The Iron Pillar of Delhi shows the amazing skills of Indian blacksmiths from a long time ago. These skills were developed and perfected during the Neolithic Period. This period happened at the same time as the Indus Valley civilization. Indian blacksmiths started making and using iron objects around 1800 B.C.E. This was during the second millennium B.C.E. The emergence of iron technology was a result of indigenous developments. Iron melts at 1539 degrees Celsius, a temperature that Indian furnaces of that era could not achieve. To overcome this difficulty, Indian blacksmiths developed forge welding technology. These artisans used this method to make various iron objects. They created tools for farming and large items like the Delhi Iron Pillar. This 7-meter-tall Delhi Iron Pillar, weighing approximately 6 tons, is a huge iron structure in India.
Its most striking feature is its remarkable resistance to corrosion. This raises the question: why has this ancient iron structure remained rust-free for over 1600 years. Ancient Indian blacksmiths used advanced techniques to work with metals. This skill helped their creations last a long time resisting corrosion.
Corrosion happens when metal breaks down due to reactions with the environment. For example, iron rusts when it interacts with water and oxygen.
Who made the Iron Pillar of Delhi?
Chandragupta II Vikramaditya created the Iron Pillar of Delhi around AD 402. He made it as a ‘Standard of Vishnu’ at a place called Vishnupadgiri or Udaygiri. The pillar had a religious and astronomical purpose. In the early morning during the summer solstice, the pillar’s shadow moved toward the foot of Vishnu’s image. This happened in one of the temples at Udaygiri. The summer solstice is the longest day of the year, usually around June 21. Around AD 1233, Sultan Iltutmish moved the pillar from Udaygiri to its current location in the Qutub Complex.
How was it originally fabricated?
As noted, Indian blacksmiths developed forge welding technology to manufacture metal objects. This method involves forging iron pieces in a red-hot condition to create a joint between them. On a charcoal bed, blacksmiths heat the pieces to a high temperature. Then they join all these pieces one by one by hammering. Finally, chiseling and burnishing the pillar’s surface gave it a decorative appearance. They used cold dies to engrave Sanskrit inscriptions on their surfaces. They must have finally fitted the decorative bell capital onto the upper section. This technique allowed the Delhi Iron Pillar to resist rusting for over 1600 years.
Why does the Iron Pillar of Delhi resist corrosion?
Metals extracted from their ores have non-thermodynamic stability. Through the process of rusting or corrosion, they tend to achieve a lower energy state. We commonly observe that any iron object in a moist atmosphere quickly rusts. If the humidity in the atmosphere exceeds 60%, iron objects tend to corrode. If this is the case, why does the Iron Pillar of Delhi resist corrosion? This is primarily due to the high phosphorus content of anciently produced iron. The table below details the weight percentage and chemical composition of Delhi Iron Pillar.
The high phosphorus content converts unstable oxyhydroxides in the rust layer on the pillar into stable ones. After some time, this transforms into a stable magnetite layer. Additionally, phosphoric acid reacts with iron. This reaction forms a layer of crystalline iron-hydrogen phosphate hydrate on the surface. This layer fills cracks and voids in the coating that is already on the pillar surface. As a result, it reduces electrochemical reactions. Electrochemical reactions are processes where electricity causes chemical changes.
Therefore, we can say that the crystalline iron hydrogen phosphate hydrate is responsible for its excellent corrosion resistance. Alternate wetting and drying cycles help form a protective passive layer. This layer protects the pillar from corrosion. Delhi’s dry atmosphere significantly aids in this.
Some scientists have another theory. They suggest that soil rich in beneficial bacteria surrounds the Iron Pillar. This soil could help form protective layers on the metal surface. These microorganisms may help prevent rust by forming biofilms that shield the iron from corrosive agents.
Closing Remarks
Researchers have suggested several explanations for the pillar’s impressive corrosion resistance. However, they have not yet agreed on a final, full-proof theory. Research on the Delhi iron pillar paved the way for a number of ideas. For instance, blacksmiths from medieval India used a method called direct reduction. This method helped them get iron with a high phosphorus content. They then used this special iron to make RCC bars. RCC bars are reinforced concrete bars that are strong and durable. Another idea is to improve the ductility of high-phosphoric irons. This can be achieved by heating, soaking, and then air-cooling wrought iron. To sum up, the Delhi iron pillar has remained a mystery for many centuries.
Reference
Balasubramaniam R., Delhi Iron Pillar: New Insights, Indian Institute of Advanced Studies, Shimla, 2002.
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