How to tell the Kizu flaws of a blade.
Kizu, or flaws, can occur during the forging and tempering process of a sword. These can be caused by impurities that were not completely removed or small mistakes made during the repeated forging process. While kizu can be a sign of the swords repeated forging, it may cause problems during actual use and for authentication. Its important to examine kizu objectively and rationally.
One type of kizu is hagire, which is a crack that appears in the tempered edge and extends to the blade. These are considered fatal flaws and are unacceptable in a sword. If hagire occurs, the blade must be scrapped as it is very dangerous and can break during practical use. Hagire is caused by hardening failure and can appear not only on the cutting edge but also on the shinogi or mune. If hagire occurs, the blade should never be sold, even at a discounted price.
Kitae-ware, or horizontal "lines" (splits) on the blade, is a common imperfection that can occur during the forging process due to a poor weld between layers of steel. These splits may appear anywhere on the blade, such as on the mune (back), tate (vertical), or shinogi (ridge) areas. Although this kind of flaw is visible and can affect the appearance of the blade, it usually does not significantly affect its performance or safety.
In our experience, small kitae-ware flaws resulting from fold welding work do not necessarily compromise the quality of the steel. Instead, we assess the quality of the steel based on the color and brightness of its particles, which indicate its hardness and durability. Therefore, we consider kitae-ware as an aesthetic imperfection rather than a critical flaw in most cases.
To ensure the quality of our blades, we have a rigorous quality control (QC) process that determines whether a blade is acceptable or not based on the size of its kitae-ware. Blades with kitae-ware over 10mm are usually scrapped, as they may affect the integrity of the blade. Blades with kitae-ware less than 5mm are considered acceptable, while blades with kitae-ware between 5-10mm may be sold at a discount, depending on their overall quality.
In summary, while kitae-ware is a common imperfection in forged blades, it usually does not affect their performance or safety, and its severity can be assessed based on its size and location.
During the tempering process, air or carbon pockets may appear in the steel due to a bad weld between layers when the blade was forged. This is known as fukure in Japanese and it is more common in tamahagane steel. Although these pockets will not likely affect the structural integrity of the sword, the blade will still be scrapped if the hole diameter is over 1mm. It is considered acceptable if the hole diameter is less than 0.5mm.
Nioi-gire is a flaw that can occur in blades that undergo a clay tempering and differential hardening process. When the hamon (temper line) is interrupted or runs off the blade at any point, including the boshi, it is considered a major flaw that renders the blade unsuitable for practical cutting practice. Blades with this flaw are typically only suitable for display purposes or iaido practice.
Its important to note that nioi-gire can be difficult to detect and may not be immediately apparent upon inspection. Therefore, its essential to have a skilled and experienced evaluator inspect the blade to ensure its quality and authenticity. If the flaw is significant, heavy discounts may be offered for the blades sale as a decorative piece or for iaido practice.