Delhi Iron Pillar - A Metallurgical marvel.

Delhi Iron Pillar – A Metallurgical Marvel. 

Image Credits : Wiki Commons.

Image Credits : Regional Science Centre, Dharwad and National Science Centre, Delhi

Delhi, the nations capital, was once again in the news for its infamous air pollution, which was so alarmingly dangerous that the government was  forced to close schools. Even the odd even measures seem not to have helped in improving air pollution, which was choking the health of Delhiites. Amidst such adverse environmental conditions, the Delhi Iron Pillar - aptly classified ‘The Rustless Wonder’ by the doyen of metallurgy, Dr T R Anathraman - has been majestically standing tall without any rusting for more than 1600 years. The credit for this monumental metallurgy craftsmanship must go to those Indian iron smiths of yesteryear’s, whose metallurgical knowledge has ensured that the Delhi Iron Pillar has remained corrosion less. Scholars like Dr T R Anathraman, Prof R Balasubramaniam (Bala) have extensively researched on the Delhi Iron Pillar to shed new light into the reasons for DIP to remain rustles for centuries. During the course of my curating of ‘Our Technology Heritage’ exhibition, I was privileged to work with Prof Balasubramaniam and also had the honour to publish a popular article on the Delhi Iron Pillar (DIP) in one of the leading monthly magazine - One India One People -  in 2005. Prof R Balasubramaniam, former Professor at the IIT Kanpur has extensively researched and published on the DIP and his book ‘New Insights into the Delhi Iron Pillar’, a publications of the Indian Institute of Advanced Studies, Shimla is an outstanding book on the DIP. Professor Balasubramaniam's illustrious career was most tragically cut short by his untimely demise on December 9, 2009. I am writing this blogpost on the DIP as a tribute to Prof Bala and have extensively relied on the joint article that I published with Prof Bala.

Delhi, India’s capital city, has a long and rich heritage. The city has ancient ruins, monuments and contemporary buildings that stand side by side, providing a glimpse of its glorious past and tumultuous history. Of all the ancient monuments in the city of Delhi, the Qutab Complex (where the Qutub Minar is located) houses a metallurgical wonder and India’s unique technological marvel - the Delhi Iron Pillar. Its uniqueness in remaining without significant signs of corrosion for more than 1600 years has baffled modern scientists and metallurgists from across the world. The DIP embodies the rich technological achievements of ancient Indian blacksmiths. The sight of the DIP standing in the courtyard of the Quwwat-ul-Islam mosque, adjacent to the Qutub Minar, is breath-taking and cannot be missed by any visitor. It is testimony to the marvelous metallurgical knowledge that existed in ancient India, which should rightfully considered as one of the metallurgical wonders of the world.

Ancient Indian Iron

The use of metals and knowledge of metallurgy is inextricably linked to the history of civilization. Material progress of any country, apart from other things, depends upon mining, metallurgy and metal industries. The credit of providing impetus to material progress, in modern times, no doubt, goes to scientific technology. Even the cursory survey of history of mankind reveals fairly well that people all over the world, specially in India, and much before the dawn of modern scientific age, have been exploring the possibilities conducive to their material progress. Modern metallurgy has witnessed unprecedented growth beginning with the Industrial Revolution. However, many modern concepts in metallurgy owe their genesis to ancient practices that pre-date the Industrial Revolution. Artifacts, tools, objects, etc. made of metals for both artistic and utilitarian purposes have been in vogue ever since antiquity. The commonly used metals, from a historical standpoint, include gold, silver, copper, iron, tin, lead, zinc and mercury.

