Clay-tempered blades

Discussion in 'Japanese Swords and Sword Arts' started by shesulsa, Jan 12, 2007.

  1. shesulsa

    shesulsa Columbia Martial Arts Academy

    • MartialTalk Mentor
    • LifeTime Supporting Member
    • Martial Talk Alumni
    Joined:
    May 27, 2004
    Messages:
    27,172
    Likes Received:
    460
    Trophy Points:
    193
    Location:
    Not BC, Not DC
    Can someone please enlighten me to this process?
     
  2. Flying Crane

    Flying Crane Sr. Grandmaster

    • Supporting Member
    Joined:
    Sep 21, 2005
    Messages:
    11,816
    Likes Received:
    2,030
    Trophy Points:
    263
    Location:
    San Francisco
    This is a traditional technique in Japanese sword making, and has perhaps permeated into other sword making, but would only be done among high-end smiths. Certainly not something mass-produced.

    When the blade is ready to be tempered, the smith coats it with a layer of special clay. He then goes back along the edge of the blade, and scrapes some of it away, so the edge has a thinner coat than the rest of the blade.

    The blade is then heated to proper temperature, and quenched.

    The clay affects the heating and cooling of the blade. The thicker clay on the back of the blade causes it to cool somewhat more slowly when quenched, and that portion of the blade retains a bit of springingness, which adds to the toughness. If the blade is too hard, it becomes brittle, and this prevents that from happening.

    The thinner coating on the edge causes that portion of the blade to cool more quickly when quenched. This causes the edge to be harder, allowing for a much sharper edge.

    This method combines extreme hardness enabling a very sharp edge, with a softer springiness, which prevents brittleness, increases overall toughness, and prevents the blade from breaking.

    The technique also causes a pattern to appear on the blade. You can actually see the line where the clay was thicker vs. thinner. This is known as the "hamon", or temper line. A skilled smith can create a specific pattern, adding to the beauty of the piece, and even use it as a sort of "signature" of his work.

    many of the cheap, junky Japanese swords that you might find in places like San Francisco's Chinatown, meant to be sold to the turists, have a fake hamon. they just grind a pattern on the edge of the blade, and call it a hamon. If you have ever seen a real hamon, you will never be fooled by the fake ones. You can often see a fake one from across the room, and they tend to be very regular and even in their appearance. A real one is more subtle, you need to look more closely to see it, and it doesn't look at all "manufactured", or ground into the blade.

    I have a book that outlines the steps in manufacturing a Japanese sword. I don't remember the name, but I'll post it later when I check it.
     
  3. howard

    howard Brown Belt

    Joined:
    May 12, 2004
    Messages:
    469
    Likes Received:
    16
    Trophy Points:
    18
    Sure... as long as you'll settle for a layman's version. ;~)

    This is how the edge of a traditionally forged sword is hardened, while leaving the upper part of the blade softer and more pliable.

    The smith coats the blade with a clay slurry. He makes it much thinner along the cutting edge than along the upper part of the blade. He then heats the blade to a specific temperature range (usually somewhere between 700 and 800 degrees C, depending on the part of the blade we're talking about). Due to the differing thickness of the clay coating on the upper part of the blade and the edge, the edge will get hotter during this process than the upper part.

    When the smith feels that the blade is at the right temperature (which the traditonal smiths judge by the color of the steel), he plunges the blade into a trough of water.

    Because the clay is thicker along the upper part of the blade, this area will cool more slowly than the cutting edge. The edge will cool quickly enough that its steel will assume a molecular structure called austensite, which is harder than the upper part of the blade. The cutting part of the blade will therefore hold an edge better than the upper part would. Also, the upper part will be more yielding to impact than the edge, which will reduce the probability of shattering the blade under hard impact. The smith can also make narrow lines of thicker clay that descend vertically to the edge to create points of softer steel in the edge. This is done in order to try to localize any area of damage that results from too much impact, torsion, etc. on the blade.

    The line along which the smith changes the thickness of the clay coating appears on the finished blade as the hamon. The hamon is the visible manifestation of a functional feature of the blade, but it is also a very important aesthetic feature to serious collectors.

    Hope this helps... I'm sure there are plenty of members who know much mroe about this than I, and can elaborate on / correct this explanation.
     
  4. Flying Crane

    Flying Crane Sr. Grandmaster

    • Supporting Member
    Joined:
    Sep 21, 2005
    Messages:
    11,816
    Likes Received:
    2,030
    Trophy Points:
    263
    Location:
    San Francisco
  5. howard

    howard Brown Belt

    Joined:
    May 12, 2004
    Messages:
    469
    Likes Received:
    16
    Trophy Points:
    18
    :wink:

    It sure is... in fact, it's the basis of what I posted above.
     
