Ancient swordsmiths used nanoparticles to strengthen steel. Ancient chemists used them to dye hair and wool. How did these skilled yet unscientific craftsmen unwittingly learn to use what has become one of the 21st century's most scientifically sophisticated technologies? The answer is being sought by nanotechnology scientists. And along the way, they are gaining new insight into how particles measuring less than a millionth of a meter interact with other constituents to give composite materials useful new properties.
Take the classical Damascus sword, for example. It is famous for its strength, flexibility, and ability to keep a keen edge. Specimen swords have been studied a long time without uncovering the swordsmiths' secret. When Peter Paufler and colleagues at Germany's Technical University Dresden took a look at a sample of Damascus sword steel with an electron microscope, they found evidence of carbon nanotubes and nanowires of the brittle iron-carbon mineral cementite. Reporting this three weeks ago in Nature, they interpreted their findings as "indicating that these wires could have become encapsulated and protected by the carbon nanotubes." Carbon microstructures strengthen some modern composite materials. They may have been key to the Damascus swords' unique properties.
The blades were forged from steel cakes, called wootz, made in India. Repeated annealing with high heat and forging produced a blade in which many foldovers of the steel made a distinctive banded pattern. Impurities in the wootz may also have been important. The research team notes that the secret of the swordmaking appears to have been lost about 300 years ago when some of the traditional materials used to make wootz became scarce. The impurities may have helped the nanostructures to form. Whatever happened, the team concludes that "by empirically optimizing their blade-treatment procedure, craftsmen ended up making nanotubes more than 400 years ago."
Mediterranean dye chemists learned to use nanoparticles 2,000 years ago. Philippe Walter at the Center for Research and Restoration of Museums in Paris led a French-US team that has studied the chemistry involved. The dye was used to turn wool and human hair black. The team found that the key to the dye's effect is the formation of lead sulphide particles only five billionths of a meter in diameter. The team explained last October in Nano Letters how lead atoms penetrate hair and bond with sulfur atoms from proteins. The resulting particles form crystals that absorb light, thus turning the hair black.
The online nanotechnology information portal Nanowerk calls this "a remarkable illustration of synthetic nanoscale biomineralization." Dr. Walter expressed admiration for ancient craftsmen who used simple chemistry and low-cost materials to practice a form of nanotechnology millenniums ago. Nanowerk says that today, its database contains more than 1,300 nanotech products from 90 suppliers. Your sunblock or toothpaste may be examples of products that use nanotech ingredients.
Ancient craftsmen profited from stores of trial-and-error knowledge. They were not practicing materials science as we know it. Yet by looking at their work through modern scientific eyes, today's materials scientists can profit from their example.