Iron, however, has its own unique place amongst all the metals. Iron stands for power and strength as evidenced from the title given to Sardar Patel, one of the architects of freedom movement in India, who was called the Loha Purush - the Iron Man of India. The significance of iron for human kind can best be described in the words of Pliny the Elder (AD 23-79), the Roman naturalist and writer who wrote about iron: "Iron mines bring man a most splendid and most harmful tool. It is by means of this tool that we cut into the ground, plant bushes, cultivate flourishing orchards and make grape vines with grapes. By this same tool we build houses, break stones and use iron for all such purposes. But it is also with the help of iron that we fight and battle and rob. And we do it not only at close quarters, but giving it wings, we hurl it far into the distance, now from embracers, now from powerful human hands, now from bows in the form feathered darts. This, I think, is the most infamous invention of the human brain. For in order to enable death to catch up with man faster, it has given it wings and armed iron with feathers. For that many the blame rests with man and not with the nature."

One of the civilizations that invented iron and refined the technique for extraction from iron ore, was India. The earliest date for iron in the Indian subcontinent is about 1600 BC, which is being pushed further back by many scholars. Indians discovered the beneficial aspects of intentionally alloying carbon into iron, thereby producing stronger and tougher steel. Technically, steel is defined as an alloy of carbon in iron. This steeling of iron, which was discovered in the Indian sub-continent sometime in 800 BC, resulted in clearing of dense forests leading to the subsequent second urbanization of India along the banks of the Ganga and Yamuna. The first urbanization of India was the establishment of planned settlements along the once-mighty Saraswati river during the  Indus Valley or Harappan Civilization which dates back to 3000 BC. The shifting of feeding river channels away from the Saraswati river (due to tectonic motions), and its subsequent drying up, resulted in the disbursement of the inhabitants of these settlements to other corners of India, by about 1500 BC thus ending the Harappan era.

It may interest reader that with the advent of carburization of iron, a special type of high carbon steel was produced in India from as early as the 4th century BC. This steel was known as ‘wootz steel’ and it was much prized by warriors due to tough swords that could be wrought from wootz steel. This wonder material of the Orient was held in great esteem by the medieval warriors and European scientists in the 19th century. Studies conducted by eminent scientists like Michael Faraday for understanding the mystery of wootz steel laid the foundation of modern metallurgy. The story of wootz steel, however will not be touched in this article. 

Given this rich metallurgical tradition in iron and steel making, it was but natural that the Indian sub-continent was home to the creation of marvelous iron objects. It must be mentioned here that ancient Indian iron was extracted in small furnaces by solid state reduction of the ore. The end product was an iron lump which was later forged together in white heat to produce different shaped objects. This method of working with iron was practiced for a long time in Indian history, all the way up to the British period. One marvelous example of a wrought iron product is a gilded wrought iron image of Buddha, now in the Lucknow State Museum. This 18 cm high image was discovered from Azamgarh and dated to the Gupta period (320-600 AD). Let us now turn our attention to the real metallurgical marvel of the Gupta period – the Delhi Iron Pillar.

Early Studies of the Delhi Iron Pillar

The Delhi Iron Pillar is singularly featured on the emblems of several Indian institutions (for example, the National Metallurgical Laboratory at Jamshedpur), thereby signifying its prime identity as the country’s metallurgical pride and heritage. The first detailed scientific study of the Pillar was carried out by an eminent British metallurgist, Hadfield, in the year 1912. Ever since, there have been a growing number of studies in which several “mysteries” of the Pillar have been unraveled. The Archaeological Survey of India studied the Pillar with the co-operation of the National Metallurgical Laboratory (NML) in 1961. The results of these scientific studies were summarized in a special issue of the NML Technical Journal (Volume 5, 1963). A review of the pillar’s corrosion resistance appeared in 1970. Professor Anantharaman, one of the doyens of modern metallurgy in India, has published known scientific facts about the Pillar in a book titled The Rustless Wonder – A Study of the Iron Pillar at Delhi in 1996. Several new insights on the historical, scientific and technical aspects of the Pillar have been researched by Prof R Balasubramaniam and have been published in National and international journals. In addition, the research findings of Prof R BAla have been compiled in two books titled Delhi Iron Pillar – New Insights (2002) and Story of the Delhi Iron Pillar (2005). This blogpost provides a brief summary of all aspects related to the Delhi Iron Pillar.

Pillar’s History

Among the several inscriptions on the pillar, the oldest (and also the largest) is a three-stanza six-line Sanskrit-Brahmi inscription, at a level of about 7 feet from the stone platform. This inscription records that the pillar was set up by Chandra as a standard of Vishnu at Vishnupadagiri. The monarch’s conquests have also been poetically described in the inscription. Based on the nature of the characters, the inscription can be dated to the period between 400 to 450 AD. This inscription appears to have been embedded using specially prepared dies (i.e. by die-striking operations). One close-up view of the inscription provides some details. Specific die shapes were used for making the inscription. The monarch Chandra has been identified unambiguously with Chandragupta II Vikramaditya (375 AD-414 AD), based on careful analysis of archer type Gupta gold coins. A possible image of how Chandragupta II Vikramaditya would have looked like can be hypothesized. The Ajanta cave paintings are believed to have been executed during the Gupta period. In one of the Ajanta paintings, the king is seen having his royal bath and this shows the appearance of a monarch of this period.

The original location of the pillar, Vishnupadagiri (meaning “Vishnu-footprint-hill”) has been identified as modern Udayagiri, situated in the close vicinity of Besnagar, Vidisha and Sanchi. These towns are located about 50 km east of Bhopal, in central India. There are several aspects to the original erection site of the Pillar at Udayagiri, which cannot discussed in detail here. However, it must be worth noting that Vishnupadagiri is located on the Tropic of Cancer and therefore was a centre of astronomical studies during the Gupta period. The Iron Pillar served an important astronomical function, when it was originally at Vishnupadagiri. The early morning shadow of the Iron Pillar fell in the direction of the foot of Anantasayain Vishnu (in one of the panels at Udayagiri) only in the time around summer solstice (21 June). The creation and development of the Udayagiri site appears to have been clearly guided by a highly developed astronomical knowledge. Therefore, the Udayagiri site, in general, and the Iron Pillar location in particular, provide firm evidence for the astronomical knowledge that existed in ancient India, around 400AD. The flowering of astronomical knowledge under prominent astronomers like Aryabhata, Varahamihira and Brahmagupta during the Gupta period is well known.

Why is the Pillar now located at Delhi? Who moved it here and When was it moved? Based on careful study of history of the Qutub Complex, it can be reasonably concluded that the Pillar was erected at its current location in the Quwwat-ul-Islam mosque by Iltutmish (1210-1235 AD). Iltutmish was the first Delhi Sultan to invade Malwa in 1233 AD. After his capture of Vishnupadagiri, several objects were carried to Delhi and the Pillar was one such object. In fact, the Quwwat-ul-Islam mosque was completed by Iltutmish and it is certain that he planted the wonderful Iron Pillar in the centre of the mosque courtyard. Ever since, the Pillar has remained at this location.

Engineering Design

Let us explore some engineering aspects of the DIP. It is now certain that the current burial level of the DIP  was not the original burial level of the pillar, when it was located at Udayagiri. The rough portion of the pillar was originally buried in the courtyard of the temple and later left exposed outside when the iron pillar was displaced from its original position. A closer look at the DIP, one can see the hammer-marked cavities, which are visible on the surface of the pillar in the rough region just below the smooth surface-finish region. This rough surface is the original manufactured condition of the pillar. It was left rough in order to aid gripping of the pillar to the buried underground region in Udayagiri.

There is a stone platform currently seen surrounding the pillar base, which was set by Beglar in 1861. Early sketches and published photographs of the pillar, before the construction of the stone platform, attest to the absence of the stone platform before Beglar’s excavations. One example is the free hand sketch of the pillar by an artist named Mirza Shah Rukh Beg, commissioned for publication in Syed Ahmed Khan's Urdu work Athar'al-Sanadid in 1846. A critical analysis of the dimensions of the main body of the pillar provides conclusive evidence for the original burial level of the pillar and also an appreciation of the pillar's symmetrical design. Considering the basic unit as U, the rough surface occupies one-fourth (60U) and the smooth surface three-fourths (180U) of the pillar’s main body length, excluding the decorative top. The burial of the pillar body to one-fourth of its height would have provided the necessary stability to the structure. The unit U is equal to 1 modern inch. The angulam, the ancient Indian unit of measurement, equals 0.75 of the modern one inch.

The buried underground region was excavated in 1961. The base of the pillar was flat. Eight small projections were seen at equal intervals. These projections appeared from the sheet of almost pure lead, placed at the bottom of the pillar. A heavy slab of stone was found placed horizontally on the original upper layer of the temple floor on which the lead sheet rested. It appears that the two iron rods were placed parallel to each other and another two iron rods were placed above these, such that they were perpendicular to the initial rods. This provided a grid-like structure. The iron pillar base was then fixed atop this iron grid structure thereby providing necessary support at the bottom. Why was the lead plate provided at this location? The lead sheet would have acted like a cushion in case of seismic disturbances. However, the main purpose of the lead sheet appears to be to grip the pillar to the supporting stone underneath.

A coating of lead was present when the underground regions were excavated. In 1961, the year I was born, the surface was cleaned and the cracks were consolidated. A new lead coating was provided on the pillar in the buried underground region and the pillar again buried under the ground.

Iron of Delhi Pillar

The underlying metal of the pillar would be discussed briefly in order to elucidate its characteristic features. Incidentally, these features are also characteristic of ancient Indian irons. Several composition analysis of the iron of Delhi Pillar is available. the average composition of the pillar iron is 0.15%C, 0.25%P, 0.005% S, 0.05%Si, 0.02% N, 0.05% Mn, 0.03% Cu, 0.05% Ni and balance Fe. The high P content of the pillar iron must be particularly noted. Careful compositional analysis near surface regions proved that there was no surface coating provided specifically for enhancing the corrosion resistance of the pillar. The pillar is a solid body with good mechanical strength. In fact, a cannon ball fired at the Delhi iron pillar in the 18th century (either by Nadir Shah in 1739 AD or Ghulam Quadir in 1787) failed to break the pillar. The marks of this cannon ball shot can be seen in the southern face of the pillar half way up the height of the pillar.

The presence of phosphorus (P) is crucial to the corrosion resistance of Iron Pillar. In ancient iron making practice, limestone was not added. The absence of lime resulted in a higher amount of phosphorus in the metal. It must also be noted that there are indications that Phosphorus addition may also have been intentional. We have some evidence for this based on observations recorded during the detailed travels of an early British explorer - Francis Buchanan. In his detailed description of steel making at Devaraya Durga in Karnataka in the 18th century, Buchanan describes an Indian wootz steel making furnace. According to Buchanan, conical clay crucibles were filled with a specific amount of wood, from the barks of a plant cassia auriculata, pieces of wrought iron, then sealed and fired. Interestingly, the bark of this plant contains a high content of P, extracted by osmosis from the ground.

Manufacturing Methodology.

The Iron Pillar weighs almost 6 tonnes. Even in modern times it is It is a real challenge to manufacture such a large object. Therefore, given the time period of manufacture of the Iron Pillar, its construction must be considered a real engineering marvel. Based on careful analysis of several aspects of manufacturing methodology, gleaned from careful observation of the surface of the pillar, the following conclusions can be reached.

The starting material for the forging of the Pillar was iron lumps, obtained from bloomery furnaces. They weighed between 20 to 40 kilograms. They were joined together by forge welding. This was the method used in ancient and medieval India to manufacture large iron objects. Forge welding is an operation in which iron lumps are joined together by forging them in the hot state (high temperature of about 700 to 900 degree Centigrade, so that they fuse together. This process initially involves heating of the lumps to a relatively high temperature in a bed of charcoal, in order to make them soft and amenable for deformation. One iron lump is then placed on top of another and force is applied in order to weld them in the solid state. As the force is dynamic in nature, it is called forge welding.

For manufacturing the Pillar, the heated iron lumps were placed on the side surface of the pillar and hammered on to the same by the use of hand-held hammers. The addition of metal was sideways with the pillar placed in the horizontal direction. The pillar's vertical and horizontal movements would have been aided by handling clamps provided on the surface of the pillar, the protruding portion of which must have been chiseled away during the surface finishing operations. Visual proof for the presence of these clamps is available at two locations on the pillar. Finally, the surface of the pillar (that was supposed to be exposed) must have been smoothened by chiseling and burnishing the surface of the pillar, thereby providing it a smooth tapered cylindrical appearance. Finally, the Sanskrit inscriptions might have been inscribed on to the surface of the pillar. Cold dies must have been used for inscribing the inscriptions with the metal surface being inscribed possibly being locally heated before inscribing. The decorative bell capital must have been finally fit on to the top portion of the Delhi Iron Pillar before its erection, possibly in the royal presence of Chandra.

Decorative Bell Capital

The top decorative capital of the Iron Pillar is a wonderful engineered structure. The decorative bell capital consists of seven distinct parts. The bottom-most part is the reeded bell structure which has been manufactured by utilizing iron rods of uniform diameter. Atop this comes the slanted rod structure. The presence of a black filling in between the joints can be seen in the bell capital. This filling has been identified as a lead-based solder. The next three members are rounded structures, with the top one being only half rounded, because when the pillar is viewed from the bottom, this part would appear curved when viewed in perspective from the bottom. A round disc comes above this and finally the box pedestal is placed on the top of the capital. The box capital contains holes that are empty at the four corners and these could have been originally utilized for holding different animal figures, depending upon the season of the year. The top of the pillar presently contains a hollow slot in which a chakra (discus) image must have been originally fitted.  Interestingly, the image of the Delhi Iron Pillar capital’s box pedestal along with the chakra image is depicted in one of the Vishnu panels in cave 6 at Udayagiri itself and this is the most forceful argument for the chakra image atop the Delhi Iron Pillar, capital. The circular disc is also in tune with the cut that is seen on the top surface of the capital.

Corrosion Resistance

The real fascination of the Delhi Iron Pillar is its remarkable characteristics of corrosion resistance in the atmospheric environment. We know that the pillar is at least 1600 years old based on the identification of Chandra of the Gupta-Brahmi inscription on the Iron Pillar with Chandragupta II Vikramaditya (375-414 AD). Corrosion is a common menace, which eats away and eventually destroys metals and alloys by a electrochemical attack. The rusting of ordinary iron and steel is the most common form of corrosion. Rusting takes place in moist air, when the iron combines with oxygen and water to form a coating of brown-orange deposit, which in common parlance is termed as rust (hydrated iron oxide). The rate of corrosion increases where the atmosphere is polluted with sulphur dioxide. We come across the menace of rust in our day to day lives. Even modern cars and other gadgets are not spared from this menace. Salty road and air conditions accelerate the rusting of car bodies. It may be noted here that because of oxidation the extant articles made of iron in antiquity are extremely rare. We tend to live with rust even in modern times and take rusting for granted as a natural phenomenon. However it is baffling to note that the Iron Pillar has largely remained rustless for all of 1600 plus years. Several theories have been proposed to explain the pillar’s excellent corrosion resistance. They can be broadly classified into two categories: environmental and material theories.

The proponents of the environment theory state that the mild climate of Delhi is responsible for the corrosion resistance. It is known that atmospheric rusting of iron is not significant for humidity levels less than 70%. That the environmental theory may not be important, is attested by the presence of ancient massive iron objects located in areas where the relative humidity is high all round the year. Good examples for this case are the iron beams in the Jagannath temple at Puri, the Sun temple at Konarak and the iron pillar at Adi-Mookambika temple at the Kodachadri Hills. Puri and Konarak are situated on the western coast of India while Kodachadri is on the eastern coast. The distance from the location of these massive iron objects to the actual seacoast is less than 20 kilometers, thereby implying that these iron objects are constantly subjected to a saline environment due to proximity to the seacoast. There are other examples of massive iron objects in several other parts of the Indian sub-continent that have successfully withstood atmospheric corrosion. These include the iron pillar at Dhar and the numerous large forge-welded iron cannons scattered all over the Indian sub-continent. Two examples are the 22-ton Rajagopala cannon at Thanjavur in Tamil Nadu and the 20-ton Dalmardan cannon at Bishnupur in West Bengal.

Advocates of the materials theory stress the construction material’s role in determining corrosion resistance. The ideas proposed in this regard are the relatively pure composition of the iron used, presence of phosphorus, and absence of sulfur and manganese in the iron, its slag particles and formation of a protective passive film. The passive film component of the theory stems from Prof Bala’s research.

The literature does feature other, less-widely held theories about the pillar’s corrosion resistance. These suppositions include: the mass metal effect, initial exposure to an alkaline and ammoniacal environment; residual stresses resulting from the surface finishing (hammering) operation; freedom from sulfur contamination both in the metal and in the air; the “cinder theory,” which holds that layers of cinder in the metal stop corrosion from advancing; and that surface treatments of steam, and slag and coatings of clarified butter were applied to the pillar after manufacture and during use, respectively. The use of surface coatings is readily discounted because a freshly-exposed surface attains the color of the rest of the pillar in about three years time.

Scientific research by Pro Bala has revealed that the excellent atmospheric corrosion resistance of the Delhi Iron Pillar is due to the formation of a protective passive film on the surface of the pillar. In other words, the Iron Pillar does rust, but the passive rust is protective and thin such that significant rusting is not realized. Philosophically, this is very akin to the protective clothing that one wears in the cold season. The jacket or sweater, that one wears when the mercury dips down, “prevents” the body heat from being dissipated and therefore keeps the person warm. In a similar manner, the protective passive film on the surface of the Pillar does not allow corrosion of the underlying metal of the iron pillar, by preventing. moisture from contacting the bare metal surface. The relatively high phosphorus content of the Delhi Pillar iron plays a major role in aiding the formation of the protective passive film.

When viewed from a nonscientific standpoint, the Delhi Iron Pillar’s ability to resist corrosion has often been called a “mystery.” This notion must be dismissed. There is nothing mysterious about the Iron Pillar. The remarkable corrosion resistance can be understood by applying the basic principles of corrosion research. The direct reduction technique used to produce the iron is no mystery, either. The established scientific facts notwithstanding, there is one aspect that is not well-understood and this may be called a mystery, in one sense. This is the method by which the iron lumps were forge-welded to produce the massive 6-tonne structure.

On to the Future

India is again on the forefront of steel making as very much evident by the stellar rise of the Indian steel magnate - Mr Mittal. It is time that we also paid more careful attention to the Delhi Iron Pillar, which serves as a guidepost for metallurgists in the 21st century and beyond. It is hoped that research on the Iron Pillar will motivate others to explore the potential uses of phosphorus-containing iron. There are so many wonderful options available with phosphoric irons. The Iron-Phosphorus alloys deserve as much attention as the more popular Iron-Carbon alloys (i.e. steels). There is an exciting future in developing phosphoric irons, particularly for corrosion scientists and engineers. 

The beacon of light showing the way to the future is the Delhi Iron Pillar, with its tested proof of corrosion resistance. Long live the Delhi Iron Pillar and Prof R Balasubramaniam, who spent years and years researching the Delhi Iron Pillar to shed new insights into this metallurgical marvel of yesteryear’s.