Forge, Furnace, What's the Difference?
Medieval ironmakers often used the terms "bloomery," "forge," and "furnace" interchangeably. The terms were, in fact, the names given the three major mechanisms employed during the course of the Middle Ages for smelting (separating metallic iron from its ore). Although all used charcoal for fuel, there were significant differences in methods, scale of operation and efficiency. The evolution from bloomeries, a legacy of the ancient world, through the Catalan and other forges, to the first blast furnaces has already been traced in the In-Depth Article "The Rise and Fall of a Medieval Iron Technology." What remains to describe is how the forge, as it was displaced by the blast furnace in smelting, became the "finery" forge where most pig iron was taken to be refined into wrought iron.
What was this "refining" and why was it necessary? The pig iron coming out of a furnace was an alloy — roughly four percent carbon and other impurities and 96 percent iron. So long as it was in a molten state it could be shaped in moulds or as it cooled by hammering. Once cold, however, it was no longer malleable and could no longer be shaped by hammering. It was brittle and if struck by a hammer would crack, break or even shatter. Increasingly the market demanded wrought iron which was malleable and could easily be fashioned by blacksmiths into shoes for horses, iron tires for wagons and carts, every day tools, nails and spikes, and a a hundred-and-one other products.
The basic equipment of such a forge consisted of two or more small furnaces or fires: a "finery" (or "refinery") and a "chafery," both fueled by charcoal. There were two or more large trip hammers driven by water-power, carts for moving stock around, assorted hand hammers, anvils, tongs, and assorted other tools for manipulating hot bars of iron. Usually these forges were housed in large shed-like buildings of wooden frame construction. The refining was done by two-man teams, each working with anvils and one of the large trip hammers. Usually one of the team members was experienced, the other learning the process. The number of employees depended on the number of fires and hammers operated and orders to be filled.
The "refining" process began by "softening" (reheating) a batch of cast iron sows and pigs from the casting room floor in a small "finery" furnace. The finery furnace's strong blast of air oxidized part of the carbon and impurities in the pigs as it softened them. A skilled "finer," working the softening, semi-molten iron with a long iron bar, shaped it into a large chunk called a "half-bloom." The half-bloom was turned over to one of the teams operating a large, water-powered trip hammer. The experienced forgeman would place the half-bloom on an anvil and indicate to his assistant where it was to be pounded by the trip hammer. The pounding would continue until the half-bloom cooled. The half-bloom was repeatedly heated and hammered until it became a thick flat bar of wrought iron called an "anconie" which had knobs on both ends.
There was a market for anconies so some were sold at this point. Most, however, went to the "chafery" hearth where they were reheated and hammered into long bars of "decarbonized" iron, tailored to various lengths, sizes, and shapes required by those who were buying them: blacksmiths, coopers, tool makers, and wheelrights who made them into products they would sell to their customers.
The years brought changes at the finery forges. Early in the 19th century they began to produce not only wrought iron bars, but larger slabs called "blooms" that were forwarded to rolling mills for further refining. There the blooms were compressed by heavy rollers rather than hammered. By the 1850s half of Pennsylvania's forges operated outside the medieval pattern, using steam engines rather than water wheels to power their hammers and generate their air blasts.