The Manufacture of Charcoal Iron at Centre Furnace
Centre Furnace first went into "blast" and began producing iron in 1792 using very simple technology. Workers layered the raw ingredients into this 35-foot furnace stack, made of native stone. First a burning hearth was created in the stack by filling it with charcoal. Then the furnace was "put in blast" by releasing the water wheel-powered bellows to blow a blast of air through the hearth. Iron ore, charcoal, and limestone were added from the top in alternate layers.
For up to a year at a time, twenty-four hours a day, constantly renewed ingredients were added to the burning hearth until pure iron separated from its impurities. Slag, the by-product, floated to the top of the molten iron, while the heavier liquid iron sank to the bottom.
The iron was drawn off, or "tapped" from the bottom of the stack, twice a day. It flowed into sand molds in a casting house. The molds were shaped into a large trough in the center, and smaller troughs on either side at right angles. The shape of the molded iron reminded early furnace men of a sow suckling her piglets, which in turn introduced the term "pig iron." Before the iron cooled, the smaller pigs were separated from the sow and the sow was broken up into smaller pieces. After cooling, the pigs were shipped to forges to be worked into bars of wrought iron.
While initially some iron was forged at Centre Furnace, most pig iron was transported to forges along Spring Creek to be transformed into iron goods. Or, the iron was carried on mule back or by wagons to be shipped on flat boats, called arks, west to Pittsburgh, or south and east to Philadelphia and Baltimore.