  6. shesulsa

    shesulsa Columbia Martial Arts Academy

    • MartialTalk Mentor
    • LifeTime Supporting Member
    • Martial Talk Alumni
    Joined:
    May 27, 2004
    Messages:
    27,172
    Likes Received:
    460
    Trophy Points:
    193
    Location:
    Not BC, Not DC
    Thanks, y'all! Very informative!
     
  7. pgsmith

    pgsmith Master of Arts

    Joined:
    Jun 1, 2005
    Messages:
    1,587
    Likes Received:
    478
    Trophy Points:
    123
    Location:
    Texas
    A couple of things just because I'm a geek. :)

    The proper term is differential hardening. Tempering is what a smith does by reheating the steel to a lower than critical temperature in order to relieve the stresses created in hardening. It's the same way many people will call the hamon a temper line, which is actually quite wrong.

    How a sword is hardened (quick and dirty version) ... the steel (alloy of iron and carbon, with sometimes other elements thrown in to slightly change its properties) is heated above critical temperature. This is the temperature at which the carbon atoms in the steel structure basically become free flowing so they align with the internal crystal lattice. If the steel temperature is lowered quickly, called quenching, then the carbon is trapped where it is, resulting in a very tight crystal lattice, or much harder steel. The slower the cooling, the more opportunity for the carbon atoms to migrate back out of the lattice resulting in progressively softer steel. As the gentlemen have pointed out previously, the clay is there to allow the back of the sword to cool down marginally slower in the quench which results in softer, more ductile steel. The harder steel is, the better it will hold a sharp edge, but the more brittle it is also. This is why stainless steel makes for lousy sword blades. Stainless is very easy to harden, but it gets very hard. It is extremely difficult, and requires quite a bit of special equipment, to make stainless steel harden enough to hold an edge, but not so hard that it's brittle. I have heard of a couple of American sword makers that have done this in the past, but it's quite expensive and time consuming. All of the inexpensive swords on the market that are made of stainless are either too hard and will shatter (pieces of sharp steel whizzing through the air!) or they are unhardened and will dent upon contact with anything. Not to mention that they are generally poorly built and likely to fall apart. :)

    People often talk about "spring steel" as if it is something different. This is actually just steel that is hardened to the middle range. Soft steel will tend to bend and stay deformed if bent. Hard steel will tend to break if bent. Middle range steel is soft enough to bend, but still hard enough to not deform. Therefore, it will spring back to its original position ... spring steel. Hardness of steel is usually measured by the RC scale (or Rockwell hardness). European swords are generally hardened into the spring region around 50 RC. Japanese swords are generally hardened with the edge in the low 60's, and the spine in the mid 40's. this means that, in general, European swords aren't as sharp as Japanese swords, but they usually won't bend the way a Japanese sword will.

    Sorry, I got on a roll there!
     
  8. Flying Crane

    Flying Crane Sr. Grandmaster

    • Supporting Member
    Joined:
    Sep 21, 2005
    Messages:
    11,816
    Likes Received:
    2,030
    Trophy Points:
    263
    Location:
    San Francisco

    Yup, me too!!
     
  9. Touch Of Death

    Touch Of Death Sr. Grandmaster

    • Martial Talk Alumni
    Joined:
    May 6, 2003
    Messages:
    11,610
    Likes Received:
    844
    Trophy Points:
    263
    Location:
    Spokane Valley WA
    An interssting fact I came across is that a major part of the secret of Japanese blade making was the high quality steel imported from India. Were it not for thes imports the tempering process would not produce the results we have come to expect.
    Sean
     
  10. howard

    howard Brown Belt

    Joined:
    May 12, 2004
    Messages:
    469
    Likes Received:
    16
    Trophy Points:
    18
    Are you talking about modern Japanese blades, or old ones?

    Old Japanese blades were made from native steel. The ore was usually in the form of a black sand called satetsu. It's found in river and stream beds.

    The differential heating process should work with any good-quality "high carbon" steel (around 0.7% carbon), regardless of the source of its ore.
     
  11. Ninjamom

    Ninjamom 2nd Black Belt

    Joined:
    May 29, 2006
    Messages:
    882
    Likes Received:
    84
    Trophy Points:
    28
    Location:
    Solomons, MD, USA
    The ores found in India had a natural mix of carbon and other impurities in just the right size (microparticles) and proportions to make the wavy pattern seen in good hamons. When differentially hardened, it forms 'wootz' steel, and it retains a fine, hard edge. The Indian ore was also imported into the Mediterranean region, giving rise to the 'Damascus Steel' of medieval crusades fame.

