Photo by Hep Svadja.

At the start of the Industrial Revolution, the state of the art in iron metallurgy was the puddling furnace. Ironworkers loaded crude “pig iron” ingots into the furnace and then continually stirred the molten metal through a small hole. Puddling was a hot and difficult task, and an experienced puddler was considered a highly skilled craftsman.

As the puddler stirred, solid chunks of refined iron would appear in the liquid mass. He’d gather these and work them under a forge hammer to yield a slab of hot wrought iron which, after all that work, still lacked the strength or utility of steel.

Steel is iron alloyed with carbon and it’s superior to plain iron in every way. But prior to 1856 there was no practical way to control the percentage of carbon in iron, so there was no way to manufacture steel at a price industry could afford.

The railroads were booming in the mid-19th century, but their wrought iron rails were too soft — on busy stretches the rails had to be replaced every 6 to 8 weeks. Steel rails would be far more durable, but they were too expensive.

Then, a smart fellow named Henry Bessemer came on the scene. Bessemer, an English engineer and metallurgist, would receive 129 patents in a variety of engineering disciplines. But the invention for which he was knighted and the one that made him rich was the one that involved turning iron and coke (carbon) into steel.


Photo by Alfred T. Palmer.

While looking for a way to strengthen cannon barrels, Bessemer discovered that the carbon dissolved within molten pig iron unites readily with oxygen. Knowing this, he determined that if he could blast a jet of air through the molten pig iron, then he could convert it into much stronger alloy steel by accurately controlling its carbon content.

Bessemer built an experimental furnace at his laboratory in London, with a high-temperature heating chamber 4 feet high, and a 12-horse-power steam engine to run the air injector. When the pig iron in the chamber liquefied and he turned on the blower, a fireball erupted from the top. But when he sluiced the molten metal into ingot molds, he gazed with delight upon the “limpid stream of incandescent malleable iron almost too brilliant for the eye to rest upon.” Bessemer had found a way to make cheap steel.

In 1856 Bessemer designed what he called a converter, a large, napiform (turnip-shaped) receptacle with holes (“tuyeres”) at the bottom where pumps could inject compressed air. Bessemer filled his converter with molten pig iron, blew compressed air through it, and found that the pig iron was indeed emptied of excess carbon and silicon in minutes. From that time onward, affordable carbon steel could be made abundantly. The Age of Steel had begun.

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Image by James Burke.