The mystery of a legendary type of sword is gradually being unlocked by scientists and smiths. A sword of Damascus steel was said to be so sharp that it could cut through a gauze kerchief – or a steel helmet – and so flexible that it could bend through 90 degrees without breaking.
But the last of them was made around two centuries ago, and no one could work out the secret of their manufacture – even with samples available to study.
Now, however, clues have been emerging, and one of the keys is the source of the steel – which came from India.
[title size=”2″]The source of the steel[/title]
It’s called wootz steel, from a word for steel in South India – ekku. India has had a reputation for its iron and steel for a long time – since around 300 BC, and some Indian steel was said to have been presented to Alexander. The ancient Greeks and Romans sourced steel from India, and the Arabs took it to Damascus, where it was used for many centuries. ‘It is impossible to find anything to surpass the edge from Indian steel,’ observed the 12th-century Moroccan geographer al-Idrisi.
The story of Indian steel can be found in a fascinating book available online, written by two leading researchers in the Indian Institute of Science at Bangalore, S. Srivavasan and S. Ranganathan.
Many researchers tried to find out what was the key to the qualities of wootz steel. Michael Faraday – the son of a blacksmith – brought in a master cutler and surgical instrument-maker, James Stodart to help him, and they tried adding various metals to iron. They found that noble metals such as platinum and silver did indeed produce a harder steel and they thereby laid the foundations of the technology of alloy steel making – but they could not reproduce the wootz qualities.
But a clue to the nature of steel had already been found by the Swedish chemist Tobern Bergmann – it’s the carbon content that gives it the familiar qualities of strength and flexibility. Today the carbon content of steel rises from around 0.1% (low-carbon steel) up to 1-2% (ultra-high carbon steel) – and an analysis of Damascus steel shows that its carbon content was up at around 1.5% – in other words, right up with the finest steels that we can make today.
The Indian steelmakers introduced the carbon from carefully-selected plant material, such as the bark of an evergreen shrub called Cassia auriculata (used in tanning), and the leaves of a species of milkweed. The reason why carbon from plant leaves is particularly good is that when green leaves are burned they also generate hydrogen, which helps the take-up of the carbon into the iron.
[title size=”2″]Reinforcing fibres[/title]
One of the ways in which carbon produces the qualities of steel is through the bonding or iron and carbon atoms to form a very hard substance called cementite (iron carbide). Very fine strings of cementite particles act like reinforcing fibres running through the body of iron. With the right mix of these strings, an ultra-high carbon steel can show superplastic properties – can be extremely bendable.
And in Damascus swords, the strings of cementite particles form up into bands that you see as the typical wavy pattern on the blade.
But to get the right amount of these fine cementite strings is very difficult. Too high a temperature will prevent their formation. One reason why Europeans failed to reproduce wootz steel was that they sought very high temperatures, up to white heat, in addition to their failure to get a high enough carbon content.
And the wavy pattern only develops during the process of forging – when the steel is hammered into a blade. Nobody was able to work out how the pattern formed. There was speculation that it could be caused by some kind of impurity, and substances like silicon, sulphur and phosphorus, all known to be present in ancient wootz steels, were tried – without success.
[title size=”2″]How the fibres form[/title]
Then the American materials scientist John D. Verhoeven had a breakthrough. He happened to take for his source of iron a material called Sorel metal. This is a type of iron with a higher carbon content than steel (around 4 to 4.5%), referred to as ductile iron. Sorel metal is produced from a large deposit of the ore ilmenite on the St Lawrence River in Canada, ilmenite being an oxide of iron and titanium. The titanium is taken out for practical uses, and the iron left behind is very pure.
But remarkably, one impurity in that source of iron turns out to be the secret of wootz steel. This is vanadium. In Sorel metal, its concentration can be as little as 0.003%. But that amount, when added to high-purity iron-carbon allows, can produce good banding. Titanium also contributes, but the big influence comes from vanadium.
It seems that the very small traces of vanadium somehow segregate out of the iron when it cools, and that these traces form very thin lines along which the cementite fibres can form. Iron in liquid form can hold more impurities than can iron when it’s solid. And so every time that molten iron cools, a very little of the impurities will be forced out of it, and the impurities build up like a string of pearls. Forging and hammering seems to concentrate the bands further, with Verhoeven working over many years with a blacksmith, Alfred H. Pendray. You can see photos of Pendray making wootz steel in the story that Verhoeven wrote for Scientific American.
So the vanadium helps the strengthening fibres of cementite to form, in just the right type of band structure, and careful forging and hammering builds this up into the well-known patterns of Damascus steel.
[title size=”2″]Going to an even finer level [/title]
But a further suggestion for the cause of the cementite fibres has come from a German team led by Marianne Reibold of the University of Dresden. They have examined a 17th-century Damascus sabre and found – cementite nanowires and carbon nanotubes. A carbon nanotube is an incredibly thin tube, so thin that its diameter is only one-millionth of a millimetre; its walls are formed of one-atom-thick carbon. This is the first time that carbon nanotubes have ever been found in steel. The image is beautiful.
The Dresden University team found that some of the nanowires of cementite were encased in carbon nanotubes. So, they suggest, the process of forming the Damascus steel may start with the impurities, which help the carbon nanotubes to form. The nanotubes act as a kind of matrix within which the cementite nanowires can form, and the nanowires develop into larger cementite particles.
Further, they say, it was known that the swordsmiths of old used to etch their blades with acid. This brings out the characteristic wavy light and dark lines. And in addition, they add, the acid would dissolve a very little of the surface, exposing the acid-resistant carbon nanotubes which would protect their cementite cores, and creating an effect of a myriad of tiny saw-like teeth.
And so a whole new field of investigation is opening up, bringing out subtler and subtler properties of iron and impurities at a molecular and atomic level – to understand the skills of the ancient Indian steelmakers of more than two thousand years ago and the smiths of Damascus who turned that steel into the swords of legend.