, there is always a danger of “tendering” and burning the fibre, if too much oxidizing agent is present, or of not developing a full black, but a dark green color, if the oxidizing agent is not active enough. We have, after a great deal of experimenting, worked out a formula which, with reasonable care in steaming, will give a good full black, absolutely fast to light and washing, upon cotton, linen, and silk, without any tendering of the cloth. And, by dividing up the component parts into two separate pastes, which are kept in separate tubes or bottles, and are mixed together only when about to be used, we have gone far to solve the important problem of keeping. The use of this Black stencil paste is very simple. It comes in two tubes or bottles marked A and B. When the cloth, free from dressing and slightly dampened, is all ready, equal amounts are taken from each of the two tubes, and mixed together in a watch-glass or small glass or porcelain dish with, if necessary, a drop of water to soften them if they have dried up at all. This mixed paste is then brushed on to, and into, the cloth, and, as soon as dry, is steamed as before described. The black color will develop almost immediately, and, after a few minutes’ steaming, will be found fast to hard washing as well as to light. CHAPTER XVI