When the Persian invader, Nadir Shah, who occupied Delhi in 1739, visited the Qutb Minar
complex, he was dismayed to find an artefact made by Kafirs (unbelievers) occupying pride
of place. No-one knows who exactly had the iron pillar currently located in the complex
made. Based on the name, Chandra that figures in one of the inscriptions on it, the pillar is
believed to have been forged at the behest of Chandragupta II Vikramaditya, the great Gupta
emperor, about 1600 years ago. It is likely the pillar had been brought to the Qutb Complex
from elsewhere by one of the early Sultans of Delhi to plant as a trophy: the pillar is located
in the courtyard of the Quwwat-ul-Islam (“Might of Islam”) Mosque. However, unlike the
fragments of temples that had had any Hindu or Jain imagery defaced before being used in
the Islamic structures in the complex, the iron pillar does not seem to have been changed in
any noticeable way. This is probably what met with Nadir’s disapproval.
Not one to lose time, he lined up a cannon and had a ball fired at the pillar, believing –
not unreasonably – that it would not survive the explosion. After the smoke from the gunshot
had cleared away sufficiently, Nadir found, to his great shock, that the ancient iron pillar had
not only withstood the impact of the cannonball but the ball had ricocheted off the pillar, and
fallen on the Quwwat-ul-Islam mosque, destroying its southwestern wing in the process. One
can see both the minor concave dent and a fissure in the pillar from the shot as well as the
damaged portion of the mosque today. Nadir dared not fire a second ball at the pillar, lest it
fall on him in its rebound. Nadir was not one to brook any defiance, having ordered an
infamous general massacre of Delhi’s citizens when his soldiers faced minor skirmishes,
which any hostile foreign army can expect. However, when it came to this ancient kafir
artefact, he quietly decided that discretion was the better part of valour!
Above: The corrosion-resistant Iron Pillar in the courtyard of the Quwwat-ul-Islam mosque
in Mehrauli, Delhi (picture courtesy: Wikipedia).
“Rustless” wonder
One is struck by the absence of any loose rust on the surface of the pillar, which hints that it
has not been weakened by weathering in all the centuries that it has stood exposed to the
elements. Compare this with the Eiffel Tower, whose rivets have to be changed every few
years. Indeed, can you think of any stainless steel objects in your home that will not start
rusting if they are immersed in water continuously for a few days (steel is basically an alloy
of iron and carbon, and some other elements)?
However, the fact that the iron pillar has been so resistant to corrosion is not the only
thing wondrous about it. The technology by which the pillar was forged to make it extremely
strong – indeed so strong a cannonball barely dented it 1300 years after it was made – is
remarkable too.
Sponge Iron and Forge-welding
Iron was traditionally made by what was known as the bloomery process. Pulverised iron ore,
charcoal and a mixture of minerals known as fluxes – often other iron oxides occurring in the
ore itself – were charged into a furnace. The charcoal would be partially oxidised to carbon
monoxide, which would then reduce the iron ore to iron. To maintain a sufficiently high
temperature in the furnace, the blacksmith would blow air through holes at the bottom of the
furnace using bellows. If the temperature in the furnace rose very high, the charcoal would be
oxidised to carbon dioxide, and no iron formation would happen. Thus, the fluxes, which
catalysed the formation of iron at suitably low temperatures, and mopped up the molten
byproducts and removed them in the form of slag, were extremely important. At the end of
this process, the furnace would yield a welter of iron and slag, which is a mixture of all the
byproducts of the reaction. This welter was known as a bloom. Some communities that
smelted iron this way referred to this forbidding-looking mass as hagora (the furnace’s
excreta). The bloom would be heated back to a red-hot state and, in this semi-molten state,
would have all the slag hammered out of it, leaving only the metal behind. The iron obtained
in this way is known as sponge, bloomery or wrought iron. The iron could also contain a
certain amount of carbon.
