In the courtyard of the Qutb Minar complex in Delhi there is an iron pillar 23 feet tall, weighing about six tons, with a Sanskrit inscription that dates it to around 400 AD, the reign of Chandragupta II. It has stood in the open air through 1,600 monsoon seasons. It does not rust.
That should not be possible. Iron made with the technology of the fourth century, forged by hammer welding small lumps together, should have been a streak of red dust centuries ago. The pillar’s surface is dull gray and smooth. Metallurgists at the Indian Institute of Technology finally worked out why in 2002: the iron is unusually high in phosphorus, and over its first few years a thin protective film of a rare iron compound formed over the surface and sealed it. The ancient smiths, in other words, made a kind of iron that modern steelmakers would have called a mistake, and it turned out to be the secret.
How they forged a single six ton piece is a separate question. Analysis shows it was built up from hundreds of small forge welded pieces, worked so cleanly the joints are invisible. Nobody in Europe could make a single iron casting that size for another thousand years. And the pillar is not alone: there are similar rustless iron beams in temples at Konark and Dhar.
Whether the smiths understood what they were making, or stumbled onto it and kept the recipe as a secret of the trade, nobody knows. The recipe was lost. The pillar is still standing. What do you think?
