In the older methods of heating, hardening, tempering and annealing steel, results depend entirely upon the eye and practice of the workman. To insure uniform results, various methods of heating and hardening have been devised which eliminate judgment, and secure automatically the same results in each case.

In manufacturing establishments, tools and small articles to be hardened are usually heated in a bath of red-hot molten lead. Heating may also be. done in closed iron tubes or in small muffle furnaces. A pyrometer must be used to regulate the temperature of a heating furnace or a lead bath.

To avoid burning off the sharp points of file teeth and the sharp cutting edges of threading dies, etc., during heating or while transferring these articles from the heating bath to the quenching or hardening bath, the articles are dipped before heating into a thin hot paste of salt, flour and charred leather, or into a hot salt solution. This mixture dries at once and remains on the surface until after the articles are hardened.

For hardening steel, quenching baths of fish oil, petroleum residue, or brine are much used, and are kept at constant and uniform temperature by agitation or by a constant flow from one receptacle to another. These quenching baths are not so sudden in their cooling effect as is pure water, hence the shock to the metal is less, and the degree of hardness given is sufficient. The consistency and temperature of a quenching bath is determined by experiment to give the degree of hardness required without further process.