Chromium steel - like the tool steel know today - was made in Persia a millennium ago, says S.Ananthanarayanan.

Just as the Bronze Age and the Iron Age marked stages of development since ancient times, widespread use of steel is the mark of the modern age. And, yet, the material has been known for a long time

What has been known for long, however, is iron in a crude form. Its refinement into steel, with the addition of different substances to improve its properties, is a modern development. Yet, Rahil Alipour, Thilo Rehren and Marcos Martinon-Torres, from University College, London, The Cyprus Institute, Nicosia and the University of Cambridge, UK, write in the Journal of Archeological Science, that over ten centuries ago, the Persians could make chromium steel, a sophisticated material we have believed to be of recent origin.

In the ancient world, the discovery of fire led to baking pottery and extracting metal from ores. As the fires were wood fires, only metals that have low melting points could be extracted. Typically, these are copper and tin, and the alloy, which is bronze. There was also silver and gold, but these were rare and not sturdy or useful. It was only after the discovery of coal, and higher temperature became possible, that iron could be extracted. The discovery of iron expanded the scope of agriculture, hunting, attack and defense, and the Iron Age (1200 to 600 BC) is another phase of civilization.

Iron occurs in the ore in combination with oxygen, in the form of its oxides. The process of extracting the metal is to heat the ore with carbon, so that the oxygen atoms change partners and become carbon dioxide, leaving behind the metallic iron. As we can expect, there is much carbon left in the material created - in fact, there is 3.8% - 4.7% of carbon, and the carbon is dissolved in the iron. This crude form of iron, just extracted from the ore, is called pig iron and is the starting point for a purer form of iron, which is called wrought iron.

Thanks to the high carbon content, Pig iron is brittle and of limited utility. It is purified by melting and exposure to a strong current of air, while it is stirred and agitated, when impurities are oxidized and can be removed. Ironically, pure iron is ductile, or easily bends and changes shape. The solution is to add carbon, but less than what there is pig iron, to give it strength.

The crystalline structure of iron is ‘cubic’, either with atoms at the corners of cubes or in the middle of the faces of the cubes. The result is that the cubes slide over one another. Adding other substances, mainly carbon, helps hold the cubes together and improves the strength of iron. Iron with up to 2.4% of carbon is steel, the wonder material of the modern world. Not that steel was not made earlier, but working with iron, in coal furnaces, was tiresome, and steel was made only for special purposes, like swords or suits of armour. This changed with coking coal and plentiful coal, when mining methods improved. Better grades of furnaces led to more production of steel, and there was expansion in many fields, of construction and manufacture, when steel was freely available.

There was, nevertheless, the industry of steel making in the ancient world. The method used was to mix pig iron and wrought iron in the melt, to reduce the carbon content by dilution. As the process was in a crucible, the product is called crucible steel. The paper in the Journal of Archeological Science says there is evidence for two crucible steel making traditions - the wootz steel made in south India and Sri Lanka, since the third century BC, and in later centuries, in Central Asia (the Persian pulad). Wootz steel from India became famous for its strength and hardness and was in demand in the west for the manufacture of Damascus steel and swords for the Crusades. The Indian method used ores from particular sources, with a number of ingredients with the ore during smelting, followed by reducing the carbon content by dilution with purer material. The reason for the strength, and very high corrosion resistance (the Iron Pillar near New Delhi, of the 4th and 5th century, AD, for example) is understood as partly the organic materials added during smelting, but largely the trace impurities present in skeins of ore that were used. When these sources depleted, the supply of this steel also came to a stop.

As for the industry in Central Asia, the paper discusses the archeological finds in a site, Chahak, in southern Iran, which is described in historical reports as a centre of steel production. A specific report is the work, al-Jamahir fi Marifah al-Jawahir (A Compendium to Know the Gems), by Abu Rayhan al-Biruni, the famous scientist, scholar, historian, theologian, traveler and linguist, around the turn of the first millennium. While other reports speak of steel being made in Chahak, Al Biruni describes the process used and the materials, some of which are not identified, as additives.

The paper says that most crucible steels found at historical sites have greater carbon content than ideal, some phosphorus, which adds some properties, but scant traces of other metals, like manganese, vanadium or chromium, which we find in modern steels. In the samples of steel, as in suits of armour, swords, blades and daggers, where we do find trace of such materials, we cannot be sure of their place or time of origin, the paper says. And the sole plausible instance of an object with chromium content greater than 1% has not been matched with evidence that the chromium was added during steel production.

It is in this context that the authors of the paper report physical evidence of production of low chromium crucible steel, with conscious addition of a chromium mineral, in Chahak, the only known archeological site in present day Iran. While the site has been mentioned in manuscripts, no excavation was conducted until the present work. The current study has discovered and analysed crucible steel making waste, rubble of broken crucibles, with the slag that is left after smelting, and evidence of smithing workshop and furnaces, the paper says.

Analysis of bits of steel that were found indicated positive chromium content at levels that affect the performance of steel. Again, there was high content of chromium oxide in the slag, which separates from the metal during smelting. However, there was no chromite in the material of the crucibles or in the smithing workshop. The presence of chromium in the products must hence arise from deliberate addition, the paper says.

Modern stainless steel is an alloy of steel with chromium as an important component. The chromium adds to strength, raises heat resistance and creates a surface film that prevents the formation of iron oxide, and hence corrosion. Modern steel is the result of metallurgy and scientific advances of recent times. It seems some method of product development in Persia had reached the same place a thousand years ago.

Pig iron
The first, crude melt, when iron is made, used to be poured into sand moulds that were shaped like a stem with branches – it looked like a sow suckling piglets. The branches, which were broken off for purification or working on, were called ‘pigs’.

How did it disappear?
We can understand that the quality of wootz steel was not the result of process design and hence died out. But this is not true of chromium steel of ancient Persia, where there was deliberate addition of a chromium mineral. A reason its use declined could be the dark years and breaking of communications that followed the sacking of Constantinople, the rise of the Ottomans and the Black Death, in the 12th and 13th centuries. .

------------------------------------------------------------------------------------------ Do respond to : response@simplescience.in -------------------------------------------