Damascus Sword - An Ancient Product of Nanotechnology
Emeritus Professor, Department of Materials Science, School of Chemistry,
Madurai Kamaraj University, Madurai – 625 021, India
Swords were used as weapons in battlefields till 19th century and the use of better version of daggers was invariably regarded as an advantage for a victory. In this respect Damascus swords were considered to be the best and the strongest swords ever made in history. These swords, which possess distinguishing water marking in the blade, probably involved complex metallurgy and sword smithing. It is believed that these swords are manufactured from wootz steel which confers the great hardness, incredible super elasticity (unequaled by any other steel) and the ability to cut other swords in half without dulling the blade.1 It is claimed that a Damascus steel blade could cut a piece of silk in half as it fell to the ground. Damascus swords became very valued possession because of their mechanical strength, flexibility and sharpness. Though initial development of these sword is accredited to India (about 300 AD), Syria took the lead and introduced to the Western world between 1000 AD and 1300 AD. During Crusade times, the Christian warriors of Europe encountered Muslim armies and the Europeans discovered that the Muslims had steel swords superior to their own steel swords. Historical accounts claim that the swords were encountered by Europeans in Damascus.
Several reasons are attributed for the name Damascus. It refers to swords forged in Damascus, Syria. Another reason is that the swordsmith, Damasqui made this type of blades. In Arabic damas refers to the surface pattern of moiré ripples, which resemble turbulent water and is also found in some Damascus swords (Figure 1). The beautiful Damascus sword has a wavy pattern on its surface and it looks like wood grain. Some of the old swords are kept in museums like Berne Historical Museum, Switzerland.2
In the production of steel, if iron is loaded up to 2 % carbon, hard and brittle steel will be produced while soft and malleable steel is obtained by the addition of about 0.5% carbon. The Damascus steel is both hard and malleable.1 These features are important – hard to hold an edge once sharpened, but malleable so that it would not break when hitting other metal in combat. This was not possible with normal processes. It is hard to believe that the blades of these swords can be bent to about 90o.
Figure 1. (a) Damascus sword and (b) the wavy pattern in the sword
It is learnt that the swords were prepared by forging small cakes of steel called wootz steel manufactured in southern India and exported to other countries.1 Wootz is the anglicized version of ukku in many south Indian languages of a term denoting steel. A systematic survey of literature indicates that the steel from the southern part of the Indian subcontinent was exported to Europe, China, the Arab world and the Middle East. It is astonishing to note numerous early literary references to steel produced in India are from Mediterranean sources including one from the time of Alexander (3rd c. BC) who was said to have been presented with Indian steel.3 There was a great reputation of Indian iron and steel in Greek and Rome in that period and perhaps that promoted the export of high quality iron and steel from ancient India.4 Archaeological evidence from the region of Tamil Nadu suggests that the Indian crucible steel process is likely to have started before the Christian era from that region.5,6 The manufacture of steel in south India by a crucible process at several locales including Mysore, Malabar and Golconda was observed by various European travellors.7,8 By the late 1600’s shipments running into tens of thousands of wootz ingots were traded from the Coromandel coast to Persia.
Wootz steel was one of the advanced materials of early period exhibiting properties such as superplasticity and high impact hardness. The recipe for the manufacture wootz steel was an enigma. In the Indian method of preparation of wootz steel cake, it is believed that some particular ingredients were essential like wood from Cassia auriculata and leaves of Calotropis gigantean and ores from particular mines. Wootz steel was produced as roughly 2.3 kg ingots, commonly referred to as cakes, which are solidified in a closed crucible. It was a relatively high-purity iron steel with 1.5% carbon. The cakes were shipped to Damascus. The smiths repeatedly heated and hammered the cake till it was stretched and flattened into a blade. During this process the wavy pattern was formed on the surface of the blade. Verhoeven found that the swords contained band of iron carbide particles, Fe3C, known as cementite.1 It is a mystery how the inherent brittleness of cementite was overcome by the Indians in their preparation of wootz steel. The production of this type of steel almost vanished possibly because of the depletion of the particular ores. Unfortunately, the technique of producing wootz Damascus steel blades is a lost art. The date of the last blades produced with the highest-quality damascene patterns is uncertain, but is probably around 1750. Debate has persisted in the metallurgy community over the past 200 years as to how these blades were made and why the surface pattern appeared. Success eluded the hands of European swordsmiths to produce steel similar to wootz. Recently, Vorhoeven, an Emeritus Distinguished Professor of Materials Science and Engineering at Iowa State University, USA, produced a steel which when forged into a blade had all the characteristics of Damascus blade.1,9 Their recipe includes iron, carbon and other elements in trace amounts such as vanadium and molybdenum (which are referred to as impurity elements) in addition to rare-earth elements.
It is strange that in spite of the presence of about 1.5% carbon in wootz steel, the blades produced from it is not only strong but also malleable. This appears to be a mystery. Does the carbon play a new role? To understand this let us examine the recent studies on various forms of carbon. Buckminsterfullerene (C60), the third form of carbon, was first reported from Rice University, Houston in 1985 by Smalley and co-workers.10 After the announcement of the large-scale preparation of C60 by electric arc discharge method,11 several amazing discoveries followed soon. It is not an exaggeration to state that the invention of fullerene is solely responsible for the discovery of carbon nanotubes (CNTs). A careful examination of the carbon cathode used in the arc discharge process for preparing small carbon clusters by Sumio Iijima12 in 1991 resulted in the historical discovery of CNTs, the name of ultra-thin carbon fibres with nanometre size diameter and micrometre size length. Iijima originally obtained only multiwalled carbon nanotubes (MWCNTs) and that is indeed a milestone in the study of different forms of carbon. Subsequently, Iijima and Ichihashi12 and Bethune et al.14 reported the production of single-walled carbon nanotubes (SWCNTs). CNTs have been recognized as the quintessential nanomaterials and have acquired the status of one of the most active fields of nanoscience and nanotechnology. The MWCNT is composed of 2–30 concentric graphitic layers, the diameters of which range from 10 to 50 nm and length more than 10 μm. On the other hand, SWCNT is much thinner, with diameter ranging from 1.0 to 1.4 nm (Figure 2). CNTs exhibit unique electronic, mechanical and thermal properties. CNTs are very strong and the Young’s modulus of them is almost 6 to 10 times that of steel. Tensile strength of CNTs is about 20 times higher than that of steel. Thus CNTs are strong, even though they are light weight. When CNTs are bent, they are very resilient They buckle like straws but do not break and can be straightened without any damage.
Figure 2.Structrues of (a) SWCNT and (b) MWCNT
The high mechanical properties and flexibility features of Damascus blades resemble those of CNTs and these characteristics probably motivated German scientists Reibold et al. to probe whether a genuine Damscus sabre contains CNTs by using high-resolution transmission electron microscopy (HRTEM).15 A specimen was taken from one of the swords kept in Berne Museum, Switzerland and dissolved in hydrochloric acid and the remnants examined by HRTEM revealed the presence of MWCNTs with the characteristic distance of 0.34 nm and also bent CNTs (Figure 3a & 3b). Figure 3c shows remnants of cementite nanowires encapsulated by CNTs which prevents the wires from dissolving in acid.
Scientists are not surprised to find the presence of CNTs in these swords as it is now well known that CNTs can be produced from carbon at high temperature – the laser ablation and arc-discharge methods involve high temperature. Probably the repeated heating and hammering (forging) results in band formation from segregation at a microscopic level of some impurity elements (metals). These elements may also be responsible for the growth of CNTs which in turn initiate formation of cementite nanowires and coarse cementite particles. A question to be answered is whether the high mechanical strength and flexibility of Damascus blade arise due to the presence of CNTs. It is needless to state that further detailed studies may provide answer to the question. However, we can be proud of the fact that even several centuries ago Indians are aware of the importance of wootz steel and Damascus sword, which are now proved to contain carbon nanostructures.
Figure 3. HRTEM images of remnants from the dissolution of a sample of genuine Damasus sabre in hydrochloric acid. a, b MWCNTs with the characteristic distance of d = 0.34 nm. In b, the tubes are bent like a rope. c. Remnants of cementite nanowires encapsulated by CNTs, which prevent wires from dissolving in acid. Scale bars: 5 nm (a) & (c) and (b) 10 nm reproduced from ref. 15 ( with permission from P. Paufler).
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