Etna Furnace in blast was a spectacular sight. The furnace itself was impressive enough --a massive stack built of roughly finished, rectangular-shaped pieces of limestone, each weighing several tons. It stood 35 feet high and had an interior 9 feet across at its widest point. The process of blowing in started with slave workmen filling the furnace completely with charcoal. It was lighted at the top and took several days to burn to the bottom. When the fire reached the base of the furnace, slaves refilled the stack with charcoal and the fire gradually worked its way back up to the top. At this point, the furnace was charged, always from above, with measured quantities of iron ore, limestone, and charcoal, and the manufacture of iron began.The burning charcoal supplied the heat needed to melt the ore, the limestone provided the flux that carried most of the impurities in the ore to the top of the mass of liquid iron, and molten iron generally accumulated in a hearth at the base of the furnace. Blasts of air, needed to sustain temperatures high enough to melt the ore, were forced into the interior of the furnace by huge bellows operated by a waterwheel --thus the essential requirement of adequate waterpower at any furnace site. When enough molten iron settled in the hearth area, the furnace was tapped, usually twice a day, and the iron drawn off into sand molds and cast in one of two ways. Sometimes it was made directly into hollowware, items like skillets, pots, pans, kettles, and the like; more frequently, it was cast into pigs, which required further processing in a forge before they could be worked into usable articles... The refining and chaffery stages of the forge operation took out enough of the impurities present in the brittle pig iron to render the metal malleable, and therefore ready to be worked by a blacksmith into useful articles.
Once a furnace was blown in, it operated almost continuously, day and night, until the blast was completed. Blasts at southern furnaces frequently lasted for 6 to 9 months... (pp 29-30)
[describing the forge]
...charcoal fires burning at white heat, the slave refiner and his underhand working bars of pig iron in those fires until the iron turned into a ball of glowing, pasty metal. The refiner would then sling this semi-molten mass of iron onto his anvil, where it was pounded and shaped under the rythmic blows of the huge, water-powered hammer. Through successive reheatings and poundings, the refiner removed enough of the impurities in the pig iron to work it into something called an 'anchony'. Turning out high-quality anchonies was the most important single job in the forge... a piece of malleable iron about 6 inches square weighing between 80 and 150 pounds...Pounding out anchonies was the most critical part of the forge operation, but it was only the first half of the manufacturing process. The final stage came when a second group of operatives, the hammermen, reheated the anchonies and reworked them at another forge called a chaffery. The hammermen produced iron bars of various standard shapes, sizes, and lengths -'merchant bars', in the language of the iron trade... Blacksmiths hammered these bars into the things needed on or off the farm that had to be made out of iron: horse and mule shoes, wagin tires, nails, tools, agricultural implements... (pp10-11)