The Iron Pillar of Delhi

Sixteen centuries in the open air, and the iron still has not rusted

A column of iron 7.21 metres tall and more than six tonnes, standing in the Qutb complex at Mehrauli, that has stood through some sixteen centuries without rusting away. Its oldest inscription honours a king named Chandra, generally identified as the Gupta emperor Chandragupta II, who reigned about 375 to 415, and records it as a standard of Vishnu raised on a hill called Vishnupada; it was probably first set up somewhere else, perhaps outside the Udayagiri Caves, and moved here in the 11th century. Its refusal to corrode has drawn archaeologists and materials scientists, and the answer they found is a tribute to the smiths who made it. The iron is rich in phosphorus, and in Delhi’s climate it grows an even, protective skin of crystalline iron hydrogen phosphate. That is also why the inscription reads so clearly after all this time: the metal it is cut into will not rust.