In this era iron was made in stone blast furnaces, about twenty-five feet high, in which a mixture of iron ore, charcoal, and limestone was burned. A blast of air generated by water-powered bellows kept the fire burning and the temperature in the furnace high enough to melt the product iron. In the furnace the combustion of the charcoal generated carbon monoxide that reacted with the iron oxide ore to make metallic iron. The limestone absorbed impurities from the iron to create slag which could be skimmed off the molten iron. Iron from a blast iron was called pig or cast iron. The term pig iron probably came from the way in which the molten iron from the blast furnace was allowed to run through channels dug in the sand cast floor to form approximately ninety pound "pigs" of iron. Apparently, someone once thought that this scene resembled young pigs suckling a sow–and the name stuck. Cast iron refers to the properties of blast furnace iron; it melts at a relatively low temperature. This type of iron was useful for making items such as pots and stoves, but because it was brittle it could not be used for tools or weapons. Cast iron could be converted to aptly named "wrought" iron by repeated heating and hammering. Wrought iron was much tougher than cast iron, but was too soft to make good knife or sword blades. These uses required steel, which has a combination of toughness and hardness. It was made in very small quantities by heating wrought iron bars buried in charcoal. Chemists and metallurgists would later explain the differences between these types of iron. Cast iron contains about 4% carbon, absorbed in the blast furnace, and wrought iron has very little carbon in it. Steel refers to a wide variety of materials containing between 0.1 and 1% carbon, although it the complex crystal structures of steels that are responsible for their properties. One important limitation of colonial technology was that furnace temperatures could not be raised high enough to melt wrought iron or steel, so both had to be made from small solid bars.
Iron was not only difficult to work, it was also expensive to make. Because of limited transportation on rough roads and shallow rivers, iron could only be made where iron ore, limestone, extensive forests for charcoal, and fast-flowing streams to provide water-power were located. So-called iron plantations soon became known for their destruction of forests. A typical 18th century blast furnace that produced about 600 tons of iron per year would clear 300 acres of trees annually. Wood was converted to charcoal by a controlled burning process that had to be tended around the clock. Charcoal had to be supplied to the blast furnace continuously because once lit, it was in blast for many months at a time. The scale of blast furnaces, and iron production, was limited by the amount of nearby timber that could be converted to charcoal.