They may be heated in a hollow or in an open fire, exposed upon a hot plate, or in a dish with charcoal in an oven, or upon a gas stove.
Small articles may be heated by being placed within a nick in a red-hot bar.
In hardening steel care must be taken not to overheat the metal before dipping. In case of doubt it is better to heat it at too low than too high a temperature.
If, however, the steel has not been thoroughly hardened it cannot be tempered. The hardness of the steel can be tested with a file.
The process of hardening often causes the steel to crack. The expansion of the inner particles caused by the heat is suddenly arrested by the crust formed in consequence of the cooling of the outer particles, and there is a tendency to burst the outer skin thus formed.
When the whole bulk of any article has to be tempered, it may either be dipped or allowed to cool in the air. "It matters not which way they become cold, providing the heat has not been too suddenly applied, for when the articles are removed from the heat they cannot become more heated, consequently the temper cannot become more reduced." But those tools in which a portion only is tempered, and in which the heat for tempering is supplied by conduction from other parts of the tool (as described at p. 293), "must be cooled in the water directly the cutting part attains the desired colour, otherwise the body of the tool will continue to supply heat and the cutting part will become too soft." 1
1 Edes, 80.
When toughness and elasticity are required rather than extreme hardness, oil is used instead of water both for hardening and tempering, and the latter process is sometimes called toughening.
The steel plunged into the oil does not cool nearly so rapidly as it would in water. The oil takes up the heat less rapidly. The heated particles of oil cling more to the steel, and there is not so much decrease of temperature caused by vaporisation as there is in using water.
Sometimes the oil for tempering is raised to the heat suited to the degree of hardness required.
When a large number of articles have to be raised to the same temperature they are treated in this way.