Agriculture Reference
In-Depth Information
in the twelfth century, describes small tools made entirely of steel and provides
guidance for hardening larger files without scaling or decarburizing their fine
cutting surfaces (always a danger in the oxygen rich forge fire).
This wondrous material, steel, was a boon to the smith, the consumer and to
the tool user. Bar iron will simply make poor edge tools. Like most metals, iron
will stiffen from cold hammering, but it will never take an effective cutting edge.
Steel, on the other hand, can be forged to shape, slow cooled to leave it soft and
then filed, polished, engraved or otherwise dressed and improved. Then - as a
final step - it can be re-heated and fast-cooled to produce the desired hardness
for a given application: from glass-hard, through springy, to soft but tough.
As in all things, there is no free lunch. The harder the steel the more brittle
it becomes. While the same steel might make a 'burin' ['graver'] for copper-
plate engraving (very hard) or a spring (soft but elastic), each use demands a
different degree of hardness, each has a different tolerance for brittleness. Well
before the eighteenth century smiths discovered that they had great control
over the final hardness of steel if they first made it as hard as possible by quick
quenching from red heat and then carefully re-heated the steel to a lower tem-
perature in the 300 to 600 degree F range (149 to 316ºC). This pulls away the
excess hardness that weakens the tool and leaves behind just the needed hard-
ness for the tool to function well. To quote Moxon concerning newly hardened
steel: '….you must
let it down
(as Smiths say) that is, make it softer by
tempering
it' (Moxon, 1678, 61).
This second heating, 'tempering', in Moxon's terminology, modern smiths
also call 'drawing' or 'drawing the temper'. The higher this second temper-
ing heat, the softer the steel was left. The smith judges forging temperature
by the incandescent colour of the heated iron; tempering heats are judged by
the development of oxide colours on polished steel: yellow or straw, bronze
or brown, purple, peacock blue, pale blue and then grey as the temperature
increases. Smiths could also selectively temper different parts of a tool leaving
some areas relatively soft and tough, other parts harder.
While I mentioned that the early bloomery furnaces could make steel by
direct reduction of iron ore, a more readily controlled process called 'cementa-
tion' developed as well. I think this accounts for most large-scale steel produc-
tion by the Renaissance. In this process a piece of quality iron was surrounded
by a carbonaceous material - often charcoal - and packed inside a heat resist-
ant, closed container to exclude oxygen. The container and its contents were
brought to glowing red temperature in a furnace and held at temperature for
a lengthy time, perhaps days. Heated in the absence of free oxygen the iron
absorbs carbon from the materials packed around it. Heated long enough, the
iron will absorb carbon into its core. The result is a low grade of steel with
uneven carbon content. The British referred to this in the eighteenth century as
'blister steel'.
