The Process of Iron Smelting: Turning Iron into Usable Metal

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Iron changed the world. It was a metal that could be turned into weapons, useful household implements and frames for buildings.

The process of smelting iron has been around for centuries, but in the early days of America furnaces were built in order to refine enough iron to build a country on.

Getting the Ore

The business of Catoctin Furnace in Frederick County, Maryland, was producing iron and the furnaces and workers did that for more than 125 years.

The furnaces produced pig iron in 80-100 pound bars that were five to six feel long and six inches around. The brown hematite ore was originally mined from the ground by pick and shovel. The ore was washed to remove the clay and then the clean ore was hauled by mule to the top of the furnace where it was dumped in a 12-foot opening and melted into pig iron.

The Iron Furnaces

The stone and brick furnaces were at least 32 feet tall and somewhat smaller at the base. “If the height was greater, the weight of the iron ore and limestone tended to crush the lower layers of charcoal, clogging the furnace,” Frank Mentzer wrote in a series of articles about the history of Catoctin Furnace. (The Frederick News, March 20, 1972) The sides tapered inward. The interior of the furnaces were lined with fire bricks and a rubble chamber between the brick and the exterior walls. The inner chamber was shaped like a soda bottle, open at the top. A few feet from the bottom was an air blast nozzle. Air was blown into the furnace by a bellows worked by a water wheel.

A Recipe for Iron

To produce one ton of iron, you needed 1 ¾ tons or ore, ¾ ton of charcoal, ¼ ton of limestone and 4 tons of air.

Before the furnace was started, all of the materials had to be placed in the furnace. The first layer was charcoal, then a layer of limestone and then a layer of iron ore. Additional layers in that order could be laid down until the furnace was filled to the top.

The fire was then started. Originally, it was kept burning by a natural draft. Later forced air was used. Heating the furnace started a number of chemical reactions in the materials.

“Charcoal burns with a greater heat than wood, and when this heat reached 2800 degrees F the iron in the ore began to melt and trickle down to the hearth. In addition to providing the necessary heat to melt the iron, the burning coal gave off carbon as gas which combined with oxygen from the air to form carbon monoxide. As this gas moved upward through the iron oxide it combined with the oxygen in the ore to form carbon dioxide, freeing the iron. As the melted iron trickled down to the hearth it dissolved a small amount of the carbon,” according to Mentzer.

By adding limestone to the burn mix, impurities melted at a lower temperature than the iron. The limestone would then combine with the impurities to form slag, a waste material.

“As the contents would burn and the iron melt and settle to the bottom, more charcoal or coke, limestone and iron ore would be added. This purpose continued until there was sufficient melted iron to draw off or be cast,” Fern Rice wrote in an unpublished paper about Catoctin Furnace.

Collecting the Smelted Iron

At the bottom of the furnace was a channel. As the iron flowed to the bottom of the channel, a worker would use a long hook and open a clay valve at the bottom of the furnace.

“When the iron was cast it would flow into channels or shallow troughs previously made ready by furrowing out long lines in the ground. These channels were sprinkled with sand to prevent the iron from adhering to the ground. The product obtained from the furnace was called pig iron. Some people say it was called pig iron because as the molten metal flowed along a main channel into a series of small sand molds it made a noise which suggested pigs sucking a sow. Since the channels were marked off into sections approximately two feet long, it was easy to break the iron into ‘pigs’ and carry them when sufficiently cooled. The ‘pigs’ were stacked in rows like cordwood and were then ready for shipment,” Rice wrote.

In the iron was to be used for hollow ware, it would be caught in a ladle as it came out of the bottom of the furnace and poured into mold.

Using the Iron

Once the pig iron cooled, it could be broken into transportable bars for use in casting pots, pans, kettles, stoves, tools, farm implements, shot or shells. Iron from the furnace was also cast as a popular pot-belly stove called a “Catoctin Stove.”

The length of time the furnace burned was called a “blast.” Once the furnace was lighted, it was kept burning until the stocks of charcoal, iron ore and limestone were used up or until freezing winter weather halted the waterwheel that powered the bellows to the furnace.