The substitution of coke for charcoal greatly lowered the fuel cost for pig iron and wrought iron production.[23] Using coke also allowed larger blast furnaces,[24][25] resulting in economies of scale. The cast iron blowing cylinder was first used in 1760. It was later improved by making it double acting, which allowed higher furnace temperatures. The puddling process produced a structural grade iron at a lower cost than the finery forge.[26] The rolling mill was fifteen times faster than hammering wrought iron.[26] Hot blast (1828) greatly increased fuel efficiency in iron production in the following decades.