t shank to the cooled end. The heat becomes
equalized, and at the same time the approximate temperature for
quenching for temper is estimated by the appearance of a certain tint;
at that instant the article is plunged and allowed to remain until quite
cold. For every different class of tool a different tint is required.

"Blazing off" is a particular method of hardening applied to small
springs. The springs are heated and plunged in oils, fats, or tallow,
which is burned off previous to cooling in air, or in the ashes of the
forge, or in oil, or water usually. They are hardened, reheated and
tempered, and the tempering by blazing off is repeated for heavy
springs. The practice varies almost infinitely with dimensions, quality
of steel, and purpose to which the springs have to be applied.

The range of temper for most cutting tools lies between a pale straw or
yellow, and a light purple or plum colour. The corresponding range of
temperatures is about 430 deg. F. to 530 deg. F., respectively. "Spring
temper" is higher, from dark purple to blue, or 550 deg. F. to 630 deg.
F. In many fine tools the range of temperature possible between good and
poor results lies within from 5 deg. to 10 deg. F.

There is another kind of hardening which is of a superficial character
only--"case hardening." It is employed in cases where toughness has to
be combined with durability of surface. It is a cementation process,
practised on wrought iron and mild steel, and applied to the link
motions of engines, to many pins and studs, eyes of levers, &c. The
articles are hermetically luted in an iron box, packed with nitrogenous
and saline substances such as potash, bone dust, leather cuttings, and
salt. The box is placed in a furnace, and allowed to remain for periods
of from twelve to thirty-six hours, during which period the surface of
the metal, to a depth of 1/32 to 1/16 in., is penetrated by the
cementing materials, and converted into steel. The work is then thrown
into water and quenched.

[Illustration: FIG. 1.--Automatic Oil Muffle Furnace.]

A muffle furnace, employed for annealing, hardening and tempering is
shown in fig. 1; the heat being obtained by means of petroleum, which is
contained in the tank A, and is kept under pressure by pumping at
intervals with the wooden handle, so that when the valve B is opened the
oil is vaporized by passing through a heating coil at the furnace
entrance, and when ignited burns fiercely as a gas fl