The iron pillar (also known as the Ashokan pillar) of Delhi, India, is a 7 m (23 ft) high pillar in the Qutb complex, notable for the composition of the metals used in its construction.
The pillar, which weighs more than six tons, is said to have been fashioned at the time of Chandragupta Vikramaditya (375–413) of the Gupta Empire, though other authorities give dates as early as 912 BCE. The pillar initially stood in the center of a Jain temple complex housing twenty-seven temples that were destroyed by Qutb-ud-din Aybak, and their material was used in building the Quwwat-ul-Islam mosque and the Qutub Minar complex where the pillar stands today.
The pillar has attracted the attention of archaeologists and metallurgists and has been called “a testament to the skill of ancient Indian blacksmiths” because of its high resistance to corrosion, due to both the Delhi environment providing alternate wetting and drying conditions, and iron with high phosphorus content conferring protection by the formation of an even layer of crystalline iron hydrogen phosphate.
About the Iron Pillar
An iron pillar weighing over 6 tonnes, more than 7 metres tall is constructed in a single forge and is erected on top of the Vishnupada hill (somewhere in modern central India) with sanskrit inscriptions on it in the brahmi script about the great gupta ruler Chandragupta Vikramaditya.
More than 1600 years back, to build an iron pillar of this huge size in a single forge itself is an indication of the advanced metallurgy of the ancient Indians. Even in today’s modern technological world it is a great achievement to forge such a huge pillar in a single forge!!!
But there’s more, this pillar which contains more than 98% of pure iron, even after 1600 years has not caught rust!!! It is 100% corrosion resistant inspite of the fact that it is 98% iron!! This indicates one of the great technological achievements of the ancient Indians. Even today it is next to impossible to construct such a huge corrossion resistant iron pillar. Corrosion resistant technologists from all over the world have studied this pillar.
Modern day technology uses limestone in the blast furnaces which carries away most of the phosphrous content in the ore in the form of slag. Ancient Indians instead by solid state reduction (used charcoal as a reducing agent) to extract pure iron with low carbon content from the ore.
One of the research opinions about the iron pillar’s corrosion resistant nature is that the high amount of phosphorous (which is 1% of the pillar as opposed to modern day proportions which is 0.05%) has formed a thin protective layer on the surface of the pillar thereby making it corrosion proof. Since other ancient iron works of the same period do not contain such a high quantity of phosphorous, it indicates that the extra phosphorous was intentionally added to the iron pillar.
Another theory suggests that the relative low humidity of Delhi ensures that the Iron pillar does not corrode. But I strongly disagree with this theory and want to stress on the fact that it is not the climate of Delhi but the composition of the pillar which is responsible for the corrosion resistant nature of the pillar. This is because of a simple reason that I observed a similar ancient corrosion resistant Iron pillar when I had been on a trek to the Kodachadri hills in the west coast of South India. Also I have heard about a similar corrosion resistant Iron pillar in the konark temple of Orissa. Both the above mentioned places have a highly humid climate throughout the year and yet the iron pillars here are corrosion resistant! The spreadout of these pillars across the geographical landscape of India indicates that the Iron pillar of delhi was not a single isolated incident of an ancient genius but was a common technical knowledge of the ancient civilization in this country.