| 5. Industrial Secrecy and Espionage
The cast steel furnace belongs to a category
now known as the induced draught shaft melting furnace which first
emerged around the eleventh century. By the time of Huntsman's
experiments, furnaces of this type were already in use for glassmaking
and the manufacture of copper and brass, and there is evidence
to suggest the latter was the model upon which Huntsman based
his crucible furnace.
As the melting point of steel is over 500 degrees higher than
that of brass, significant modifications had to be made to the
design of the furnace. The application of such a furnace to steelmaking
was most likely hindered by the lack of a suitable fuel and refractory
crucible material. Charcoal was the traditional fuel for metallurgical
operations, but by the eighteenth century its use was in decline
as Britain's forests were depleted. As an alternative, pit coal
was rapidly gaining in popularity, while coke -- a derivative
of coal which burnt at higher temperatures still -- had first
been used for iron smelting by Abraham Darby I at the Coalbrookdale
blast furnaces around 1709.
The lower combustion temperatures of the traditional fuels often
necessitated the use of bellows for processes such as brass-making,
but the introduction of taller chimneys gradually removed this
requirement. Chimneys had originated simply as a means of conveying
the smoke and heat of combustion out of a building, but were later
discovered to produce a draught, the intensity of which could
be varied with a damper and concentrated by increasing the height
of the stack. Huntsman chose not to use bellows, but to adopt
the more recent practice of the brass-founders as described in
Lazarus Ercker's treatise on metallurgy. Ercker's illustration
of "a furnace for making brass" bears a strong resemblance
to Huntsman's crucible furnace with its raised floor, furnace
"hole" with refractory lid, round crucibles and special
lifting tongs.
Nevertheless, Huntsman had to make substantial changes to the
design. The underground air passages of the brass furnace were
enlarged to become cellars, which allowed a man or boy to tend
the furnaces from beneath and to control the flow of air. Each
furnace also contained only one pot, as opposed to the eight or
nine of a brass furnace, due to the greater concentration of heat
required to melt the steel. Consequently, a number of individual
furnace "holes" were used, placed together in rows with
their flues converging into a single broad stack.
Huntsman employed the most heat-resistant materials then available,
including highly refractory local sandstone for the furnace holes
and special clay firebricks for the flues. The heat of the furnaces
also created an almost unbearable working environment. Consequently,
the windows to the furnace building had no glass, secured instead
by iron bars and external shutters, and the roof was liberally
punctured by the large hatches traditionally found in iron foundries
to let out heat and smoke.
Perhaps the greatest technical obstacle to the mass adoption of
Huntsman's process lay in the manufacture of the crucibles. When
the Swedish industrialist Robsahm visited Huntsman in 1761, he
was shown around the works and allowed to see the furnaces and
even the finished crucibles, but under no circumstances would
Huntsman show him where the crucibles were made or reveal their
composition, not even when he offered him fifty pounds. Even by
1814, the crucible making process was still sensitive enough to
be withheld from a German visitor to Sheffield.
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