Device used to convert molten pig iron to steel, via the bessemer process. Typically it is made of steel, and lined with either gannister or dolomite.

Physically speaking, the converter is a hollow, egg-shaped device capable of holding approximately 8 to 30 tons of pig iron (though the standard charge is only 15 to 18 tons). There is an opening at the top, which serves as the means by which new iron is introduced as well as by which refined steel may be extracted. The bottom contains a number of perforations (tuyères) which allow air to be forced into the charge, and the entire device is set on pivot fixtures known as trunnions.

When receiving the charge, the converter is tilted to accept the pig iron. It is then turned upright for the "blow". This is when air is forced through the tuyères, where it makes contact with the iron so as to cause many of the impurities to oxidize (see: bessemer process). An amount of carbon monoxide is also burned away. The air then bubbles from the surface of the iron, and the slag falls to the bottom. At this point the converter is tilted again so as to allow the refined steel to be poured out. The process takes about 15-20 minutes, depending upon the size of the charge.