The Iron Pillar located in Delhi, India, is a 7 m (23 ft) column in the Qutb complex, notable for the rust-resistant composition of the metals used in its construction.
The pillar has attracted the attention of archaeologists and materials scientists 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 (23.7 ft), 1.12 m (3 ft 8 in) of which is below ground. Its bell pattern capital is 1.07 m (3 ft 6 in) in height, and its bulb-shaped base is 0.71 m (2 ft 4 in) 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 420 mm (17 in), and its upper diameter 306 mm (12.0 in). 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. There is a popular tradition that it was considered good luck if one could stand with one’s back to the pillar and make one’s hands meet behind it. The practice led to significant wear and visible discoloration on the lower portion of the pillar.
The first location of the pillar has been debated.
While the pillar was certainly used as a trophy in the building the Quwwat-ul-Islam mosque and the Qutb complex, its original location, whether on the site itself or from elsewhere, has frequented discussion. A summary of views on this subject and related matters was collected in volume edited by M. C. Joshi and published in 1989. More recently, opinions have been summarised again by Upinder Singh in her book Delhi: Ancient History.
- Balasubramaniam explored the metallurgy of the pillar and the iconography based on analysis of archer-type Gupta gold coins. In his view, the pillar, with a wheel or discus at the top, was originally located at the Udayagiri caves, situated near Vidisha in Madhya Pradesh. This conclusion was partly based on the fact that the inscription mentions Viṣṇupadagiri (meaning “hill with footprint of Viṣṇu”). This conclusion was endorsed and elaborated by Michael Willis in his Archaeology of Hindu Ritual, published in 2009. The key point in favour of placing the iron pillar at Udayagiri is that this site was closely associated with Chandragupta and the worship of Viṣṇu in the Gupta period. In addition, there are well-established traditions of mining and working iron in central India, documented particularly by the iron pillar at Dhar and local place names like Lohapura and Lohangī Pīr . 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 honour of Viṣṇu. It also praises the valor and qualities of a king referred to simply as Candra, now generally identified with the Gupta King Candragupta II. Some authors attempted to identify Candra with Chandragupta Maurya and yet others have claimed the pillar dates as early as 912 BCE. These views are no longer accepted.
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. Thanks to the tablets installed on the building in 1903 by Pandit Banke Rai, the reading provided by him enjoys wide currency. His interpretation has, however, been overtaken by more recent scholarship. The 1903 tablets read as follows:
He, on whose arm fame was inscribed by the sword, when, in battle in the Vanga countries (Bengal), he kneaded (and turned) back with (his) breast the enemies who, uniting together, came against (him);-he, by whom, having crossed in warfare the seven mouths of the (river) Sindhu, the Vahlikas were conquered;-he, by the breezes of whose prowess the southern ocean is even still perfumed;-
(Line 3.)-He, the remnant of the great zeal of whose energy, which utterly destroyed (his) enemies, like (the remnant of the great glowing heat) of a burned-out fire in a great forest, even now leaves not the earth; though he, the king, as if wearied, has quit this earth, and has gone to the other world, moving in (bodily) from to the land (of paradise) won by (the merit of his) actions, (but) remaining on (this) earth by (the memory of his) fame;- (L. 5.)-By him, the king,-who attained sole supreme sovereignty in the world, acquired by his own arm and (enjoyed) for a very long time; (and) who, having the name of Chandra, carried a beauty of countenance like (the beauty of) the full-moon,-having in faith fixed his mind upon (the god) Vishnu, this lofty standard of the divine Vishnu was set up on the hill (called) Vishnupada.
The inscription has been revisited by Michael Willis in his book Archaeology of Hindu Ritual, his special concern being the nature of the king’s spiritual identity after death. His reading and translation is as follows:
[khi]nnasyeva visṛjya gāṃ narapater ggām āśritasyetarāṃ mūrtyā karrmajitāvaniṃ gatavataḥ kīrtyā sthitasya kṣitau [*|]
śāntasyeva mahāvane hutabhujo yasya pratāpo mahān nādyāpy utsṛjati praṇāśitaripor yyatnasya śeṣaḥ kṣitim [||*]
The residue of the king’s effort – a burning splendour which utterly destroyed his enemies – leaves not the earth even now, just like (the residual heat of) a burned-out conflagration in a great forest. He, as if wearied, has abandoned this world, and resorted in actual form to the other world – a place won by the merit of his deeds – (and although) he has departed, he remains on earth through (the memory of his) fame (kīrti).
He concludes: “Candragupta may have passed away but the legacy of his achievement is so great that he seems to remain on earth by virtue of his fame. Emphasis is placed on Candragupta’s conquest of enemies and the merit of his deeds, ideas which are also found in coin legends: kṣitim avajitya sucaritair divaṃ jayati vikramādityaḥ, i.e. ‘Having conquered the earth with good conduct, Vikramāditya conquered heaven’. The king’s conquest of heaven combined with the description of him resorting to the other world in bodily form (gām āśritasyetarāṃ mūrtyā), confirms our understanding of the worthy dead as autonomous theomorphic entities.”
One of the later inscriptions dates to A.D. 1052 mentions Tomara king Anangpal II. This has suggested by some, without any substantial basis, that the pillar was installed in its current location by Vigraha Rāja, the ruling Tomar king.
The pillar was manufactured by the forge welding of pieces of 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.
Lepidocrocite and goethite are the first amorphous iron oxyhydroxides that appear upon oxidation of iron. High corrosion rates are initially observed. Then, an essential chemical reaction intervenes: slag and unreduced iron oxides (second phase particles) in the iron microstructure alter the polarization characteristics and enrich the metal–scale interface with phosphorus, thus indirectly promoting passivation of the iron (cessation of rusting activity). The second-phase particles act as a cathode, and the metal itself serves as anode, for a mini-galvanic corrosion reaction during environment exposure. Part of the initial iron oxyhydroxides is also transformed into magnetite, which somewhat slows down the process of corrosion. The ongoing reduction of lepidocrocite and the diffusion of oxygen and complementary corrosion through the cracks and pores in the rust still contribute to the corrosion mechanism from atmospheric conditions.
The next main agent to intervene in protection from oxidation is phosphorus, enhanced at the metal–scale interface by the same chemical interaction previously described between the slags and the metal. The ancient Indian smiths did not add lime to their furnaces. The use of limestone as in modern blast furnaces yields pig iron that is later converted into steel; in the process, most phosphorus is carried away by the slag. The absence of lime in the slag and the use of specific quantities of wood with high phosphorus content (for example, Cassia auriculata) during the smelting induces a higher phosphorus content (> 0.1%, average 0.25%) than in modern iron produced in blast furnaces (usually less than 0.05%). One analysis gives 0.10% in the slags for 0.18% in the iron itself. This high phosphorus content and particular repartition are essential catalysts in the formation of a passive protective film of misawite (d-FeOOH), an amorphous iron oxyhydroxide that forms a barrier by adhering next to the interface between metal and rust. Misawite, the initial corrosion-resistance agent, was thus named because of the pioneering studies of Misawa and co-workers on the effects of phosphorus and copper and those of alternating atmospheric conditions in rust formation.
The most critical corrosion-resistance agent is iron hydrogen phosphate hydrate (FePO4-H3PO4-4H2O) under its crystalline form and building up as a thin layer next to the interface between metal and rust. Rust initially contains iron oxide/oxyhydroxides in their amorphous forms. Due to the initial corrosion of metal, there is more phosphorus at the metal–scale interface than in the bulk of the metal. Alternate environmental wetting and drying cycles provide the moisture for phosphoric-acid formation. Over time, the amorphous phosphate is precipitated into its crystalline form (the latter being therefore an indicator of old age, as this precipitation is a rather slow happening). The crystalline phosphate eventually forms a continuous layer next to the metal, which results in an excellent corrosion resistance layer. In 1,600 years, the film has grown just one-twentieth of a millimetre thick.
Balasubramaniam states that the pillar is “a living testimony to the skill of metallurgists of ancient India”. An interview with Balasubramaniam and his work can be seen in the 2005 article by Veazy. Further research published in 2009 showed that corrosion has developed evenly over the surface of the pillar.
It was claimed in the 1920s that iron manufactured in Mirjati near Jamshedpur is similar to the iron of the Delhi pillar. Further work on Adivasi (tribal) iron by the National Metallurgical Laboratory in the 1960s did not verify this claim.