The manufacture of cast iron and its transformation into wrought iron is a comparatively recent technological development dating from the end of the 15th century. In prehistoric and early historic iron smelting, temperatures of around 800 degrees centigrade in the forge produced a spongy mass of iron and slag with many impurities. This was then reheated and hammered to consolidate the iron and remove the slag. With the development of bellows powered by water wheels the melting point of iron (1540 degrees centigrade) was reached and the liquid iron run off into moulds to produce cast iron. About 1.3 tons of charcoal was needed to reduce 2.5 tons of ore into 1 ton of cast iron, with the addition of 5 tons of air.
In England, the demand for cast iron was such that timber prices to make charcoal rose sharply. Even before water-powered bellows were adopted in blast furnaces, the demand for charcoal in the medieval period had already seriously denuded large areas of natural forest and, as early as the 13th century, measures had been taken to limit the destruction and wastage of the forests.
In the 17th century, therefore, English iron masters sought alternative sources of timber and Ireland, with its enormous expanses of natural forest, became increasingly the focus of their attentions. Although timber was plentiful and cheap in Ireland, there was no existing tradition of large-scale smelting. The technology was, therefore imported by settling English iron-makers into the country. In the south west, industrial colonies were created in Cork and Waterford by the Earl of Cork, and in Kerry by the Petty and White families.
The blast furnaces were located in heavily wooded areas with access to a constant water supply. This ensured a reliable energy source for the water-powered bellows and, of course, a convenient supply of charcoal.
Approximately 1.7 tons of charcoal were required to produce 1 ton of wrought iron.
The yearly consumption for the two smelters in the Killarney valley was
52 tons of wood per day = 5 trees per day
= 2.6 tons of charcoal per day
= 2 tons of iron per day
= 600 Oak trees per year
=20 acres of mature forest per year.
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