It’s 1600 years old. It’s made of wrought iron. It stands pristine amid the ruins of an ancient temple. And it refuses to rust.
The origin of the remarkable Iron Pillar, part of the world-heritage-listed Qutub funerary complex in India’s Delhi, is lost to time.
All sorts of ideas have been put forward over the decades.
Evidence of a lost prehistoric civilisation?
Proof of an ancient super-race?
Alien intervention?
A fingerprint of the gods?
Turns out, it’s just human ingenuity learnt from the “school of hard knocks”.
But it took more than a century for modern science to figure out the properties that give its iron – a material that rapidly rusts away when exposed to the elements – its uniquely resistant character.
A new study has pulled together all the evidence extracted from its structure in a bid to understand the complex chemistry at work beneath its dull grey surface and cast new light on how it was made.
“The pillar, renowned for its craftsmanship, flawless surface and resistance to corrosion behaviour, is a prominent monument demonstrating an indisputable record of the ancient Indian metallurgical and engineering marvel,” the researchers write.
Ancient origins
“In the center of the mosque is the awe-inspiring column of which nobody knows of what metal it is constructed,” the traveller Ibn Battista wrote of his visit to Delhi about 1333 AD. “One of their learned men told me that it is called “Haft Jûsh”, which means ‘seven metals’, and that it is composed of these seven. A part of this column, of a finger’s length, has been polished, and this polished part gives out a brilliant gleam. Iron makes no impression on it.”
It stands about 7.3m tall and 41.6cm in diameter at its base, tapering to about 30.5cm with the ornamental structure at its top. It’s made of solid wrought iron weighing 6.5 tonnes.
And it has been dragged across India several times over the centuries as loot from battles of conquest.
One account states that the Delhi-based Sultan Iltutmish had the Iron Pillar removed after sacking the Hindu city of Madhya Pradesh in 1228 AD.
Another tradition describes the Hindu Tomara Dynasty king Anangapala II taking it as a trophy of war and moving it to Delhi in around 1050AD. But the pillar itself is much older than that.
Several inscriptions have been etched on its surface. The oldest – a six-line verse – is in the ancient Indian language Brahmi. It names a king called Chandra.
Was this the Gupta Dynasty King Chandragupta II Vikramaditya (375 to 415AD)?
Or Emperor Chandragupta Maurya (350 to 295 BC)?
Researchers say the poetic writing style and form of Sanskrit text used in the inscription are most likely from the reign of the Gupta king.
The inscription describes Chandra dedicating the pillar as a “Vishnudhvaja” – a banner pole for the god Vishnu – and erecting it at a site named “Vishnupada” (translating to “a hill with the footprint of God Viṣṇu”).
Where this was remains uncertain. Though it is generally argued to have been among the Udayagiri cave temples in Madhya Pradesh.
Metallurgical marvel
India’s ancient metal smiths purified their ironwork to a remarkable 98 per cent. Exactly how they did so was a closely guarded secret, jealously protected by a few communities and handed down through generations of the same families.
“The tribal people of Odisha produce iron from locally available raw materials using primitive processing methods dates back at least to 1300 BC to 1200 BC, which continued well into the 19th century AD,” an Odisha province report reads.
The new critical review of the Iron Pillar’s properties by Assistant Professor Nityananda Agasti and Professor Balaram Pani finds these metal smiths extracted the iron from iron ore using charcoal furnaces.
The 1400C these generated was not hot enough to entirely melt the metal. Instead, it was extracted as a “soft spongy mass which was hammered to be shaped into the desired objects”.
Lumps of this spongy iron were laid out and hammered repeatedly to separate the fluidic slag (mineral impurities) and formed into a sequence of “pancake” shapes.
“The heated iron pancakes were then placed one over another, and both were joined by hammering using hand-held hammers,” the study states.
Microstructural analysis of this forged iron structure has revealed a remarkable convergence of properties that results in resistance to corrosion.
The hammering did not remove the slag evenly. This left a patchwork of tiny, phosphorus-rich slag particles spaced among the iron.
This creates a distributive network of anode-cathode electrical conductors and triggers a series of chemical reactions.
“The corrosion resistant property of the sample of Iron Pillar in Delhi was mainly due to the mode of its fabrication resulting in high slag inclusions dispersed in three dimensions around the metal,” the study says.
“The P (phosphorus) present in FePO4 (ferric phosphate) is oxidized to phosphate which acts as an inhibitor and promotes the formation of protective oxide films for preventing corrosion.
“The protective film on the pillar decreases the corrosion rate from 0.395 mm to 0.040 mm per year.”