. Oil, gas and coal or coke are
most common but there is a steady growth of the use of electric
furnaces.

[Illustration: FIG. 92.--Standard lead pot furnace.]

TYPICAL OIL-FIRED FURNACES.--Several types of standard oil-fired
furnaces are shown herewith. Figure 92 is a lead pot furnace, Fig.
93 is a vertical furnace with a center column. This column reduces
the cubical contents to be heated and also supports the cover.

[Illustration: FIG. 93.--Furnace with center column.]

A small tool furnace is shown in Fig. 94, which gives the construction
and heat circulation. A larger furnace for high-speed steel is
given in Fig. 95. The steel is supported above the heat, the lower
flame passing beneath the support.

For hardening broaches and long reamers and taps, the furnace shown
in Fig. 96 is used. Twelve jets are used, these coming in radially
to produce a whirling motion.

[Illustration: FIG. 94.--Furnace for cutting tools.]

[Illustration: FIG. 95.--High-speed steel furnace.]

Oil and gas furnaces may be divided into three types: the open
heating chamber in which combustion takes place in the chamber
and directly over the stock; the semimuffle heating chamber in
which combustion takes place beneath the floor of the chamber from
which the hot gases pass into the chamber through suitable openings;
and the muffle heating chamber in which the heat entirely surrounds
the chamber but does not enter it. The open furnace is used for
forging, tool dressing and welding. The muffle furnace is used for
hardening dies, taps, cutters and similar tools of either carbon
or high-speed steel. The muffle furnace is for spring hardening,
enameling, assaying and work where the gases of combustion may
have an injurious effect on the material.

[Illustration: FIG. 96.--Furnace for hardening broaches.]

[Illustration: FIG. 97.--Forging and welding furnace.]

[Illustration: FIG. 98.--Semi-muffle furnace.]

[Illustration: FIG. 99.--Muffle furnace.]

Furnaces of these types of oil-burning furnaces are shown in Figs.
97, 98, and 99; these being made by the Gilbert & Barker Manufacturing
Company. The first has an air curtain formed by jets from the large
pipe just below the opening, to protect the operator from heat.

[Illustration: FIG. 100.--Gas fired furnace.]

[Illustration: FIG. 101.--Car door type of annealing furnace.]

Oil furnaces are also made for both high- and low-pressure air,
each having its advocates. The same people