Iron pillar of Delhi
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 hydrate forming on the high phosphorus content iron, which
serves to protect it from the effects of the local Delhi climate.
The pillar weighs over 6,000 kg, and is thought to have
originally been erected in what is now Udayagiri by one of the Gupta monarchs
in approximately 402 CE, though the precise date and location are a matter of
dispute. It was transported to its current location in 1233 CE.
Description
The Iron pillar stands within the courtyard of
Quwwat-ul-Islam Mosque
Original location
The first location of the pillar has been debated. While the
pillar was certainly used as a trophy in building the Quwwat-ul-Islam mosque
and the Qutb complex, its original location, whether on the site itself or from
elsewhere, has been discussed frequently. 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.
R. 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.
Inscriptions
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 Chandragupta 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 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:
One of the later inscriptions, dated to 1052 CE, 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.
Scientific analysis
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.
Evidence of
cannonball strike
A significant indentation on the middle section of the
pillar, approximately 400 cm (156 in) from the current courtyard ground level,
has been shown to be the result of a cannonball fired at close range. The
impact caused horizontal fissuring of the column in the area diametrically
opposite to the indentation site, but the column itself remained intact. While
no contemporaneous records, inscriptions, or documents describing the event are
known to exist, historians generally agree that Nadir Shah is likely to have
ordered the pillar's destruction during his invasion of Delhi in 1739 CE, as he
would have considered a Hindu temple monument undesirable within an Islamic
mosque complex. Alternatively, he may have sought to dislodge the decorative
top portion of the pillar in search of hidden precious stones or other items of
value.
No additional damage attributable to cannon fire has been
found on the pillar, suggesting that no further shots were taken. Historians
have speculated that ricocheting fragments of the cannonball may have damaged
the nearby Quwwat-ul-Islam mosque — which is known to have suffered damage to
its southwestern portion during the same period — and the assault on the pillar
might have been abandoned as a result.
