Know why THIS UNESCO-listed Delhi iron pillar has stayed rust-free for 1600 years — IIT-Kanpur cracks the code

In the heart of Delhi’s Qutub Minar complex stands a remarkable iron structure that has left scientists across the world amazed — a 1,600-year-old iron pillar that has never rusted. Despite being exposed to the open air and weather changes for centuries, the pillar remains in surprisingly pristine condition. How has it withstood the test of time without corrosion? Scientists from the Indian Institute of Technology (IIT) Kanpur finally cracked the mystery in a 2003 study.

The iron pillar, which predates the Qutub Minar complex itself, is 7.2 metres tall and weighs around six tonnes. It carries inscriptions that honour Chandragupta Vikramaditya of the Gupta Empire, placing its origins as far back as the 4th century. Typically, iron and its alloys rust over time when exposed to moisture and oxygen. However, this pillar has defied the usual rules of corrosion — and not because of modern paints or protective coatings.

The mystery has fascinated experts since 1912. British-era scientists like Murray Thompson from Roorkee Engineering College and Percy from the School of Mines were among the first to analyse the pillar. They confirmed that it was made of wrought iron with a specific gravity of 7.66. Still, the exact reason for its rust-free state remained unknown until IIT-Kanpur researchers published their findings.

The 2003 study revealed that the iron pillar’s incredible resistance to rust is due to two key factors: its high phosphorus content and the presence of a protective layer called “misawite” — a compound made of iron, oxygen, and hydrogen. Unlike modern-day iron, the ancient iron used in this pillar contains almost 1% phosphorus, and lacks elements like sulphur and magnesium. This phosphorus encourages the natural formation of a stable, thin film over the iron’s surface, which acts as a protective barrier against moisture and oxygen.

Another interesting discovery was the method used to build the pillar — forge welding. This ancient process involves heating and hammering the iron, which helps retain the phosphorus and strengthens the structure. According to archeo-metallurgist Professor R. Balasubramaniam, who led the research, this traditional technique played a major role in the pillar’s long-lasting durability.

In short, the rust-free status of the Iron Pillar of Delhi is not a miracle, but a testament to the advanced metallurgical skills of ancient Indian craftsmen — a blend of high-phosphorus iron, expert forging techniques, and time-tested science.