An example of an architectural wonder is the Iron Pillar at Delhi’s Qutub complex at Mehrauli. It will initially appear to be nothing more than an iron pillar standing on ancient earth. Though, The pillar has mostly resisted rusting for many centuries despite withstanding Delhi’s climate.
Delhi’s Iron Pillar has more amount of Phosphorus than the usual.
If you melt pig iron and add phosphorous in it, it will start breaking up. It would never come together and make bond with each other. So, what the manufacturers do is that they take out phosphorous out of the molten iron. (Just like you filter out tea leaves out from your tea, where tea leaves are your phosphorous molecules in an analogy.)
What ancient Indians did was they never took out the phosphorous out of the pillar. They welded the cylindrical pieces of wrought iron. To remove the phosphorous from it, they started hitting it from all sides with a hammer. Phosphorous got pushed towards the surface of the cylinder from inside. So inside had no more phosphorous while outer surface had a protective layer of phosphorous.