We now come to the cooling or quenching, which requires as much skill as the heating, to prevent warping and cracking, and to straighten the article as much as possible during the cooling process. The cooling should be performed with a view to prevent the contraction of the metal from warping the weaker parts; and to aid this, those parts are sometimes made a little hotter than the more solid parts of the article, the extra heat required to be extracted compensating in some degree for the diminution of sectional area from which the heat must be extracted. Water for cooling must be kept clean, and in that case it becomes better from use. It may be kept heated to about 100° F. (38° C), which will diminish the risk of having the article crack. Cracking occurs from the weaker parts having to give way to suit the contraction of other parts, and usually takes place in the sharp corners or necks of the articles, or through the weakest section; hence, in articles found to be liable to crack, such corners are made as rounded as possible. If the water is very cold, and the heat is hence extracted very rapidly from the outside, the liability to crack is increased; and in many cases the water is heated to nearly the boiling - point, so as to retard the extraction of the heat.

Since, however, the hardening of the steel is due to the rapid extraction of its heat, increasing the temperature of the water diminishes the hardness of the steel, and it is necessary to counteract this effect as far as possible, which is done by adding salt to the water, the steel hardening more thoroughly in the saline mixture. To assist the hardening, various ingredients are sometimes added to the water, such as fullers' earth, cyanide of potash, etc. All articles that are straight or of the proper form when leaving the fire should be dipped vertically, and lowered steadily into the water; and if of weak section or liable to crack or warp, they should be held, quite still, low down in the water until cooled quite through to the temperature of the water. If the article is taken from the water too soon, it will crack; and this is a common occurrence, the cracking often being accompanied by a sharp audible "click." Pieces of blade form should be dipped edgeways, the length of the article lying horizontally and the article lowered vertically and held quite still, because, by moving it laterally, the ad - vancing side becomes cooled the quickest, and warping and cracking may ensue. Straight cylindrical pieces are dipped endwise and vertically.

When, however, the dipping process is performed with a view to leave sufficient heat in the body of the article to lower or temper the part dipped, the method of procedure is slightly varied.

Since the hardening of steel consists of first heating and then rapidly extracting the heat, it follows that this latter part of the process may be performed otherwise than by the use of water - such, for example, as by placing the article in a current of cold air, or, if it is thin, by placing it between 2 cold iron plates. In these processes, however, the heat is not extracted quickly enough to give a great degree of hardness; hence, cold plates are rarely used, unless in cases where straightness and truth are of primary importance, and where straightening processes, to be applied after hardening, are inadmissible. When extreme hardness is required, it is not uncommon to quench the steel in mercury, which will harden to a much greater degree than water. To increase the efficiency of water, it is not unusual to boil it, which draws off the air contained in it; and there is no doubt that the superiority of water which has been long used for hardening is largely, if not altogether, due to its comparative freedom from air.