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238 — MECHANICAL DENTISTRY AND METALLURGY.
always passes before it fuses, that two pieces may be
brought together and made to cohere by pressure or ham-
mering, the process known as welding.
It is not upon the physical properties alone that the value
of iron depends, as it enters into many compounds which
are of much use to the arts, and its chemical relation to
carbon is such that the addition of a small quantity of the
latter will convert it into steel, which is harder and more
elastic than iron, while the addition of a larger quantity of
carbon produces cast-iron, which is more fusible and brittle.
Thus we have three distinct grades or modifications of
iron, cast-iron, zvroughf-iroii, and steel.
Cast-iron.—This is the product obtained by smelting
ores of iron in the blast furnace, and is commonly known
as pig iron. It usually contains 2 to 4 per cent, of carbon,
0.2 to per cent, of silicon and about i per cent, of
3
manganese, with small amounts of sulphur and phosphorus.
It differs from wrought iron or steel in being non-ductile,
more brittle, therefore not so tough, and is harder than
malleable iron.
Wrought-iron.—Wrought-iron, or malleable iron, is
probably the nearest approach to pure iron which can be
produced in a commercial way. This form of iron con-
tains from 0.05 to 0.3 per cent, of carbon, and is com-
paratively soft, very malleable, ductile and tenacious. By
this we understand that as far as practicable the foreign
substances have been removed from the pig iron by a
process of refining, known as puddling or Bessemerising.
Wrought-iron may be magnetised by keeping it in con-
tact v^'ith a magnet; it loses the property however as soon
as the magnet is removed. Heating to redness and
plunging into cold water does not harden it, as steel is
hardened when similarly treated.
Steel.—The marked apparent difference in steel and