The blast furnace was the heart of any iron-making establishment. Lined with heat-resistant brick, known as refractory, the stack was filled from the top with alternating layers of iron ore, anthracite and limestone.
As the raw materials worked their way down inside the stack, the heated air blast caused the coal to burn, which in turn melted the ore and limestone. the calcium in the limestone acted as a flux, which removed impurities from the melted ore and formed slag. Just below the melt zone, the furnace walls formed a funnel, which direct the flow of molten iron droplets into the hearth at the bottom. Since the molten slag was lighter, it floated above the molten iron and was tapped off first.
Putting a furnace into blast marked the beginning of another campaign. During this period, usually ten to twelve months at the Scranton Works, a furnace would be fired continuously and tapped every ten to twelve hours. Depending on the size of the furnace, an average of seven tons of iron was smelted per tap. Between campaigns a furnace was relined, the hearth rebuilt and any necessary repairs were made. Due to the number of furnaces at Scranton - five by 1854 - it was rare that production stopped entirely.
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