Where Do Little Pig Irons Come From?
The clanging of the furnace bell signals that the blast furnace has completed its work on another batch of iron and stands ready to be tapped. The Casting House, directly beneath the Charging House, is essentially a large empty room with a floor of mixed sand and clay. It now becomes the center of attention. All hands not needed elsewhere make their way there to help. The business end of the Casting House is in the Cast Arch of the furnace; there the tapping will occur.
During the shift, a keeper or two were stationed in the Casting House, peering over the tymp stone, monitoring the crucible where the molten iron and slag were accumulating. At the Founder's command, a keeper knocks a clay plug from the higher of two holes in the dam stone, allowing the slag to run off in a trough to one side. When it cools it will harden, be broken up by sledge hammers, and hauled away as unwanted waste. The keeper, meanwhile, returns to the dam stone and knocks out the lower clay plug, releasing the flow of molten iron into molds that the gutterman and his helpers prepared earlier.
The shape of the molds is traditional: a string of large molds each about four feet long, four or so inches wide and six inches deep, running end to end from a point a few feet from the tymp stone, halfway across the Casting House floor. These are referred to as "sow molds." Leading off from the sow molds at right angles are a series of smaller ones, side by side, called "pig molds." The gutterman and his helpers guide the flow into the first of the sow molds. When it fills, the molten tide is guided into the pig molds, then into the second set of molds and so on until the entire batch is accommodated. To people living in an agricultural society, the pattern of the molds resembles a very common scene: litters of small piglets suckling their mothers. From that similarity the output of the iron furnace derived its name: "pig iron."
Although the output of furnaces differed widely, between 1 and 1½ tons per tapping, 2 to 3 tons per furnace per day, and 600 to 900 tons per year was fairly common. The iron pigs, weighing well over 50 pounds each, were stacked in the Casting House until they could be taken to the forge for further work. Because pig iron still contained carbon and other impurities, once it cooled it became unmalleable and brittle. This made pig iron all but useless to blacksmiths and others who fashioned iron goods.
Even so, pig iron could be used for many needed things; frying pans, cooking pots, ornamental fences and railings, and fireplace screens to name a few. Molders and their helpers, having taken orders for such items beforehand, now used patterns of such to fashion molds for them in the sand floor. As the tapping proceeded, they carried ladles of molten iron from the furnace and poured it into the patterns. Pig iron used in this way was referred to as “cast iron.”
However, the great bulk of iron coming from the furnace was shaped into pigs. When they cooled they were hauled to a forge. There they would be worked on to reduce their carbon content and shaped into wrought iron bars that were malleable and much sought by blacksmiths and other workers in iron.