The blast furnace evolved naturally from earlier direct reduction smelters. As the furnace was made taller in order to increase the temperature and yield, the carbon pick-up of the iron would increase to the point where it would form molten cast iron. This substance could not be forged (due to brittleness) but has a lower melting point than steel, allowing it to be cast into various shapes with the technology of the time.
Principles of operation
A blast furnace in its most basic form consists of a tall tube with several tuyeres (nozzles) through which air or oxygen is blown into the furnace. The furnace is loaded with layers of coke or charcoal, iron ore and fluxes such as dolomite or lime. As the fuel burns, carbon monoxide (and tiny amounts of hydrogen impurity) gas reduces the iron oxides in the ore and deposits additional carbon in it. This high-carbon alloy then melts and collects at the bottom of the furnace along with a layer of molten slag, which floats on top of it. The metal is removed by periodically breaking open a tap hole at the bottom of the furnace.
The blast furnace must be pre-heated by burning an initial charge of fuel without any ore or flux.