The Iron Pillar of Delhi: The 1,600-Year-Old Column That Barely Rusts

In the courtyard of a ruined mosque in south Delhi, a few steps from the soaring red tower of the Qutb Minar, stands a dark column of iron. It is a little over seven metres tall, smooth and almost black, with a ribbed, bell-shaped top.

For generations, visitors queued to stand with their backs against it, stretch their arms round behind them and try to make their fingertips meet. Success, the guides said, meant good luck.

What should have puzzled them more is that the pillar is about 1,600 years old, has stood in the open through centuries of monsoons, and has barely rusted.

A king called Chandra

Cut into the pillar's surface is an inscription of six lines in Sanskrit verse, written in a script of the Gupta period. It praises a king named Chandra, who conquered his enemies from Bengal to the far side of the Indus, and says that this "lofty standard of Vishnu" was set up on a hill called Vishnupada.

The inscription gives only the short name, Chandra. Most historians identify him with Chandragupta II, one of the great Gupta emperors, who ruled from around AD 375 to 415. His coins use the same short name, and he was a known devotee of Vishnu. On that basis the pillar is usually dated to around AD 400. Some scholars think the inscription may have been carved after the king's death, as a memorial.

Reading it took time. The British scholar James Prinsep published copies in the 1830s, but it was the Indian scholar Bhau Daji Lad who, in 1875, first read the king's name correctly.

How did it get to Delhi?

The pillar almost certainly did not start where it stands now. There is no Vishnupada hill in Mehrauli, and the mosque around it was built in the late 12th and early 13th centuries, long after the pillar was made.

Where it came from is still argued over. One old suggestion is Mathura. The metallurgist R. Balasubramaniam and Meera Dass argued in 2004 for Udayagiri, a Gupta-era site of rock-cut temples near Vidisha in Madhya Pradesh. Who moved it, and when, is also debated. A short later inscription has been linked to the Tomar king Anangpal in the 11th century, while other historians think one of the early Delhi sultans, possibly Iltutmish, brought it to the mosque as a trophy in the 13th century.

Made by hand, one lump at a time

The pillar is about 7.2 metres long, with roughly 1.1 metres hidden below ground, and about 41 centimetres across at the base. It weighs more than six tonnes.

It was not cast. Furnaces of the time could not melt iron in such quantities. Instead, smiths heated lumps of iron, known as blooms, and hammered them together while hot, building the column up piece by piece. This method, called forge-welding, leaves small traces of slag in the metal. Making a solid shaft of this size by hand was a remarkable feat. Very few iron objects anywhere in the world approached this scale for centuries afterwards.

About four metres up, the shaft has a deep dent. It was made by a cannonball fired at close range, most often attributed to Nadir Shah's invasion of Delhi in 1739. The pillar cracked slightly on the far side but survived.

The science of a rust-proof pillar

For a long time, the pillar's resistance to rust was treated as an unsolved mystery. In 1969 the writer Erich von Däniken even cited it in Chariots of the Gods? as possible evidence of visitors from space. In a 1974 interview he said he no longer considered it a mystery, and metallurgists had been working on a down-to-earth answer for decades.

The most complete explanation came from R. Balasubramaniam, a metallurgist at the Indian Institute of Technology, Kanpur, who published a series of papers around 2000 to 2002. He found three factors working together.

First, the iron is unusually rich in phosphorus, about 0.25% on average, compared with typically less than 0.05% in modern blast-furnace iron. Ancient smelters did not add lime to their furnaces, which in modern ironmaking removes phosphorus into the slag. Second, the tiny particles of slag and unreduced iron oxide left by forge-welding help set off the right reactions at the surface. Third, Delhi's climate alternates between wet and dry, which lets a protective film build up and then harden.

In the first years after the pillar was set up, a thin layer formed between the metal and its rust. At first it contained misawite, an amorphous iron oxyhydroxide. Over time, crystalline iron phosphates developed too. This film is dense and sticks tightly to the metal, so water and oxygen struggle to get through. According to the IIT Kanpur work, it has grown only about a twentieth of a millimetre thick in some 1,600 years.

In other words, the pillar does rust, but its rust protects it. Later studies by other Indian researchers broadly support this picture.

Legends and hugs

The pillar has gathered stories. One tradition says it was driven so deep that it pinned down the head of the serpent who holds up the world. Another, the most famous, is the hugging custom. Visitors would stand with their backs to the pillar and try to clasp their hands behind it. Some guides said success brought luck, others that it proved a lover's faithfulness.

The custom did real damage. Years of hands and shoulders polished and wore the lower part of the shaft. A fence was put up around the pillar, in 1997 according to most accounts, and hugging is no longer possible.

Where it stands now

The pillar is part of the Qutb Minar and its Monuments, a UNESCO World Heritage Site in Mehrauli, south Delhi, and is cared for by the Archaeological Survey of India. It stands in the courtyard of the Quwwat-ul-Islam mosque, ringed by its fence and by the carved pillars and broken arches of the mosque.

It is easy to walk past. Next to the 72-metre Qutb Minar, a seven-metre column looks modest. But it is one of the oldest large iron objects still standing in the open anywhere, and a direct record of the skill of Indian ironworkers about 1,600 years ago.

The smiths who hammered it together could not have known about phosphates or protective films. They knew how to make good iron, and they made it well enough that the metal is still doing its quiet work in the Delhi air.