Keys to the Past

Glossary

Blast furnace

A furnace for smelting ironstone at high temperatures beyond the melting point of Iron, (Fe, 1540°C). These furnaces were used to produce cast iron. The furnace was a tall cavity funnelling out from the base into which layers of ironstone, fuel (either charcoal or coke) and limestone were added. The limestone was used to remove the unwanted components of the ironstone. Coal was not used as a fuel - as it contains Sulphur (S) which made the metal unworkable. The inside of the furnace was lined with fireclay bricks or ganister which could withstand the temperatures.

The cast iron and slag were removed by taking out part of the furnace base. As these were both molten liquids these both ran from the furnace into shaped moulds, (called pigs for the cast iron), from an alcove called a casting arch. The floor where these moulds were is called the casting floor. These smelting operations could last for long periods of time such as weeks; additional fuel and ironstone being added as necessary, from the top, (where the furnace might be cut into slope, or reached from a ramp). At the end of the smelting the lining was removed and replaced as necessary. The temperature was held at high temperatures by bellows blowing the fire - these were often powered by waterwheels.

Blast furnace sites included the stores for the ironstone and fuel. They could also include calcining kilns, workers accommodation, and provision for the water-powered elements - such as reservoirs and leats. Blast furnaces developed from the high bloomery style of furnace. Examples in Northumberland date to the later 16th century, (Wheelbirks, Stocksfield) and the 18th century, (Allensford). Other later examples were built in the region - but little remains to be seen.

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