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 resmelting. Closure The Derbyshire lead industry declined after the late 18th century because of worked-out veins, increased production costs and the discovery of much cheaper foreign sources. The industry was protected from this foreign ore by import duty in the late 18th and early 19th centuries. A reduction in the duty in 1820 and its abolition in 1845 brought a steep rise in the volume of lead imported into England and accelerated the local industry's decline. There were still bursts of high production, and indeed the output of certain mines during the 18th and 19th centuries exceeded anything achieved in the 17th century; over 2658 loads (about 641 tons or 651 metric tonnes) were mined at Brassington, traditionally an area of low output, in 1862. At a meeting of the Barmote Court in Wirksworth in 1862 one mine owner announced "that by perseverance for upwards of twenty years, they had at last found the long sought for treasure, which he hoped would be prosperous, and they should be able to continue employing, as they are at the present time, upwards of 100 men at one mine in Brassington". However, by 1901 the number of men employed in all the Derbyshire lead mines had fallen to 285 most of whom worked at the Mill Close Mine at Darley Bridge. Mill Close, the biggest lead mine in the country, took the Derbyshire lead industry into the 20th century, and just before its enforced closure in 1939, caused by flooding, employed about 600 men. The smelter at Mill Close, established in 1934, was bought in 1941 by H J Enthoven and Sons, a London-based lead producer, and still operates. See also Beans and Bacon mine Magpie