The Iron Pillar of Delhi

Out in the open for sixteen centuries, and barely rusted

The pillar stands 7.21 metres from its top to the bottom of its base, 1.12 metres of it underground, and weighs more than six tonnes. It was made by forge-welding pieces of wrought iron, and it has stood in the open for about sixteen centuries without rusting away.

Its oldest inscription, Sanskrit verse in the Gupta script, praises a king named Chandra and records that the pillar was raised as a standard of Vishnu on a hill called Vishnupada. The king is now generally identified as the Gupta emperor Chandragupta II, who reigned from about 375 to 415: the script, the poetry and the place names all fit his reign, and his gold coins carry the same short name. Where Vishnupada lay is debated. Meera Dass and R. Balasubramaniam placed the pillar first at the Udayagiri Caves in Madhya Pradesh, where Chandragupta's dated inscriptions were cut. The route to Delhi is just as uncertain. Alexander Cunningham credited the Tomara king Anangpal in the eleventh century, on the strength of a short inscription dated Samvat 1109; other scholars say Iltutmish carried it off in the thirteenth century and set it up as a trophy beside the Quwwat-ul-Islam mosque, where it stands today inside the Qutb Minar World Heritage Site.

The pillar does rust, slowly and evenly. Balasubramaniam, at IIT Kanpur, showed why the rust stops at a thin skin. Indian smiths of the period did not add lime to their furnaces, so phosphorus that a modern blast furnace would carry off in the slag stayed in the metal, at several times modern levels. As Delhi's weather wets and dries the surface, that phosphorus gathers between metal and rust and builds a continuous film of crystalline iron hydrogen phosphate. In 1,600 years the film has grown about a twentieth of a millimetre thick. The same protection keeps the inscription legible, though here and there the iron closed over strokes as they were cut.

The column carries one visible scar. A deep dent about 4 metres up marks where a cannonball struck at close range; it cracked the metal on the opposite side, but the column held. No record of the shot survives, and historians generally blame Nadir Shah during his invasion of Delhi in 1739. In 1969 Erich von Däniken offered the rustless pillar as evidence of visitors from space. Told five years later that it does rust and that its making was well understood, he said he no longer considered it a mystery.