<P> Reverberatory furnaces (in this context, usually called air furnaces) were formerly also used for melting brass, bronze, and pig iron for foundry work . They were also, for the first 75 years of the 20th century, the dominant smelting furnace used in copper production, treating either roasted calcine or raw copper sulfide concentrate . While they have been supplanted in this role, first by flash furnaces and more recently also by the Ausmelt and ISASMELT furnaces, as they are very effective at producing slags with low copper losses . </P> <P> The first reverberatory furnaces were perhaps in the medieval period, and were used for melting bronze for casting bells . They were first applied to smelting metals in the late 17th century . Sir Clement Clerke and his son Talbot built cupolas or reverberatory furnaces in the Avon Gorge below Bristol in about 1678 . In 1687, while obstructed from smelting lead (by litigation), they moved on to copper . In the following decades, reverberatory furnaces were widely adopted for smelting these metals and also tin . They had the advantage over older methods that the fuel was mineral coal, not charcoal or' white coal' (chopped dried wood). </P> <P> In the 1690s, they (or associates) applied the reverberatory furnace (in this case known as an air furnace) to melting pig iron for foundry purposes . This was used at Coalbrookdale and various other places, but became obsolete at the end of the 18th century with the introduction of the foundry cupola, which was a kind of small blast furnace, and a quite different species from the reverberatory furnace . </P> <P> The puddling furnace, introduced by Henry Cort in the 1780s to replace the older finery process, was also a variety of reverberatory furnace . </P>

Why silica is added to the sulphide are of cu in the reverbatory furnace