Blast furnaces were developed in the early 1800s. These “modern” furnaces could produce a much larger amount of iron. The iron produced in blast furnaces was cast iron, often called “pig iron.”

Blast furnaces were large structures, usually built at the bottom of a hill and river. A bridge allowed workers to reach the top of the furnace. Workers would load raw materials into the core of the blast furnace. The raw materials included 1500 pounds of ore, 150 pounds of limestone, and 30 bushels of charcoal.

These materials were heated by hot air blown into the bottom of the furnace. The limestone mixed with waste materials and rose to the top of the furnace. Workers would remove this waste material, or slag, every few hours.

The iron ore was heated to the melting point. The molten iron would settle toward the bottom of the furnace. Every eight to ten hours the workers would open a tap to release the iron. The iron flowed down a long sand channel from the furnace into a casting house. The melted iron flowed from the main channel, which was called a “sow,” into smaller, rectangular molds called “pigs.” The iron would cool and then be removed from sand molds.

While blast furnaces could produce large quantities of iron, the quality was not as good as the iron made in bloomery forges.