The Iron Age began around the first millennium BC. And at first, iron was not very popular, since the process of obtaining and processing it was much more complicated than smelting copper or obtaining bronze. Iron had an undoubted advantage in comparison with bronze - products made from it had less wear resistance, however, oddly enough, their service life was much less than bronze ones. The reason for this was the low corrosion resistance of iron. The technologies existing at that time for the production and processing of iron could not ensure the long-term safety of tools and weapons.
However, mankind had not so much to switch to iron in connection with its strength, how much to move away from bronze due to the depletion of easily accessible deposits of tin, which are its unchanging component. When it became clear that the number of iron deposits was several times greater than that of copper, the fate of the bronze tools was decided. And mankind had no choice but to look for ways to protect iron from the effects of air and water.
It must be said that for over a thousand years nothing new in the production of iron has been invented. All the time, up to the 5th century AD, the standard cheese-blown method of iron production was used: billets were smelted from the ore, from which excess impurities in the form of slags were subsequently removed by forging. The quality of the produced iron depended only on the number of methods of reforging of blanks; In fact, no one has thought of any additions to the alloy. When we watch films about Ancient Rome or the early Middle Ages, we should not be fooled at the sight of the shiny iron swords waving the heroes. None of this was even close - the weapons of that period were very rough and often bent and broke when used. And due to the lack of a sufficient number of blacksmiths, soldiers with their own hands, often with the help of ordinary stones,straightened their swords, as Gaius Marius wrote about.
And nevertheless, the technologies of that period could present very interesting results that were capable of confusing not only contemporaries, but also us. One such enigmatic piece of industry from that period is the Delhi pillar made of iron. Its main feature is a truly fantastic resistance to corrosion, which allowed it to exist from the 5th century AD to the present day.
The column was erected in honor of the king of North India, Chandragupta II. He was a brilliant ruler, ideally combining all the necessary qualities of a monarch. For 20 years, he was able to conquer vast territories and create a prosperous state on them. The king in every possible way contributed to the development of the sciences and arts, equalized all religions in the state and monitored the observance of the laws introduced under him. His coronation day is still celebrated in parts of East Asia as New Year's Day. Such a ruler fully deserved a corresponding monument.
It will seem strange to the layman that ordinary iron was chosen as the material, however, at that time, the price of iron products was slightly different. And a very important role was played not only by its quality, but also by its quantity. The cost of iron products grew in power-law dependence on the mass. That is, several hundred swords made from a five-hundred-kilogram blank would cost much less than, for example, a gate of the same mass.
The fact that the column was built over 1500 years ago has never been disputed by scientists. There are several chronicles in which it is mentioned and radiochemical analysis carried out today confirms the timing of its manufacture.
For a long time it was believed that the column was made from a single piece of iron, however, detailed research showed that it was most likely made from about a hundred blanks weighing from 30 to 100 kg. This version is supported by the fact that individual parts of the column have a somewhat heterogeneous composition and a low sulfur content, which would have been impossible when it was made from a single billet - in this case, the sulfur would not have been completely removed from the billet. Also on the column there are numerous traces of forging and welding at the joints of the blanks. The column is practically free of nickel and manganese.
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There are different opinions about how the column has survived to this day in almost its original form. The main version assumes the appearance on the outer layer of the column of a thin protective film of iron oxide, which prevents its further oxidation. Also, due to the imperfection of metallurgical technologies of that time, a significant amount of phosphorus compounds is present in the column material. Only in the 30s of the XX century was the unique ability of phosphorus compounds to act as a catalyst in the appearance of that form of iron oxide that prevents its corrosion. Thus, on the example of a column from Delhi, one can observe a phenomenon unique in its kind, when the imperfection of technology contributes to better preservation of the material.
Also, some conditions inherent in the area where it is located played an important role in the safety of the column. For example, the air in Delhi, due to the large concentration of people and animals, contains a relatively large amount of ammonia. The combination of a warm climate and an increased concentration of ammonia made it possible to obtain a complex compound of iron oxide and iron nitride on the surface of the column, which provides additional protection.
Religious ceremonies held at the column sometimes included covering the column with aromatic oils (this use of oils and incense is generally characteristic of Hinduism), which contributed to additional protection of the column from moisture and air.
Well, one should not exclude such a version that the column could simply be a unique work of engineering art, during the creation of which ancient engineers quite accidentally were able to choose the composition and technology that could form a protective layer on the surface of the product. After all, there are several of these columns in India, slightly differing in composition. But this "little" was quite enough for only one of them to be preserved in almost its original form; the rest of the columns are currently in a much more deplorable state.