Agriculture Reference
In-Depth Information
not chemically allied to the iron but ran as long, thread-like inclusions through
the bar giving it a linear grain similar to a piece of wood. Also like a piece
of wood, the bar was relatively strong when stressed in one direction relative
to the grain and weak when stressed in the other. In the better grades of bar
iron, the silica content was minimized by working the material at white heats
to squeeze out as much of the liquefied silica as possible and reduce what
remained to finer and finer threads evenly distributed through the bar.
This was the basic bar stock of the smith from the beginning of the Iron
Age until the 1870s. It was tough material when well refined, soft under the
hammer when hot and it welded at the forge beautifully. The best grades are
a dream to work. As wonderful as this material is, it makes poor quality edge
tools. There is, however, an ancient ferrous material called 'steel' that can be
manipulated to produce long-wearing and effective cutting tools. Make no mis-
take, steel is nearly as ancient as iron in human history.
Technically speaking, steel is iron plus a relatively small amount of carbon.
Even trace amounts of carbon can strengthen a bar a little, harden a bar some
and lower its melting or burning temperature by a couple of hundred degrees.
Yet, material with such low carbon content would not have been thought of as
steel in the pre-industrial age.
Steel, in the historical context, is a ferrous material that was capable of hard-
ening by quick cooling from a red heat - an effect imparted by carbon content
in excess of about .35% (that is thirty-five one hundredths of one percent by
weight) and to perhaps as high as 1.25% carbon. To a point, the more carbon
in the steel, the harder it will become. Also, up to a point, the quicker you
can cool it, the harder it will be rendered: cool in the air and leave the bar
soft; cool in water and make it quite hard; cool in oil and achieve a mid-range
hardness.
In the period, no one knew with scientific certainty what it was that made
steel harden in this way. Modern experiments have shown that the ancient
'bloomery' furnaces that smelted iron in the Middle Ages were perfectly cap-
able of making steel, too, when operated in an appropriate manner. Yet, why
some bar was soft to hammer and others resisted the hammer blow, why some
bar burned at a lower temperature than others and why some would get brittle
hard when quenched in water from a red heat was little understood. To add
to the mystery, smiths found that once a bar of steel was made hard through
quenching, the effect was completely reversible - just heat the bar red-hot
again and let it slow cool. It will be soft once more.
Yet, lack of sophisticated metallurgical science did not inhibit smiths from
practical knowledge. Moxon, writing at the end of the seventeenth century
[
Mechanick Exercises: or, The Doctrine of Handy-Works, of Smithing in General
,
d
.1678], was well aware of the characteristics of steels from different regions,
how to identify each and the sorts of work each was best for. Theophilus, writing
