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STEEL 43

carbon to iron; melting point variously estimated at from

1500-1600° C.
Soft steel contains 0.5 per cent, or less of carbon. Hard
steel contains from 0.5 per cent, to 1.5 per cent, of carbon.

Steel of an extreme degree of hardness, containing a large
percentage of carbon, is utilized for making dental cutting
instruments.
Hardening Steel.—Ordinary steel may be hardened to its
limit by heating to a full cherry red color and immediately

plunging in cold water. In this state it is extremely brittle.
The more carbon it contains, the harder and more brittle it
becomes.
Tempering Steel.—By tempering steel is meant the process
of rendering it softer, tougher and less brittle. It is ac-
complished by heating to a lower temperature than that used

for hardening steel, and then plunging in cold salt water,
slightly acidulated water, oil or mercury. Oil gives a tougher
temper than water, while mercury gives an extreme degree of
toughness. Hard steel is capable of much more temper than

soft steel.
Annealing Steel.—The operation of reducing it to its
extreme degree of soitness. This is accomplished by heating
to dull or '^incipient'' redness and cooling slowly. A greater

degree of softness is produced by covering with sand, plaster
or pumice, thus excluding the atmosphere, heating and
allowing to cool while so covered.
In the manufacture of dental instruments, hard steel
(that containing a high percentage of carbon) of a higher or
lower degree of temper is required, depending upon the pur-

pose for which the instrument is intended. The following
table shows the approximate temperatureis, with the cor-
responding colors produced, to which instrument steel is

carried in the making of various instruments: