The Delhi pillar

The oxide film formed in dry air at room temperature consists of a spinel phase, probably a solid solution of magnetite and maghemite. Such films form on magnetic tapes. They are around 1.5-2.0 nm thick, and in a dry atmosphere, can provide indefinite protection (e.g. the Delhi pillar). Ali and Wood (1969) found that with time and at a relative humidity of 46%, some hematite developed as well. At higher temperatures (200-300 °C) well defined duplex films with an inner layer of magnetite... 

Several large masses of iron are to be seen in India made many centuries ago by welding together small blooms, obtained by the direct process and weighing several pounds each. That such huge masses could be constructed is a remarkable tribute to the skill of the early Indian metallurgists. The most famous of these are the Delhi Pillar, the Dhar PillarJ and the iron beams from the Black... 

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