From Vatican Radio ...
The one used to burn the ballots is made out of cast-iron and has been used for every conclave since 1939. That was the one that elected Pope Pius XII - and there have been five conclaves since then. The second stove was added in 2005 in order to help identify the colour of the smoke from the outside. In fact, it’s not really a stove at all, but more like an electronic fumigating device that produces smoke which is either inky black (in the case of an inconclusive vote) or snowy white – heralding the announcement of the “habemus papam”.A copper duct carries the smoke up and out of the Chapel and through the chimney which is visible from outside on the roof. The duct is held in place by steel tubular scaffolding that goes all the way up from floor to ceiling, a height of some 20 meters. It keeps a safe distance away from the precious frescoes, of course, and – curiously – has been decorated with gold paint…probably to help it blend in better with its prestigious surroundings.