Cupola furnace

The top where gases escape can be open or fitted with a cap to prevent rain from entering the cupola.

During the Han dynasty (202 BC – 220 AD), most, if not all, iron smelted in the blast furnace was remelted in a cupola furnace; it was designed so that a cold blast injected at the bottom traveled through tuyere pipes across the top where the charge (i.e. of charcoal and scrap or pig iron) was dumped, the air becoming a hot blast before reaching the bottom of the furnace where the iron was melted and then drained into appropriate molds for casting.

[5][6] A modern cupola furnace was made by French scientist and entomologist René-Antoine Ferchault de Réaumur around 1720.

[7] To begin a production run, called a 'cupola campaign', the furnace is filled with layers of coke and ignited with torches.

Flammable gases also can be added to air and blown through the tuyere section of the furnace to add fuel to the fire.

During the melting process a chemical reaction takes place between the fuel, the blast air, and the metal.

Silicon carbide, ferromanganese, ferrosilicon, or other alloying agents, often in the form of briquettes, may be added to the charge materials.

Likewise, ferromanganese melts and is combined into the pool of liquid iron in the 'well' at the bottom of the cupola.

[8] The cupola tender observes the furnace through the sight glass or peep hole in the tuyeres.

The viscosity is low (with proper fluxing) and the red hot molten slag will flow easily.

With acid refractory lined cupolas a greenish colored slag means the fluxing is proper and adequate.

The width of the wedge at the point of demarcation between the white and gray areas is measured and compared to normal results for particular iron tensile strengths.