01 The numbers
It is worth starting with the figures, because they are half the story. The pillar is 7.21 metres in total height, of which 6.09 stand above ground and 1.12 are buried. It is 41 centimetres in diameter and weighs more than six tonnes. It is a solid piece of iron, not a tube and not a cladding.
The dating rests on palaeographic analysis of its inscription and places it in the early fifth century, in the time of Chandragupta II, who reigned roughly between 375 and 415. It was not originally here: it was moved to this courtyard at some point between the eleventh and thirteenth centuries, and there is no agreement on who moved it or from where.
02 Why it does not rust
The explanation has two parts. The first is composition: the pillar's iron contains a lot of phosphorus, averaging 0.25 %, against less than 0.05 % in modern blast-furnace iron. That phosphorus was not removed because the production method of the period, unlike today's, did not take it out.
The second is what that phosphorus produces. With the alternation of wet and dry — monsoon rain, dry air the rest of the year — a crystalline layer of hydrated iron phosphate forms on the surface, clinging to the interface between metal and rust. That layer seals the iron and halts corrosion instead of letting it advance.
In sixteen hundred years the rust layer has grown one twentieth of a millimetre. A modern nail left outdoors does not last a century.
03 What making it implies
There is a second feat usually overshadowed by the first: making the thing. Six tonnes of solid iron in the fifth century did not come out of a mould: they were obtained by forge-welding, hammer blow by hammer blow, an enormous number of smaller iron blooms until they formed a single six-metre body.
That demands a considerable workshop organisation: furnaces capable of producing that quantity of iron, enough people to handle incandescent masses simultaneously and a work sequence that allows no mistakes, because a bad weld shows and weakens the whole. The piece is, as well as an object, the proof of an industry.
04 How to look at it
It deserves a genuine ten minutes and not a photograph in passing, because at first sight it is an unglamorous dark column. What to look at is the surface: it does not have the flaking orange look of old iron but a uniform, smooth tone, as if waxed. That uniformity is the protective layer.
And it is worth looking at what surrounds it too. The courtyard it stands in is a ruined complex: broken arches, truncated walls, stone eroded by air and rain. All of that is twelfth-century material or later. In the middle, unprotected and unmaintained, stands a piece of iron seven hundred years older.
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