so accompanied by another
quality, namely, _toughness_. For this reason, the word _temper_, and
not _hardness_, is referred to. A lathe tool, if merely hardened, would
be useless for that purpose.

TEMPERING CONTRASTED WITH ANNEALING.--It will be observed that in
annealing three things are necessary: First, heating to a certain
temperature; second, cooling slowly; third, the particular manner of
cooling it.

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 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 o