Materialising Sheffield - re-presenting the past  
steel ingots

Benjamin Huntsman – Early Life

Benjamin Huntsman and the world of eighteenth-century science

Early Eighteenth-Century Steel Production


Industrial Secrecy and Espionage

6. Opposition of the Sheffield cutlers

7. Huntsman’s works at Handsworth and Attercliffe

Swedish Visits to the Attercliffe Works

The evolution of the Attercliffe Works in the later 18th Century



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.





illustration: Henry Horne’s Essays on Iron and Steel