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 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.
FLUXING.--The word _flux_ means to fuse or to melt, or to put into a liquid state. The office of a flux is to facilitate the fusion of metals. But fluxes do two things. They not only aid the conversion of the metal into a fluid state, but also serve as a means for facilitating the unity of several metals which make up the alloy, and aid in uniting the parts of metals to be joined in the welding of parts.
UNITING METALS.--Metals are united in three ways, where heat is used:
First: By heating two or more of them to such a high temperature that they melt and form a compound, or an alloy, as it is called.
Second: By heating up the points to be joined, and then lapping the pieces and hammering the parts. This is called forge work or welding.