    The ores available locally to Japanese smiths, however, were notoriously bad. Because of the poor qualities of the ore, the smiths had to find another method of eliminating impurities and including just the right amount of fine micro-particulate carbon. Alloying, as a science, wasn't yet known, but smiths found by centuries of trial-and-error that they could get the right form of steel (force out the unwanted impurities from bad ore) by folding the material as it was being forged. This technique also produced the same Damascene pattern as found naturally with the Indian wootz steel.
     
  12. howard

    howard Brown Belt

    Joined:
    May 12, 2004
    Messages:
    469
    Likes Received:
    16
    Trophy Points:
    18
    I'll take your word for that, but still, I've never seen a source that says that the Japanese swords of yesteryear were made of anything but local ore... can you cite a source that documents that they were using Indian steel in those days (say, any time before 1860 or so)?

    My understanding is that most of the carbon in the high-carbon steel used in traditional Japanese blades came from the charcoal used in the forges in which they heated the steel to work it. A chemical reaction would introduce carbon atoms into the iron, producing steel. Over many years of trial and error, the smiths figured out how hot to get the charcoal, what type of charcoal to use, etc., to get the carbon content of the steel to their liking.
     
  13. Touch Of Death

    Touch Of Death Sr. Grandmaster

    • Martial Talk Alumni
    Joined:
    May 6, 2003
    Messages:
    11,610
    Likes Received:
    844
    Trophy Points:
    263
    Location:
    Spokane Valley WA
    Not so, according to what I have read but, Other than providing you with the authors name I won't be able to argue this topic further. That Author was Neal Stephenson from the series "System Of The World".
    Sean
     
  14. Ninjamom

    Ninjamom 2nd Black Belt

    Joined:
    May 29, 2006
    Messages:
    882
    Likes Received:
    84
    Trophy Points:
    28
    Location:
    Solomons, MD, USA
    Here's a good intro article on the sword-making process in the Middle Ages, from someone I respect as an expert on the subject of historical swording.
     
  15. pgsmith

    pgsmith Master of Arts

    Joined:
    Jun 1, 2005
    Messages:
    1,587
    Likes Received:
    478
    Trophy Points:
    123
    Location:
    Texas
    A few more things, again because I am a geek! :)
    This is a work of fiction written by a professional author. The author is misinformed, which is not unusual.
    Close, but not exactly right either. 'Wootz' steel was NOT ore. It was already formed steel that was sold in cakes. The place where it was mostly sold from was Damascus, therefore the use of "Damascus steel". The ladder-like patterns formed in the steel were a product of how it was smelted, not by any hardening process. Today, people refer to any pattern welded steel as "Damascus" because the pattern of the folded steel resembles the patterns found in traditional wootz steel.

    Howard is absolutely correct in that most of the carbon for traditionally smelted Japanese steel was introduced through charcoal and rice straw. The folding was done for eliminating impurities in the steel, as NinjaMom said, and also for more evenly distributing the carbon throughout the steel billet so that it could be properly heat treated when finished forging.
     
  16. Touch Of Death

    Touch Of Death Sr. Grandmaster

    • Martial Talk Alumni
    Joined:
    May 6, 2003
    Messages:
    11,610
    Likes Received:
    844
    Trophy Points:
    263
    Location:
    Spokane Valley WA
    How do you know he was misinformed? Perhaps you were misinformed. Is there a source that refutes Stephenson's claims that you can cite or are you just sticking to what you have read?
    Sean
     
