The Iron Pillar located in New Delhi, India, is a 7 m (23 ft) column in the Qutub complex, notable for the rust-resistant composition of the metals used in its construction.
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. The corrosion resistance results from an even layer of crystalline iron hydrogen phosphate forming on the high phosphorus content iron, which serves to protect it from the effects of the local Delhi climate.
The height of the pillar, from the top of its capital to the bottom of its base, is 7.21 m, 1.12 m of which is below ground. Its bell pattern capital is (1.07 m in height, and its bulb-shaped base is 0.71 m high. The base rests on a grid of iron bars soldered with lead into the upper layer of the dressed stone pavement.
The pillar’s lower diameter is 16.4 in (420 mm), and its upper diameter 12.05 in (306 mm). It is estimated to weigh more than six tons. A fence was erected around the pillar in 1997 in response to damage caused by visitors.
ORIGINAL & LOCATION
In R. Balasubramaniam’s view, the pillar, with a wheel or discus at the top, was originally located at the Udayagiri caves, situated near Vidisha in Madhya Pradesh. The king of Delhi, Iltutmish, is known to have attacked and sacked Vidisha in the thirteenth century and this would have given him an opportunity to remove the pillar as a trophy to Delhi, just as the Tughluq rulers brought Asokan pillars to Delhi in the 1300s.
The pillar carries a number of inscriptions and graffiti of different dates which have not been studied systematically despite the pillar’s prominent location and easy access. The oldest inscription on the pillar is in Sanskrit, written in Gupta-period Brahmi script. This states that the pillar was erected as a standard in honor of Viṣṇu. The dating of the inscription is supported by the nature of the script and the Sanskrit poetics, both of which reflect the conventions of Gupta times.
The pillar was manufactured by forge welding and is composed of 98% pure wrought iron. In a report published in the journal Current Science, R. Balasubramaniam of the IIT Kanpur explains how the pillar’s resistance to corrosion is due to a passive protective film at the iron-rust interface.
The presence of second-phase particles (slag and unreduced iron oxides) in the microstructure of the iron, that of high amounts of phosphorus in the metal, and the alternate wetting and drying existing under atmospheric conditions are the three main factors in the three-stage formation of that protective passive film. Mr.Balasubramaniam states that the pillar is “a living testimony to the skill of metallurgists of ancient India”.