In tempering, on the other hand, three things are also necessary:
First: The heating temperature should be a dull red, which is less than the annealing heat.
Second: Instead of cooling slowly the article tempered is dipped into a liquid which suddenly chills it.
Third: The materials used vary, but if the article is plunged into an unguent made of mercury and bacon fat, it will impart a high degree of toughness and elasticity.
Materials Used.—Various oils, fats and rosins are also used, and some acids in water are also valuable for this purpose. Care should be taken to have sufficient amount of liquid in the bath so as not to evaporate it or heat it up too much when it receives the heated body.
Different parts of certain articles require varying degrees of hardness, like the tangs of files. The[p. 115] cutting body of the file must be extremely hard, and rather brittle than tough. If the tang should be of the same hardness it would readily break.
Gradual Tempering.—To prevent this, some substance like soap suds may be used to cool down the tang, so that toughness without hardness is imparted.
The tempering, or hardening, like the annealing process, may be repeated several times in succession, and at each successive heating the article is put at a higher temperature.
If any part of a body, as, for instance, a hammerhead, should require hardening, it may be plunged into the liquid for a short distance only, and this will harden the pole or peon while leaving the other part of the head soft, or annealed.
Glycerine is a good tempering substance, and to this may be added a small amount of sulphate of potash.