Iron pillar of Delhi

There two such 1,600-year-old iron pillar in Delhi. The objects show no signs of rust or decay despite the weather conditions (over 1500 monsoons). The fifth century pillar, weighing more than six tonnes are over 23 feet high and consist of a single piece of wrought iron with an iron content of 99.72 %. (It is the purest iron found in the ancient world). Their surface is smooth and only some instances of scars and weathering can be seen. But it should not be like this… Any equivalent mass of iron, exposed during 1,600 years or more, to the Indian weather would have been reduced to rust only. These two iron monuments appear to have been protected from rust by the application of a thin coating of manganese dioxide. It is one of many suggested theories. By the way was this chemical also used in any other part of the ancient world? Who were the skilled metallurgists who made the pillars?