Whenever an unfamiliar blade is unsheathed, one of the first questions to ask is has it been retempered? This may be as easy as does it have a boshi, or a welded-on nakago; or as difficult. All these questions become easier with practice and repeated detection of the defects. It is estimated that about 10% of the blades in the United States today are retempered.
We read in Yamanaka's Newsletter (Vol.IV, 9, pg. 3) that Tokagawa leyasu assigned his favorite swordsmith Yasutsugu the job of retempering famous swords burned in the seige of Osaka Castle. Among them was a previously superb Awataguchi blade by Yoshimitsu. Even after being burned and retempered it is amazingly beautiful and is considered to this day to be a great blade. This blade is known as the Ichigo Hitofura.
Yamanaka gives the credit for this to the superb steel of the Awataguchi: not the skill of the smith who retempered it. We might not be expected to recognize the subtle things that make it possible to know that this great blade is a retemper. Because even a percentage of experts would miss the retempering on this blade. This is one extreme of how difficult detection can be. Much more common is the retemper of average or poor blades: made easier by the hasty retempering by average or poor swordsmith. The purpose of the retempering was usually not to preserve the artistic beauty or to deceive the potential buyer, but to return them to utilitarian use and it is for this very reason that they exist today. The quality is usually very poor. As the existing pool of available blades is culled over by knowledgable buyers, the concentration of a retempered blade will exceed 10%.
A serious collector of blades must recognize retempering. The following is the method: The shape is always altered from the original. The burning, reheating, then quenching warps, and the result is adjusted only a little by local reheating and hammering. The resulting shape is therefore with the smallest radius of curvature at the wider and thicker parts of the blade - that is - nearer the nakago. A strange Koshizori is the usual result and reheating the back in the lower fourth of the blade after quenching is the usual method of correction. There may be evidence of this in the final product by weakening of the hamon above the ha machi. There may be hammer marks on the mune or stains from reheat treating the mune with a hot copper block; all done to reduce the curvature and to return the mune to a straight line.
The nakago color and texture are always altered. The original rust will be burned or may be removed. There may be repatinization which results in a non-shiny "flat" black color. The usual surface of a retemper has the burned and loosened rust removed and a bluish, flaky, uneven surface remains. It also literally looks burned.
The hamon shape is usually a straight suguha about a quarter inch or so wide, wider perhaps than would be expected by the total width of the blade. The widths of these blades often have been reduced by repeated polishes. The new hamon is of average width for a new blade. The new nioi line does not correspond in position with the original; noticeable if there is visible multiple plate construction.
The hamon itself has certain characteristics. There are rarely nie. The blade would have to be reheated to a greater temperature to produce nie, and it is already more likely to crack upon quenching because of the wild curvature. Detail in the hamon appears obscured. The outline is never sharp and distinct but blurred and finely frayed. It is utilitarian instead of artistic. There may be odd things at the ha machi. There may be a strange yaki dashi. There may be no hamon for some distance above the ha machi; there may be a long suguha immediately above the ha machi. The hamon line may begin by passing through the ha machi in a shortened blade.
The ji lacks detail. It appears obscured and blurred. It may have a strange bluish or greenish hue. It may be uneven in color from reheating the mune area after quenching. There are usually few ji nie just as there are fewer nie in the hamon. The Ji appears flat and lifeless. It may appear very shiny. There is the famous 45 ° degree line; the mizukage; crossing the ji just above the ha machi which is present only when the nakago was being kept cooled by water, by wet paper, rags, or radish when the sword was clay coated and being heated prior to quenching. Usually the nakago was allowed to heat with the rest of the blade as in the original tempering. The mizukage is rare in the United States. Perhaps we see mizukage in the U.S. more often in blades that have not been retempered. Remember Andy Quirt's Nobukuni in the last issue of the newsletter? There are also mizukage to be found in Hizen and Horizawa school blades. Why this happens would make an excellent research.
The boshi of a retempered blade is likewise simple, rounded, set well back from the cutting edge and also blurry and indistinct.
Worth mentioning are the instances when a known smiths name is on the nakago and the style of the yakidashi, hamon and boshi do not fit his school's work. Fortunately, these alterations to a burned, temporarily useless sword were not done to confuse the collector. They were done to provide the owner with an acceptable fighting weapon. Therefore these changes are openly made and are usually easily detected. All or most of these changes are seen on the same blade. Recognition comes from simply thinking, "is this blade a retemper?", and then looking for the typical characteristics.