Flaws in Japanese Swords
Types of Flaws
Flaws are called kizu in Japanese. There are two types of flaws: those introduced during the original making of a blade and those which came about afterwards.
Manufacturing flaws consist of forging flaws and heat treatment flaws.
The forging and repetitive folding of the steel removes impurities and evenly distributes the carbon through the steel. Incomplete or weak welds, carbon pockets, or impurities can all create flaws in the skin of the blade. Examples are ware, or longitudinal delaminations between two welds; fukure, which are bubbles or blister-like flaws in the skin; and black pits where impurities or carbon deposits formed. Many of these flaws are cosmetic; they decrease the monetary and aesthetic value of a blade, but arenÔ‡¼t critical from a practical standpoint.
Particularly difficult is the heat treatment, or yaki-ire, as it is called in Japanese. The yaki-ire process involves coating the blade with clay, heating it to the desired temperature, and quenching it in water. The timing and temperature are critical; all decisions are made through experience without instrumentation of any type. A few seconds too long in the fire may make a blade too hot and it may crack (hagire) upon quenching or have an edge that is too hard and brittle. Not enough time in the fire may create a blade with a soft, dull hardened edge. Non-uniform heating may create a blade with a hardened edge which varies in luster and hardness along its length. Traditionally, these types of flaws are more serious as they all affect the practical value of the sword as a weapon.
Swords may exhibit many different types of flaws which resulted from their use or abuse. Most commonly seen problems in the West are bends, chips ("hakobore" in Japanese), and sabi, or rust. Bends frequently induce wrinkles in the skin of the blade called shinae¬Ưand can also be a tip off to a blade with a very serious flaw called a hagire, which is a crack perpendicular to the cutting edge. Chips come in different sizes and shapes. Rust can be deep or shallow. Another type of flaw is the saiha, or rehardened blade. Sometimes blades lost their hardened edge in a fire and were re-hardened later. This is a serious flaw as the blade is no longer an original work. Unfortunately, unlike a chip or bend, this flaw is often times very difficult to detect.
While not technically considered flaws, other undesirable things one sees are altered nakago, or tangs, blades that have been reshaped, and later blades which have been shortened. Removing rust from a nakago, shortening the nakago, reshaping a nakago are all to be avoided. All of these acts drastically reduce the value of a sword. Deep rust pitting also deeply decreases the value of a blade, as does over-polishing or, as often seen in the US, poor amatuer polishing. One frequently sees older blades which have been polished many times showing core steel through the skin of the blade. This is considered a flaw as well, and the blade is said to be "tired".