The Iron Age (800 BC-100 AD) took its name from, well, iron. This kickstarted a number of technological and social changes, with centuries-old Bronze Age (2200-800 BC) civilizations based on copper and tin falling prey to invincible newcomers who wielded formidable iron weapons.
But how did people make iron in the first place? Did they suddenly have access to technology that could raise the temperature high enough to melt iron?
Well, no. Instead, people came up with an ingenious way of using existing technology in a radically different way, as Jason Almendra explains on Quora. They sourced the metal from bog iron or iron ore, both of which had been known for centuries. Now, however, they realized they could ground this to a powder. They then lit a bloomery with charcoal and raised the temperature using bellows.
Iron Age metallurgists alternated adding charcoal and iron ore powder. After a few days, they lowered the temperature and extracted a bloom, which is a mass of iron and steel mixed with slag. In the photo below, you can see what a bloom looks like: the shiny bit is steel, the dull gray part is iron, and the black bits are slag.
The bloom must be heated, hammered, and refolded to drive out the slag:
At the end of this process, you’re left with a beautiful piece of iron, ready to be forged into a tool or weapon by the local blacksmith:
The Indians took this technology one step further by accidentally developing rust-free metallurgy, as Charlie Buffet explains. In 268 BC, King Ashoka built several iron pillars with inscriptions describing their kingdom, territory, and conquests. These stunning pillars are found at various locations throughout India.
The Gupta empire pillar above is famous for the rust-resistant composition of the metals used in its construction. Standing at 23 feet 8 inches (7.2 meters) and weighing over three tonnes (6,614 lb), it still stands tall 2,400 years later in New Delhi, ignoring the wind, rain, storms, floods, and lightning.
The pillar was manufactured by forge-welding pieces of wrought iron. Strangely enough, its resistance to corrosion is due to the weakness of the manufacturing process. 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 thanks to the wood used in the furnace, and the alternate wetting and drying under atmospheric conditions are the three main factors in the three-stage formation of a protective passive film.
While high corrosion rates are initially observed, an essential chemical reaction intervenes: slag and unreduced iron oxides in the iron microstructure alter the polarisation characteristics and enrich the metal’s surface with phosphorus, thus indirectly stopping the rusting process.
Without going into too many technical details, these act as a cathode. The metal itself serves as an anode, thus creating a mini-galvanic reaction during environmental exposure that stops corrosion. This is made possible by the fact that ancient Indian smiths did not add lime to their furnaces (the use of limestone 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 during the smelting induces a higher phosphorus content than in modern iron produced in blast furnaces. This high phosphorus content is an essential catalyst in the formation of the passive protective film that forms a barrier by adhering in the meeting place between metal and rust. So slow is this film’s creation that since the pillar was built, it has grown into a thickness of just one-twentieth of a millimeter!
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