By Allison Kubo Hutchison
|Reproduced Wootz Damascus blade with ladder and rose pattern by Alfred Pendray. Photo by JD Verhoeven, AH Pendray and WE Dauksh.|
Materials science and nanoengineering are emerging areas that promise a revolution in industry, medicine and energy technologies. But our understanding of both is rooted in ancient knowledge. Would you be surprised to know that we knew and used nanoparticles as early as 400 CE? Roman technologists used gold-silver nanoparticles to make colored glass, and the Mayans used indigo dyes stored in clay nanopores to make brilliant, long-lasting pigments. One of the most powerful of these technologies was damascus steel, which was developed in AD 300 in the Middle East.
Carbon nanotubes are the source of these prized properties. The carbon nanotubes, which now cost $ 1,500 per gram and are the subject of intense research in nanotechnology, enable incredible tensile strength with the same edge. A high carbon content in the sword can be traced back to the ingots from which the Wootz swords were made. Wootz was imported from southern India (present-day Sri Lanka and Tamil Nadu). Wootz was made by heating ore in a sealed clay crucible with vegetable fiber as a carbon source. Adding fallen leaves to iron is not a recipe for good quality steel. It has to be in the right portions and also forged at the right temperatures to get the best results. Iron and carbon form a solid solution when mixed: mixing at the atomic level to form complex crystal structures. The temperature and speed influence the crystal structure and the structure controls the tensile properties of the steel.
|Steel under the reflection light microscope with cementite (dark), which is embedded parallel in ferrite crystals (white). Photo by Professor TW Clyne, Cambridge University.|
It is desirable to have several types of steel structures such as austenite and ferrite in one blade. Different crystal structures, which are stable at different temperatures and compositions, can be achieved through heat cycles and several heating and cooling cycles. This process is difficult to achieve because it requires precise temperature control. Wootz steel was also found to have trace impurities (only 40 parts by weight (ppmw)) of rare earth elements such as vanadium and molybdenum. The impurities cause certain crystal structures to form preferentially in the ribbon and not randomly. These impurities and the presence of carbon nanotubes facilitated the formation of streaks on various steel crystals, especially cementite, which is very hard but brittle. In the Damascus shovels, however, the cementite formed nanowires less than a nanometer thin, which span the shovel and increase its hardness, but allow flexibility. The microstructure woven into the blade-like cement reinforcement reinforced the steel. This was also the source of the fascinating pattern made by the most skilled blade smiths like Assad Ullah, although other blacksmiths were able to achieve a high metallurgical quality without achieving the patterns they were looking for. The patterns have been highlighted and acid etched to make the cementite appear black on the white steel matrix.
While modern Damascus is made by welding metals of different colors together, ancient Damascus’ wave patterns were formed by crystalline structures, although both are beautiful. The exact manufacture of Damascus blades was lost around the middle of the 18th century. It could be that they lost access to the special ores that contained the trace impurities necessary to form the nanowires, or that the difficult and specific heat cycling process was not passed on. Attempting to recreate the damascus steel produced throughout India and the Middle East was important in advancing the development of metallurgy as a science and continues to provide insights to this day.