| 2. Benjamin Huntsman and the world of
eighteenth-century science
At the time of Huntsman's experiments, very little
was understood of the chemical or physical composition of metals.
It had yet to be recognised that the difference between iron and
steel lay in their relative carbon content, and the Phlogiston
theory of Georg Ernst Stahl still held favour among scientists.
In practice, the techniques of iron smelting and steel making
were more an art than a science, developed in most cases by trial
and error. Had Huntsman consulted any of the available scholarship
on the subject of steel, he would have found little there to guide
him.
Huntsman's profession demanded a range of mechanical and metallurgical
skills, and his surviving clocks and watches include skilfully
executed work in brass, steel, gold and silver. Huntsman would
not necessarily have done all of the work himself, but a good
working knowledge of these metals was essential, and he was no
doubt familiar with their production, refining and alloying. Gold
and silver were still assayed and refined by the centuries-old
process of cupellation, which involved melting the impure metal
with lead and a flux of various salts in small, porous bone-ash
crucibles, or "cupels". The flux would react with the
oxidised lead and any other base metals present, before being
absorbed by the walls of the crucible, leaving behind a "button"
of the purified precious metals. The temperatures required were
easily attainable in an assaying furnace using charcoal as a fuel,
the only limitation being the small scale of the operation.
It was known in practical terms that common blister steel suffered
from a lack of homogeneity and unwanted impurities. During the
cementation process, the outside of the bar absorbed more carbon
than the centre, giving it a more "steely" character.
Its texture also became coarser and more crystalline, and the
body of the steel was peppered with tiny voids and fissures. The
subsequent forging process gave the steel a more even character,
although streaks of slag would always remain, resulting in a product
that was variable in quality and never totally sound. Huntsman's
aim was to apply the refining process to cementation steel, in
the hope that it would be similarly "purified".
Steel, however, had not previously been brought to temperatures
high enough to allow its complete fusion. Neither common cupels,
nor the larger clay crucibles used for glass making or brass founding
were able to withstand the intense heat required, or the attack
of the molten metal on the walls of the vessel. Even if a suitable
crucible were found, none of the furnaces then in use could have
produce sufficient heat, in a controlled environment, to melt
the steel. Huntsman, therefore, needed to develop both a new crucible
and a new furnace.
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