New Delhi: Experts at the Indian Institute of Technology have resolved the mystery behind the 1600-year-old iron pillar in Delhi, which has never corroded despite the capital's harsh weather.

Metallurgists at Kanpur IIT have discovered that a thin layer of "misawite," a compound of iron, oxygen and hydrogen, has protected the cast iron pillar from rust.

The protective film took form within three years after erection of the Ashoka pillar and has been growing ever so slowly since then. After 1600 years, the film has grown just one-twentieth of a millimetre thick, according to R Balasubramaniam of the IIT.

In a report published in the journal "Current Science" Balasubramanian says the protective film was formed catalytically by the presence of high amounts of phosphorous in the iron, as much as 1 per cent against less than 0.05 per cent in today's iron.

The high phosphorous content is a result of the unique iron-making process practiced by ancient Indians, who reduced iron ore into steel in one step by mixing it with charcoal.

PTI