Hard′en-ing.
1. (Metal-working.) The process of giving an additional hardness to metallic articles after leaving the hands of the workman shaper. Hammering and rolling will confer hardness, and in the reduction and shaping of articles by these means it becomes necessary to alternate annealing with the condensing processes. The hardening of steel is accomplished by heating it to a certain temperature and then suddenly quenching it in water or oil. When the article is not required to be at its ultimate hardness it is tempered by letting down the temper. This is done by heating the article, observing the change of color. See tempering. Copper alloys for gongs are hardened by another process, in some respects the reverse of that adopted for steel. See annealing. Iron is surface hardened by heating to a bright red, sprinkling with prussiate of potash, allowing to cool to a dull red, and cooling with water. Two recipes: —
| Prussiate of potash | 3 | 1 |
| Sal-ammoniac | 1 | 2 |
| Bone-dust | 2 |
2. (Hat-making.) The process of pressing together the light layer of filaments collected in the basket, so as to form it into a sheet of felt with sufficient cohesion to bear handling. The hands and then a skin are employed to produce this effect, the pressure, rubbing, and jerking causing the filaments to interlace and become felted, the body gaining consistence and hardness at the expense of its bulk and area. 3. A process of heating in the course of porcelain making. See hardening-kiln.
