Strange Metal Relic

An awe-inspiring metal column has perplexed experts for over a century.

The 1,600-year-old pillar stands amid the ruins of a world heritage-listed temple in Delhi. It’s made of iron, but it never rusts. Normally, iron is very susceptible to the elements and develops a coat of reddish-brown rust at the hint of water.

The pillar’s resistance to erosion has invited several theories over the years, but none of them could be proven. Only recently has it been discovered that the structure’s power is due to ancient Indians’ skills. Researchers have analyzed the pillar to work out its complex chemistry.

The towering column stands almost 24 feet tall. The base is around 16.4 inches in diameter and tapers to around 12 inches at the top, where an ornamental structure sits.

Despite being made of 6.5 tons of solid wrought iron, it was dragged across India several times over the centuries as spoils of battle.

Around 1333 AD, a Moroccan explorer was told the pillar was made of seven metals, but that no one knew what those seven metals were.

It has several inscriptions etched on its surface, the oldest of which is written in the ancient Indian language Brahmi and names a king called Chandra. Researchers say the style and form of the Sanskrit text suggest it was inscribed during the reign of King Chandragupta II Vikramaditya, who ruled the Gupta empire between 375 and 415 AD.

But what made the pillar so resistant?

Experts wrote that the pillar is a monument demonstrating the metallurgical and engineering skills of the ancient Indians, who purified their ironwork to a remarkable 98%. However, how they managed this was a closely guarded secret handed down through a family from generation to generation.

The heat used during these processes was not sufficient to melt the metal, so it was extracted as a soft spongy mass which was then hammered into the desired shape. Lumps of this spongy iron were laid out and repeatedly hammered to separate the mineral impurities, forming it into a sequence of ‘pancakes’. The heated iron pancakes were joined by hammering.

Microstructual analysis of this structure revealed a convergence of properties that resulted in the metal’s resistance to corrosion. It turns out the hammering did not remove the impurities evenly. It left a patchwork of tiny phosphorus-rich particles in the iron. This created a network of electrical conductors, which triggered certain chemical reactions.

The phosphorus present in the impurities are oxidized to phosphate. This acts as an inhibitor and promotes the formation of protective oxide films for preventing corrosion.

 

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