It is easy to appreciate that the process of iron making is unlikely to have been an
accidental discovery. It comprises multiple steps, none of which are intuitively obvious. In
other words, it could only have been the result of deliberate experimentation and probably
took centuries to perfect. Archaeological evidence tells us that many parts of India had
developed ironmaking processes by the early half of the 2
nd millennium BCE, which is
significantly earlier than elsewhere in Eurasia. Unfortunately, Western scholarship prefers to
ignore this evidence, and take the ironmaking process developed in Anatolia (present-day
Turkey) in around 1200 BCE as the starting point of ferrous metallurgy. It is also likely that
ironmaking technology may have spread to India to other parts of Eurasia.
To make large objects, multiple lumps of sponge iron were joined together by heating
to a red-hot state and hammering them together. At the interface, the lumps would fuse
together owing to localised melting. This process is known as forge welding. Indian
blacksmiths preferred this process to casting for making large objects such as guns and
pillars. Casting is more economical and straightforward, and can be easily scaled up for
manufacturing large objects, but also requires temperatures high enough to melt the metal till
it can be poured into a mould. Now Indian blacksmiths were perfectly capable of producing
such high temperatures. So what was the reason for choosing the highly laborious forge
welding?
Heating and hammering, as is done in forge welding, eliminates any pores or voids and
produces a denser metal. It also produces a fine-grained microstructure, which leads to better
toughness. The grains are also aligned, leading to high strength and impact resistance. In
other words, products obtained by forge welding are of a much better quality than those
produced by casting.
Above: The dent and horizontal fissure on the Iron Pillar caused by Nadir Shah’s cannon
shot. The Qutb Minar can be seen in the background. (Picture courtesy: TripAdvisor)
One may fear that, in an object produced by forge welding multiple blooms together,
the large number of interfaces may make the object more liable to fail. However, it is a
testament to the skill of Indian blacksmiths that the iron pillar, far from fragmenting along its
multiple seams, behaved as an integral whole when fired upon by Nadir Shah’s gun – and this
was nearly 1400 years after it had been forged and left exposed to the elements the entire
time.
What is even more interesting is that the pillar is made from iron, and not steel. Steel is
made by alloying iron and carbon. As a result of alloying, steel has better mechanical
properties than iron, especially strength and toughness. The mastery of the blacksmiths over
the forge welding technique is evident from the fact that the ancient pillar not only withstood
the impact of Nadir’s cannonball suffering no more than a small dent and a minor fissure, but
sent it ricocheting back.
Corrosion resistance
The most immediately observable feature of the Iron Pillar is, of course, its resistance to
rusting. This feature baffled scientists for a very long time, till it was finally understood by
Prof. R. Balasubramaniam of the Indian Institute of Technology Kanpur. The technique for
making the pillar resist corrosion has the simplicity of genius.
The pillar does have a thin layer of rust on it. Balasubramaniam analysed the rust, and
found that it contained hydrated iron hydrogen phosphate. It is known that ancient iron
produced by the bloomery process had a much higher phosphorus content than that made by
modern processes. This is because many ores used from the ancient to mediaeval times
contained phosphorus compounds, from which phosphorus made its way into the iron.
Modern processes avoid phosphorus, as it makes the iron brittle; indeed, modern processes
use limestone (calcium carbonate) as a flux, which removes phosphorus from the iron by
forming calcium phosphate. However, traditional processes worldwide preferred phosphoric
iron owing to its higher strength, even if it meant more brittleness.
While bloomery iron the world over had phosphorus, only the Indians exploited it to
confer corrosion resistance on the iron. This was done by alternately wetting and drying the
iron. When the phosphorus-containing iron was wet in the rain, rust (iron hydroxides) would
form at the surface, leading to an increase in the content of phosphorus at the iron-rust
interface. Each wetting/drying cycle would lead to a progressive enrichment of phosphorus at
the interface, eventually leading to the formation of phosphoric acid owing to the reaction of
phosphorus and water. The phosphoric acid would then react with the iron, leading to the
formation of a contiguous, crystalline layer of hydrated iron phosphate at the interface. The
hydrated iron phosphate acts as a passivating layer, which prevents further corrosion of the
iron surface. Once this layer is formed, rain will not rust the iron any more.
