Blast Furnace

A blast furnace is built as a tall shaft, wider through the middle and narrowing toward the top and bottom, loaded from above with alternating layers of ore, fuel and flux, and fed air forced in near its base through openings called tuyeres. The forced air is the whole point of the design: an open fire is limited in how hot it can burn by how much oxygen simply drifts to it on its own, but air driven continuously and under pressure into the base of the shaft lets the fuel burn far hotter and far more completely than it otherwise could.

As the charge descends through the shaft it passes through a rising column of intensely hot gas, and by the time it reaches the bottom the ore has been reduced and melted, separating into two layers that can be drawn off separately, molten iron at the bottom and a lighter molten slag, made mostly of the ore's non-metallic impurities and the added flux, floating above it. Because the furnace is loaded continuously from the top as material is drawn off the bottom, it can run for extended periods without being emptied and relit the way a batch furnace must, which is the second sense in which "blast" describes not just the air but the furnace's whole continuous, industrial-scale operation.

The earliest European blast furnaces are dated archaeologically to the twelfth and thirteenth centuries, with carbon-dated remains at Lapphyttan in Sweden among the oldest known, and the technology spread through the Bergslagen mining region, the Bergamasque area of Lombardy, and the iron districts of Champagne across the thirteenth to fifteenth centuries as part of what historians of technology call the late medieval steel revolution. The decisive step was mechanical rather than chemical: fitting a furnace's bellows to a water wheel let ironworkers scale up the draft far beyond what hand or foot power could sustain, and it was that stronger, steadier forced blast, more than any change in ore or fuel, that pushed furnace temperatures high enough to produce liquid iron continuously rather than the spongy, semi-solid "bloom" earlier bloomery furnaces produced in batches. Where nearly every other instrument in this category shrank down into laboratory glassware as chemistry separated from alchemy, the blast furnace went the other way entirely, growing continuously larger and more mechanized from the medieval water-wheel furnace through the coke-fired furnaces of the industrial revolution to the enormous computer-controlled furnaces of a modern steelworks, which still bear the name "blast furnace" and still work on the same forced-draft principle.