How Furnaces Work
|Graphic courtesy of Lancaster County Historical Society.
At the core of any smelting furnace operation was the stack. The base of heavy stone was able to withstand the extremely high temperatures generated within. The early stacks were often built against the side of a hill to facilitate loading of raw materials. In later days "skip hoists" transported the tons of anthracite coal, iron ore and limestone to the top. The coal fueled the fire that would ultimately melt the ore to extract the iron. The limestone acted as a flux which caused impurities to be separated from the metal. Anthracite furnaces differed from their charcoal fueled predecessors in that they required the blast of air that fanned the fire to be preheated. The blast was heated by a number of methods, among them the use of stoves and the capture of heated exhaust gasses. From the time a furnace was ignited until it was "blown out" after a blast was called a campaign; the length of a campaign always attracted much attention. In the days when a furnace could go bad in a day or two, an endurance record of two years was a newsworthy event. Lacking modern knowledge of chemistry, early ironmasters and their employees tended to regard what occurred inside the furnace as a combination of luck, Calvinistic predestination, and black magic. The gods that brought floods and famines were akin to the spirits operating within the fiery stacks. When a furnace was "blown in" the wife or daughter of the proprietor or of one of his honored employees performed the ceremony of igniting the furnace. Congratulations and best wishes would be offered, toasts occasionally would be drunk, and the local press would comment optimistically, as The Columbia Spy was wont to do:
The Shawnee Furnace "blew in" last week, and under the judicious management of Colonel W. C. Bradley, we trust they will have a long, successful and profitable blast. Mr. L. McMichael is the founder and he has proved himself competent.
Tapping the Furnace
Another structure common to these furnaces was the casting house. This was where the molten iron would be allowed to run out onto the sandy floor and into troughs which connected to a series of depressions. The troughs were referred to as the "sow" and the depressions were called "pigs" in reference to their visual similarity to what workers saw on the farm. This was the origin of the name given to the finished product "pig iron". Tapping probably occurred roughly every 12 hours. The first step in tapping was to open a hole in the fire brick a few feet above the base of the stack to draw off the impure iron called slag. The slag was allowed to cool and was then dumped in huge piles that surrounded all furnaces. Then a lower hole was opened to draw off the iron. Once cool the pigs were lifted out and stacked to await shipment to foundries and rolling mills where the iron would be turned into rails, bars, and other shapes and eventually into finished products.