  17. pgsmith

    pgsmith Master of Arts

    Joined:
    Jun 1, 2005
    Messages:
    1,587
    Likes Received:
    478
    Trophy Points:
    123
    Location:
    Texas
    It would have been much easier to just take my word for it. I've put a lot of research into Japanese history, and the history of the sword, over the years. It is a subject that is very interesting to me, and one that I feel everyone that is going to seriously practice the Japanese sword arts should learn. The first refutation I can give is that, before the modern era, Japan never had any trade with India. In fact, they were completely isolated by Tokugawa law from the beginning of the 1600's until the Meiji restoration of the late 1800's. The Meiji restoration outlawed the carrying of swords, so no more were made until the advent of Japanese expansion in the early 1900's. The Japanese government used tales of the samurai, and the Hagakure, to create legions of fanatical soldiers,that all had to have their own swords. These swords are referred to as "gunto" and are of almost uniformly poor quality. Following Japan's defeat in WWII, sword making was outlawed by U.S. occupation forces. The Japanese were finally allowed to begin making swords again in the mid 1950's, but the Japanese government wished them to be considered as art objects rather than merely swords. To ensure this, they created the edicts under which Japanese sword smiths still labor. First, every sword in the country must be a licensed nihonto (this rule is why we can't take our Chinese made swords to Japan with us when we go to train). Second, each smith has to be licensed by fulfilling a minimum five year apprenticeship. Third, once licensed, they may only make 2 swords per month. Fourth, swords must be made in the traditional fashion, and out of tamahagane steel provided by the NBTHK's own tatara smelters. So you see, it cannot be the "quality of Indian steel" that makes Japanese swords what they are, since they are forbidden by law to make them out of anything but local ore from the government run tatara smelters.

    To see how a Japanese sword is made, here is a good web site that list the steps involved ...
    http://www.samuraisword.com/REFERENCE/making/japanse_swordmaking_process.htm

    To learn everything you wanted to know about Japanese swords, read through Dr. Rich Stein's extremely informative Japanese Sword Index ...
    http://www.geocities.com/alchemyst/nihonto.htm

    Dr. Stein also has links to many other places on the web to gain information about Japanese swords if you're interested, although this one post is more than enough to make most people nod off! :)

    Edited because I realized I said 3 swords per month instead of 2.
     
  18. howard

    howard Brown Belt

    Joined:
    May 12, 2004
    Messages:
    469
    Likes Received:
    16
    Trophy Points:
    18
    Paul, not at all... I'm sure there are plenty of members who enjoy a good discussion of history.

    I'm not surprised to learn that Japan had no trade with India that long ago - that was definitely my suspicion, simply because of the geography. Japan's foreign trade before the 18th century was probably limited to commerce with Korea and China.

    I was not aware that licensed smiths were restricted to using tamahagane from the NBTHK's smelter - that's interesting.

    It's also interesting that the two-blades-per-month rule is still in effect. I had read that the authorities were considering relaxing that restriction. I'm sure you know that it was originally based on the output of an eminentn and extremely particular smith who is even said to have cut the charcoal that he used for yaki-ire with scissors in order to get uniform small pieces. I understand that many of the younger licensed smiths could produce three or four quality blades per month, and that some had been lobbying for a change in this rule. It's curious that the rule is still in place.

    Thanks for the history and sources.
     
  19. pgsmith

    pgsmith Master of Arts

    Joined:
    Jun 1, 2005
    Messages:
    1,587
    Likes Received:
    478
    Trophy Points:
    123
    Location:
    Texas
    Hey Howard,
    The Japanese historically traded with the Dutch and Portuguese also, but all foreigners were expelled during the Tokugawa shogunate after the unification of the country in 1600. It was the U.S. and Admiral Perry's gun ships that forced Japan to open itself to outsiders and precipitated the Meiji restoration in the late 1800's.

    The two swords per month rule is still in effect, and many prominent smiths have been lobbying hard for years to get it changed. In fact, the smith in the photos showing the sword making process that I linked in my prior post, Miyari Akihira, is the very slow and meticulous one on which the two swords per month rule was based.
     
  20. Sukerkin

    Sukerkin Have the courage to speak softly

    • MartialTalk Mentor
    • LifeTime Supporting Member
    • Martial Talk Alumni
    Joined:
    Sep 15, 2006
    Messages:
    15,316
    Likes Received:
    479
    Trophy Points:
    193
    Location:
    Staffordshire, England
    On the contrary, sir, I was thankful that you posted it as it saved me having to decide between 'informing' and 'ranting' in terms of a reply.

    No offense intended to the original questioner directly by the way.

    Why I was on the verge of a 'rantathon' was that my blood was up having come across in short order a series of posts that displayed either extreme laziness or wilful ignorance :O. It is true that asking questions is an excellent way to learn anything but it is also true that a minimal amount of self directed research on behalf of posters would reduce the 'idiocy' quotient of the Net quite markedly :D.
     

Share This Page

Search tags for this page
sword clay tempered
,
bad thing about clay tempered sword
,
clay tempered
,
clay tempered pattern
,

clay tempered vs heat tempered

,
clay tempering
,
do i leave clay on blade for tempering
,
hamon temper vs damascus
,
how to clay temper a sword
,

what does clay tempered mean

,
what is better for a sword adbrasion or clay temper
,
what is clay tempered
,
what is clay tempered mean