The iron pillar that does not rust

01The pillar and its words

The pillar is 7.21 metres tall, with about a metre more below ground, and tapers from about 42 cm at the base. Its Sanskrit inscription in Gupta Brahmi says it was set up as a dhvaja, a standard, of Bhagwan Vishnu on the hill called Vishnupada, by a king named Chandra who had crossed the seven mouths of the Sindhu and conquered the Vanga. Most scholars identify Chandra with Chandragupta II. It was probably moved to Delhi much later, perhaps under the Tomaras, and now stands in the Qutb complex.

02Why it stays bright

R. Balasubramaniam of IIT Kanpur studied the pillar for years. The iron was made by forge-welding hot lumps of bloomery iron, so it contains slag and a high level of phosphorus, about 0.1 per cent, and almost no sulphur. In Delhi’s alternating wet and dry air, phosphorus helps form a thin, dense protective film; he identified in it misawite, a form of iron oxyhydroxide, with iron phosphate. The film seals the surface. Delhi’s fairly dry climate helps, and the buried part, kept damp, has corroded.

03Its company

The Delhi pillar is not alone. The iron pillar at Dhar, in pieces, was even larger, and iron beams more than ten metres long were used at the Konark Sun Temple in the 13th century. What these show is that Indian smiths could forge-weld very large masses of iron centuries before Europe could, and that their phosphoric iron resisted rust. Claims that the pillar is of some unknown alloy are wrong; it is wrought iron, about 99.7 per cent pure.

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