When first operated in 1792 the Cupola Mill had horizontal flues linked to a chimney a short distance behind the mill. When lead is smelted some of the metal is vaporised and is lost with the smoke, to reduce this loss many smelt mill flues were extended in the 19th century.

The Cupola mill was no exception and its flue was extended four times. The long flue allows the smoke to cool and the vaporised lead condenses onto the flue sides. This could be swept out from special access points and the metal recovered.