Why Qutub Minar's 1,600-Year-Old Iron Pillar Is Still Rust-Free? Scientists Explain

An iron pillar placed in the UNESCO-listed Qutub Minar complex in New Delhi has long fascinated scientists for surviving over 1,600 years without rust. Standing tall at 7.2 metres and weighing six tonnes, the iron pillar predates the complex it resides.

Despite its long history, the iron pillar with inscriptions dedicated to Chandragupta Vikramaditya of the Gupta Empire, as per the Archaeological Survey of India, has remained in pristine condition. Usually, iron and iron alloy structures exposed to moisture oxidise over time, leading to a coating of rust, unless they are protected by layers of special paint.

So why does the iron pillar not rust?

Scientists in India and abroad started investigating the mystery in 1912. Murray Thompson from Roorkee Engineering College and Percy from the School of Mines conducted chemical analysis of the structure and established that it was wrought iron of specific gravity 7.66.

However, it wasn't until 2003 that a study by researchers at the Indian Institute of Technology (IIT)-Kanpur figured out the real engineering secret behind the structure.

The study findings revealed that the pillar is primarily made of wrought iron, having a high phosphorus content (approximately one per cent). Additionally, a thin layer of "misawite", a compound of iron, oxygen and hydrogen, was also found on the pillar's surface.

"In summary, the microstructural and compositional inhomogeneities of the Delhi pillar iron do not have any influence on the protective passive film that forms on the surface. The deciding factor this is imparting the excellent corrosion resistance is the high phosphorus content of the Delhi pillar iron," the study highlighted.

The Iron Pillar in Qutub Minar Complex, Delhi has stood the test of time without rusting for over 1600 years. It is an incredible example of ancient India's advanced metallurgy techniques which is still wondered worldwide. (1/3) pic.twitter.com/WD8cSluAce

— Archaeological Survey of India (@ASIGoI) September 14, 2024

Unlike modern irons, the structure lacks sulfur and magnesium, with ancient craftsmen using a technique called “forge-welding" to construct the iron pillar. In this technique, the iron is heated and hammered, which keeps the high phosphorus content intact -- a method relatively unseen in modern practices.

Archeo-metallurgist R Balasubramaniam, who authored the report, added that this unconventional approach contributed to the pillar's strength which has endured for centuries.