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] 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.