Iron Making

Molten iron flows from the furnace stack. Hopewell Furnace made castings and pig iron from 1771-1883.

NPS Photo

An ancient alchemy sustained Hopewell Furnace: transforming mineral into metal. Since 4,000 years ago, when humans learned how to free iron from ore, the basic process has not changed. Iron oxide is heated in an intense flame fed by carbon fuel. Oxygen in the ore combines with carbon monoxide released from the fuel and is expelled as CO₂. What is left is iron. The blast furnace’s height lets the rising gases preheat the ore and gives the iron more distance to descend as it softens – so it absorbs more carbon from the fuel. Because iron’s melting point falls as its carbon content rises, the iron becomes fully molten. A calcium-based “flux,” usually limestone, is added. The flux combines with impurities in the ore and forms slag.

The Resources

The basic ingredients of iron making – iron ore, limestone, and carbon fuel – are some of the most common on Earth, but are not found everywhere. Early furnaces were built where these materials were available.

Iron is usually found in combination in the form of hematite, the magnetite used at Hopewell, or other iron ores. Most iron ore was dug in small surface mines. Any substance that contained calcium, like seashells, could be used as a flux, but for most furnaces, limestone was cheap and abundant.

American forests were so vast – and bringing in coal so expensive before railroads were built – that early iron plantations like Hopewell made their own fuel. They slowly burned and carefully built piles of wood to make charcoal, an almost purely carbon fuel that burns with intense heat. The great demand for charcoal meant that early furnaces were sited on woodlands.

One other ingredient was needed: air. It was directed into the hearth under pressure by the water-powered blast machinery, raising the fire in the furnace to smelting temperature (around 3000°F).

The Process

Iron plantation life revolved around the always-running, roaring furnace. It shut down usually once a year – to refurbish its inner walls and hearth. While it was in “in blast,” its cycles of filling and tapping set life’s rhythm at Hopewell. It demanded close attention. Workers constantly fed it, watched its flame, and listened to the sound of its blast. For workers around the furnace, it was a hot, hard job requiring protective shoes and aprons. Every half-hour fillers dumped into the tunnel head 400 to 500 pounds of iron ore, 30 to 40 pounds of limestone, and 15 bushels of charcoal. With no gauge, the founder used his practiced eye to judge the shape and color of the flame from the chimney and the color and consistency of molten iron. This told whether the temperature was right and the proportions of ingredients correct. In temperatures that could reach 3,000°F, the molten iron flowed down toward the hearth, to be tapped when the founder judged it ready. At Hopewell, he generally tapped the furnace every 12 hours, at 6 am and 6 pm. After the guttermen drew off the slag, the iron could be tapped in two ways. It could flow directly into the “pig bed” in the cast house floor (it looked like a litter of nursing pigs), where it hardened into pig iron ready for market. Or it could be tapped into large ladles, the cast in molds. This process was repeated twice daily as long as the furnace was in blast.

 

The first known casting at Hopewell Furnace is the 1772 Mark Bird Stove, on display at the Visitor Center.

NPS Photo

The Product

To make the most money from molten iron you would cast finished products at the furnace. Moulders cast several items: plowshares, pots, sash and scale weights, cannon, and shot. But as iron stoves grew more common in 1800s homes, Hopewell built its operation on stove plates.

Cast products made profits, but the age also demanded goods that the furnace’s brittle high-carbon iron was not suitable for. The tough, malleable wrought iron needed for plow moldboards, nails, and horseshoes had to be obtained through an indirect process not in place at Hopewell. The molten iron was cast into pig iron bars shipped elsewhere for refining.

At the finery forge, the iron was remelted and much of its carbon was oxidized, raising the melting point. The iron partially solidified into a pasty lump. The lump was then beaten to drive out the slag and align the fibers, producing wrought iron. More processing converted the iron into the bars and rods used by blacksmiths.

A lengthier process was used to convert iron into steel. Hopewell’s owners often held interests in local forges and mills.