The beauty of a katana is the result of four qualities; its shape, the lie of the grain on the blade, the undulating pattern on the cutting edge and the minute dots which make up the design along the edge.
The oldest swords are also the most in demand. Until the 16th century steel was made from iron melted down with charcoal and worked into cavities made on the side of the blade exposed to the beating. The steel with a low carbon content stayed exceptionally supple. After the 16th century the use of bellows which raised the temperature of the ovens produced an alloy with a higher carbon content.
The metal obtained is then hammered until a thin sheet of steel is obtained. After being hammered and worked day after day, this sheet of steel is used to cover the body of the sword made of a softer metal. The combination of the hard and soft metals gives the blade its flexibility and its strength.
Once the sword has taken on its shape the process of re tempering begins. A clay-based compound is applied thinly to the cutting edge and generously to the rest of the sword. The undulating pattern on the cutting edge depends on the way in which this mixture is applied. For the most part this process is a professional secret and often enables one to recognise the craftsmans skill. The difference in thickness of the clay between the cutting edge and the rib gives a blade with varying grain the more tempered part being called the nie and the rib nioi. Nie suggests spirit, a virile nature, nioi signifies dignity and nobility. The synthesis between the nie and the nioi is an important criterion in evaluating a katana.