verter.]

[Illustration: FIG. 3.--Regenerative open hearth furnace.]

A diagram of a modern regenerative furnace is shown in Fig. 3.
Air and gas enter the hearth through chambers loosely packed with
hot fire brick, burn, and exit to the chimney through another pair
of chambers, giving to them some of the heat which would otherwise
waste. The direction is reversed about every twenty minutes by
changing the position of the dampers.

CRUCIBLE STEEL

Crucible steel is still made by melting material in a clay or graphite
crucible. Each crucible contains about 40 lb. of best puddled iron,
40 lb. of clean "mill scrap"--ends trimmed from tool steel bars--and
sufficient rich alloys and charcoal to make the mixture conform to
the desired chemical analysis. The crucible is covered, lowered
into a melting hole (Fig. 4) and entirely surrounded by burning
coke. In about four hours the metal is converted into a quiet white
hot liquid. Several crucibles are then pulled out of the hole, and
their contents carefully poured into a metal mold, forming an ingot.

[Illustration: FIG. 4.--Typical crucible furnace.]

If modern high-speed steel is being made, the ingots are taken
out of the molds while still red hot and placed in a furnace which
keeps them at this temperature for some hours, an operation known
as annealing. After slow cooling any surface defects are ground
out. Ingots are then reheated to forging temperature, hammered
down into "billets" of about one-quarter size, and 10 to 20 per
cent of the length cut from the top. After reheating the billets
are hammered or rolled into bars of desired size. Finished bars are
packed with a little charcoal into large pipes, the ends sealed,
and annealed for two or three days. After careful inspection and
testing the steel is ready for market.

THE ELECTRIC PROCESS

The fourth method of manufacturing steel is by the electric furnace.
These furnaces are of various sizes and designs; their size may be
sufficient for only 100 lb. of metal--on the other hand electric
furnaces for making armor-plate steel will hold 40 tons of steel.
Designs vary widely according to the electrical principles used.
A popular furnace is the 6-ton Heroult furnace illustrated in Fig. 5.

It is seen to be a squat kettle, made of heavy sheet steel, with
a dished bottom and mounted so it can be tilted forward slightly
and completely drained. This kettle is lined with special fire
brick which will withstand most