Iron ore, from which metallic iron can be extracted, is interlaced with other minerals, namely sulfur and phosphorus. Separating out the pure iron involves intermixing the iron ore with the carbonized fuel, either charcoal manufactured from wood or coke produced from coal. If the temperature is raised to 1,400œ Celsius, the pure iron can be separated from its impurities. This high temperature was first achieved in the Middle Ages by the addition of a blast of air (hence, the term "blast furnace") during the process. Furnaces needed to be located near a stream so that a waterwheel could provide power for the blast, and in proximity to deposits of ore and sources of fuel, because the cost of transporting these items any distance was prohibitive. The iron that emerged from the blast furnace could be poured into molds so that it formed "pigs," or ingots.

Pig iron's high carbon content makes it very brittle. For most uses, it has to be further processed after reheating it in a forge to remove impurities. Prior to Abraham Darby's innovation using coke-fueled blast furnaces for smelting iron, few iron products were made directly from cast iron; practically all iron products were made from reworked pigs in which the excess carbon had been removed by reheating and then pounding the pig iron until most of the carbon had been removed. Darby found that cast iron could be used to make cooking pots, which were universally used in eighteenth century England and whose brittleness was not a problem. By setting up a business to produce three-legged cast-iron cooking pots, Darby created for himself a market niche that he and his firm heavily exploited.

Under Darby's son and successor, Abraham Darby II, the business was greatly expanded to produce parts for agricultural plows, steam engines, and cannon, not to mention iron bridges. (Abraham Darby III built the Iron Bridge across the Severn River near his works at Coalbrookdale.) The Darbys were the instigators of industrial iron manufacture in England. The processes they pioneered were later introduced elsewhere in the world and laid the basis for the Industrial Revolution.

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