The First Blast Furnaces

So far, no furnace in Europe had been hot enough to melt iron to a liquid state. All that could be produced was a ‘spongy’ mass from which impurities had to be hammered out. However, design of furnaces improved over the centuries, and about the year 1400 very efficient blast furnaces were introduced by the Germans. They had found that a blast of air from water-powered bellows increased the temperature, though the iron still did not liquify. It became soft and spongy, worked its way down through the burning charcoal, and collected at the bottom of the furnace.

Furnaces were usually built about ten or fifteen feet high, but to economise on fuel a new one was built thirty feet high. Although the internal temperature in this was no higher, the iron arrived at the bottom in a completely liquid state. Not only could the metal be run off into moulds, but many of the impurities (which had previously to be hammered out) separated automatically from the melted iron. The reason for this tremendous stride in metallurgy was simple: the height of the furnace. The soft ‘sponge’ iron took so long to seep down through the charcoal that it absorbed a great deal of carbon. It became carburised, and as the melting point of carburised iron is 350^o C less than ‘sponge’ iron, it became liquid.

By about the year 1600, iron production in Britain was beginning to suffer from lack of fuel. For 3,000 years all iron-smelting, both here and abroad, had been done with charcoal. Charcoal is partly-burned wood. In Britain, timber was running short and it was impossible for the iron-makers to equal the output of a country such as Sweden, where timber was abundant.

Fortunately for Britain a Quaker, Abraham Darby, found a way to do without charcoal altogether. In his iron factory at Coalbrookdale, Shropshire, he made many experiments using coke, and finally succeeded. There were technical difficulties to overcome, and at first Darby kept the process secret for the benefit of his family. Later his methods were adopted throughout Europe. No longer dependant on dwindling forests, Britain remained her position as a leading iron producer.