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Protective Scale on Delhi Pillar 

The heterogeneous nature of the structure of the monument reveals two important points, namely, no heat treatment has been applied and the metal of the pillar has never been in the molten state, probably the last stage in the construction of so large a piece of iron at that date would almost certainly have consisted of the hammer forging together of balls of iron and thereafter repeated re-heating and hammering process to create smooth surface. This must have taken a considerable time to complete. During this time an oxide film would have formed some of which could get hammered into the surface. Slag too would have oozed out and would have joined the scale. Owing to its high heat capacity and high ambient temperature the finished iron would have taken relatively long time to cool leading to a somewhat non-homogenous normalization, the quality of the oxide layer produced by this sequence of operation would in all probability greatly promote the preservation of the pillar in pure and dry climate.

According to the second theory, the protective oxide could have formed from atmospheric exposure. Examination of small pieces of scale obtained from the iron pillar reveals that it consists of approximately 80% of an oxide of iron having the properties of the solid solution phase of mixtures of FeO and Fe2O3. About 10% of this hydrated oxide of iron, approaching Limonite (Fe2O3.3H2O) has also been reported. From the above reports it can be concluded that the scale was apparently formed under conditions of heating with significant extent of atmospheric oxidation occurring at the surface and penetrating along cracks running longitudinally in the scale. Recent measurements of scale thickness with a permanent magnet-type thickness gauge, on the pillar itself, gave readings of less than 50 mm on the polished part and 500-750 mm on the rougher portion immediately above this.

There have also been suggestions that in the past pillar was ceremonially anointed with purified butter. Tghee obtained from the milk of cow would have had a marked effect. A thin coating of linseed oil or lanoline or wool grease is well known to give good protection to steel for some months. If applied regularly and reinforced b the dust and sand which settle on it, it gives a good protective coating to the material underneath. However, the practice of ceremonial anointing would probably have been discontinued during Muslim occupation of the area in 12th century AD. (reference)

Other landmarks: Christ the Redeemer, Colossus, Delhi pillar, Eiffel tower, Golden Gate bridge, Great Buddha, Guggenheim Museum (Bilbao), Guggenheim Museum (NYC), Normandy bridge, Oresund crossing, Quebec Bridge, Statue of Liberty, Thames Barrier, Titanic, Tower of the Orologio