INTRODUCTION	23
pure form of commercial iron. It is very tough, though containing
slag streaks, and can be hammer- or forge-welded. It had a wide use
before the large-scale production of steel, but to-day it is used
chiefly by blacksmiths. Wrought iron is distinguished from steel
principally by the fact that the former never becomes molten during
manufacture, whereas steel is melted and cast and was, therefore,
once known as ingot-iron.
Steel has been defined as that form of iron containing less than
2-2 per cent of carbon (usually the range is from o-io to i -3 per
cent), produced in a fluid condition and hence practically free from
slag. It is produced by the crucible, the Bessemer and the open-
hearth processes, and also in the electric furnace. Steel may be
produced as castings, or as ingots, which are subsequently rolled or
forged to the required shapes,
The Crucible Process.—In 1740 Benjamin Huntsman developed the
first successful process for the fusion of steel on a commercial scale.
Blister steel, made by packing wrought iron in charcoal and heating
between 900° C. and 1,000° C., was broken into pieces and melted
in crucibles having a charge of about 100 lb. weight in each. This
process is now only used for the small scale production of special
tool steels.
The Bessemer Process, sometimes called converting, consists in
blowing air through molten pig iron contained in a suitable vessel
(converter), whereby the impurities are oxidized and removed.
Depending on the type of lining of the vessel there are two modifi-
cations :
(1)	The acid Bessemer process (the original process—-1856) by which
the silicon, manganese and carbon, but not the phosphorus,
are practically eliminated. This process is therefore only used for
non-phosphoric ores.
(2)	The basic Bessemer process (the Thomas or Thomas-Gilchrist
process-—1878) by which most of the phosphorus and part of the
sulphur are removed in addition to the above-mentioned ele-
ments. To do this the furnace is lined with a "base," e.g. lime-
stone, which absorbs the phosphorus and yields a basic slag.
The significance of the Bessemer process was that it enabled steel
to be produced cheaply for the first time on a large scale. The
significance of the Thomas-Gilchrist process was that it permitted
the utilization of ores of relatively high phosphorus content and
therefore released for exploitation the iron ore fields of Germany