It is a common observation that wetting leads to the formation of rust on iron surfaces.
However, rust is a mixture of hydrated iron oxides and iron hydroxides, which are porous.
They are not capable of stopping further corrosion of the iron: water can seep through the
pores and attack more iron. The hydrated iron phosphate formed on phosphorus-containing
iron, on the other hand, is continuous and impervious, and shields the iron from further
rusting.
Again, it is easy to appreciate that this discovery is unlikely to have been entirely
accidental. Only a systematic study of the effect of alternate wetting and drying cycles could
have led to the discovery of corrosion-resistant iron. Indeed, many metals acquire a thin layer
of oxide on their surfaces owing to the action of atmospheric oxygen. A prominent example
is aluminium, which grows dull over time thanks to the formation of an oxide layer. This
layer is continuous and is capable of passivation – it prevents further corrosion, even by
strong acids. Yet other metals like magnesium and iron are unable to form a continuous oxide
film which can protect them from corrosion.
Widespread technology
For long, Western researchers argued that the absence of significant rusting on the Iron Pillar
was not so much owing to any special properties of the metal as the relatively dry climate in
Delhi. However, such iron has been found in multiple places in India: the fragmented iron
pillar in Dhar (Madhya Pradesh) the combined length of which is twice the height of the
Delhi Iron Pillar, the iron frame of the Sun Temple in Konark (Odisha), the iron pillar in the
Adi Mookambika Temple in Kodachadri (Karnataka), and numerous guns made by the
Marathas that can be seen in their forts on the Konkan coast, such as the mighty Kalak
Bangadi cannon in the Murud-Janjira fort. Indeed, Indus University is home to two excellent
specimens of corrosion-resistant iron technology: the guns flanking the entrance to the main
building. Many of these places experience heavy rainfall, and some are located on the coast,
where the atmosphere is quite corrosive. Clearly, the resistance to rusting is not owing to a
benign environment, and Prof. Balasubramaniam’s explanation is likely correct.
Above: (Left) The iron pillar in Dhar, also currently located in a mosque complex after
having been moved from its original location, and (right) the Kalak Bangadi cannon in the
Janjira fort. (Pictures courtesy Wikipedia.)
How did we lose this technology, which was widespread all over India as late as three
hundred years ago? For this, we must blame the British, who were slowly beginning to
produce large amounts of iron and steel and desperately needed markets to offload these on.
However, the quality of British metal was no match for that of what was being produced in
India. The only way the British iron and steel industry could survive was by banning Indian
metal. This they proceeded to do by way of the cleverly disguised Forest Acts passed in the
19th century, which barred Indians from exploiting any resources from any piece of land that
was classified as a forest – essentially, any land that was currently not under cultivation.
Thanks to these Acts, Indian metalsmiths lost access to ore as well as wood for making
charcoal that was required to smelt the ore. Almost overnight, thousands if not lakhs of
people engaged in the trade were disenfranchised and brought to the brink of starvation.
Today, the once prosperous traditional blacksmith communities such as the Lohars of Uttar
Pradesh and the Kammari of Andhra, whose products were sought after not only all over
India but also beyond her borders, are classified as Extremely Backward Classes.
There is also reason to believe the presence of phosphorus in iron was preferred
because it conferred hardness. In this respect, phosphorus may prove to be slightly better than
carbon, which for long has been the element of choice for alloying with iron. One hopes that
in the coming years, research will unravel all the secrets of India’s corrosion-resistant iron,
which – perhaps unfairly – has been eclipsed by wootz or Damascene steel that was prized in
mediaeval times for making swords with exceptionally sharp cutting edges.
