Page "Bessemer process" Paragraph 13

According to his autobiography Bessemer was working with an ordinary reverbatory furnace but during a test, some pieces of pig iron were jostled off the side of the ladle, and were left above the ladle in the furnace's heat.

When Bessemer went to push them into the ladle, he found that they were steel shells: the hot air alone had converted the outsides of the iron pieces to steel.

This crucial discovery led him to completely redesign his furnace so that it would force high-pressure air through the molten iron using special air pumps.

Intuitively this would seem to be folly because it would cool the iron.

Instead, the oxygen in the forced air ignited silicon and carbon impurities in the iron, starting a positive feedback loop.

As the iron became hotter, more impurities burned off, making the iron even hotter and burning off more impurities, producing a batch of hotter, purer, molten iron, which converts to steel more easily.