Cupola smelting
The mills which had superseded the ancient bolehills in the late 16th century, a development described above in section 3, were themselves superseded in the 18th century by the gradual introduction of a new type of furnace known as the cupola.[37]
The old mills had a number of disadvantages. Their characteristic overheating and dissemination of polluting fumes made it necessary to close the smelter down at the end of each day’s work. The hearth burned out quickly and regular weekly repairs or rebuilding were necessary – between 24 June and 29 September 1657, for instance, thirteen new hearths were required at the Upper Mill in Wirksworth[38] Water-powered smelting mills were restricted to riverside sites and “white coal” fuel required a good supply of timber. By the 18th century timber supplies were running out and, where coke or coal was used because of timber shortages, impurities, particularly sulphur, were introduced into the lead. It was, finally, less efficient than the cupola.
The cupola was a reverberatory furnace. The fuel was burned in a combustion chamber at the side of the furnace, separate from the “charge” of ore, thus avoiding any contamination. This removed the disadvantage in using coal, which was far more plentiful than timber. The ore was loaded from a hopper into a concave furnace with a low, arched roof and a tall chimney or a flue at the opposite end from the combustion chamber. The flames and heated gases from the fuel were drawn across the charge by the draught from the chimney and beaten down by reverberation from the low roof. Slag on the surface of the molten lead was raked off and the lead itself poured into an iron pot at the side, before being ladled into moulds.
Several factors contributed to the cupola’s greater efficiency than the smelting mill. Unlike the smelting mill, the cupola could be operated continuously. Since the air flow over the ore was less powerful than that from the bellows of the blast furnace fewer lead particles were blown away. Further lead was saved by the fact that since the fuel and the charge were separate none of the lead was lost into the ash. Since no water power was needed the cupola had a fourth theoretical advantage of being freed from the riverside location of the blast furnace, and able to be placed in the most convenient site for supply of ore and coal. However the higher temperatures needed to melt the slag recovered from the primary melt required a water powered furnace and, since slag mills tended to be placed next to the cupolas, most cupolas remained in riverside sites.
Many cupolas had long horizontal flues, which were introduced to trap pollutants before they could be discharged into the air. Since the pollutants included metal vapour, the sweepings of the flue could also be recovered for re-smelting.
No comments:
Post a Comment