This time I want to sare some thoughts on utsuri. At the end of one of our last sword meetings we had a brief discussion on utsuri in the course of which the most common approaches in explaining this effect were touched. In this article, I want to forward these approaches and expand them by what I was able to find in other sources. To begin with, we all know that utsuri is a synonym for kotô Bizen swords. The common knowledge is also that utsuri „disappeared“ at the end of the kotô period and had to be „rediscovered“ by shintô, shinshintô and even by shinsakutô smiths. Well, to a certain extant, all this is kind of true. Also we know that utsuri is not a superficial thing as it remains even if a blade is polished multiple times. That means it must have a certain depth. Kapp says on p. 91 in his „The Craft of the Japanese Sword“ that „utsuri shows no evidence of containing any martensite“ and this line is what is mostly discussed and opposed when we are talking about utsuri at our sword meetings. Kapp mentions further: „The sides, where utsuri appears, are heated to 750-760°C – at this temperature, the steel is in a transitional phase from pearlite to austenite; it is presumed that utsuri is related to the complex ferrite and pearlite microstructure that obtains in this area, although close scientific analysis has yet to be done.“ Another important note in this section which must not be overlooked is: „Temperature produces other effects as well, depending on the structure of the steel. We have seen how the smith can combine blocks of varying carbon contents before finish forging. Higher carbon layers will produce more martensite steel upon quenching.“ That means not all hardened areas we see on the finished and polished blade are necessarily a result of just how the clay coat is applied. A steel with a carbon content of less than 0,5 % for example does not turn entirely into martensite when hardened. And has the steel a carbon content of less than 0,2 %, it can´t be hardened at all. The kawagane, i.e. the steel on which the utsuri appears, lies between these limitations. Apart from the carbon content, temperature is another factor which defines the resulting metallic structure after the hardening process. Is the temperature to which the blade is heated before quenching too high or too low, too much or not enough martensite is created respectively (depending as mentioned also on the carbon content of the steel). This also explains nie-utsuri, an utsuri consisting of nie particles. So for creating nie-utsuri, the kawagane has to have a high enough carbon content to produce enough martensite, i.e. nie clusters in the ji.
The appearance and lack of nie-utsuri matches also with Bizen-utsuri. That means except some early Ko-Bizen works, Bizen blades usually do not feature nie-utsuri as they are hardened in nioi-deki and feature a kawagane with a lower carbon content. This lower carbon content is usually reflected in desctiptions of polishers that the Bizen steel is „relative soft“. And with this softness, we arrive at the purpose of applying utsuri. Many experts and swordsmiths assume that it was exactly this „softness“ of the Bizen steel that required an additional treatment. Everone knows that if the steel of a blade is too hard, the sword becomes brittle and if the steel it is too soft, the blade is prone to bend. That means adding utsuri, i.e. hard but not too hard areas of steel along the ji, provides a certain torsional rigidity. But it also has to be mentioned that polishers report of Bizen blades with utsuri which were once bent but straightened. So utsuri does not reduce the risk of bending to zero. Nagayama explains utsuri in his „The Connoisseur´s Book of Japanese Swords“ on page 86 as: „The purpose seems to have been to improve the sword´s flexibility to prevent breakage during use. Usually made of steel that is softer than the rest of the blade´s surface.“ So he addresses the same challenge from the other side, i.e. not making a soft steel more rigid but a brittle blade more flexible.
The sword polisher Kurashima Hitoshi (倉島一) forwarded his thoughts on utsuri in the Tôken-Bijutsu No 470 (March 1996). He also brings it in line with steel with lower carbon content whereat he introduces the generic term „namagane“, a not so refined steel with lower carbon content. I.e. he uses namagane as „opposite“ of hagane by not making a distinction between shingane and kawagane. In short, namagane is exactly the aforementioned steel with a carbon content between 0,2 and 0,5%. Well, Kurashima actually does differentiate his namagane, namely by „type A namagane“ and „type B namagane“, terms which correspond virtually to shingane and kawagane respectively. But he goes as far as to say that namagane IS utsuri. That means he assumes that the utsuri we see is the border of the namagane meeting the hagane. In other words, the visible border of the utsuri is created by structures with more martensite than others. According to this approach, a ji with utsuri has basically three degrees of hardness: The „super hard“ ha, the hard antai (暗帯, the dark areas between nioiguchi and border of the utsuri) and the softer rest of the ji which corresponds to the utsuri. So utsuri would be the visible brighter but softer area above the hamon whereas the harder part which adds more torsional rigidity is actually the darker antai directly above the hamon. This again would correspond to Nagayama´s comment „made of steel that is softer than the rest of the blade´s surface.“
Picture 1: The visible effects and steels with different carbon content, according to Kurashima.
Another interesting approach to explain utsuri can be found in the „Kentô-kikigaki“ (見刀聞書) from Tenpô 14 (天保, 1843). Therein we read: „By plunging the heated blade into water, the clay coat contracts. When the blade is now plunged into the water a second time, the now exposed areas cool down faster than the upper areas which are still covered with clay. This produces utsuri.“ I made a sketch (picture 2) to illustrate what the „Kentô-hikigaki“ probably means. According to this approach, also the temperature difference between heated blade and water would not be that high as at the first plunging. Thus not so much martensite is produced and the utsuri area above the hamon would be a „second“, „softer“ hamon. This in turn would match with Edp-period sword publications in which utsuri is circumscribed as „kage-hamon“ (景刃文, lit. „shadow hamon“ or also „second hamon). But on the other hand, this would mean that the dark antai area above the hamon is actually harder than the whitish utsuri area. Apart from that I think that it would be very difficult to produce utsuri that way. That means to coordinate all the factors like the carbon content and desired distribution in the steel and the exact temperature is already difficult enough, i.e. adding also the factor „total control of the clay coat by plunging a heated blade two times into a trough of water“ would make utsuri in my humble opinion merely a product of chance.
Picture 2: Achieving utsuri, according to the „Kentô-kikigaki“
So the most common approach to explain utsuri would be by a case hardening or surface hardening. Here, just the surface is hardened but requires infusing additional carbon into the top layer of the to be hardened steel with a lower carbon content. Thus the deeper steel layers remain soft as it is desired with the kawagane and shingane. For adding the carbon just to the steel surface, traditionally a mixture of ground bone and charcoal or a combination of leather, hooves, salt and urine was used. But the temperature must not be that high and stay under the melting point of the iron, and left at that temperature for a length of time. The longer the temperature is held, the deeper the carbon will diffuse into the surface, whereat a typical depth of case hardening with this method is up to 1.5 mm. That means the adding of such supplements under the clay coat still requires an exact heat treatment. Thus explaining utsuri by case hardening also „solves“ the question why the effect remains to be seen even after repeated polishing. Well, at a considerable loss of material, the utsuri gets of course weak and disappears eventually.
So far so much to say on the subject utsuri from my side and I hope I provided some food for thoughts.