An experiment to see the inner effect of hardening.
Sometimes we are told by polishers that a newly produced blade is harder than old blade on their hamon.
We can see that old blades tend to have milder hamon lines than newly produced blades'. On the other hand, there are some very healthy blade from 13th century that have still tight hamon lines.
Usually old blade has been more ground than new blade. If a hamon becomes milder inside of the blade, it would be changed by grinding. We already have some happenings that suggest so.
(=> Naohiro tanto) (=> Episodes by Kokaji, grinding Iai master's blade) (=> Tanto of worn out)
Then we planed an experiment to see the inner effect of hardening. That is to grind a blade at one side and study the appearance on each surface.
The blade for this experiment should have the same quality steel on both sides, for example, made with "KOBUSE" steel construction and has a small amount of inner steel. Because it would be ground much at one side and the inner steel should not appear on the hamon. Then we will be able to see the difference of hamon between on the intact surface and on the ground surface.
However this mission means destroying blade. It might be a terrible work in a sense. We never want to destroy blades anyhow. Therefore we have been waiting for a suitable blade coming from our smith's forge.
Fortunately, or unfortunately, a suitable tanto has appeared. It was made by the smith but he was not satisfied with the result. Then he accepted to grind it for this experiment.
The tanto has been thardened and done rough polishing to see the result of the hardening work. It has very active choji hamon pattern on both sides.

blade length 24.5cm,  curvature 1mm,  width at the base 3.1cm,  weight 310g

thickness at the base on the back 8.5mm

thickness of cutting edge 2mm,
It is going to be ground on the obverse side.
The tanto has been ground much on the obverse side. The thickness of the back was removed with 1.5mm. The weight was removed with 55g. The width is not changed.

weight 255g

thickness at the base on the back 7mm
The cutting edge still has a small amount of thickness.

The blade has bent to the ground side. Because the pressure of the hardened area was decreased by grinding.
Then the blade is going to be straighten and shaped.
The blade has shaped.

weight 230g

thickness at the base 6.5mm

width at the base 3.0cm
At the drawings bellow, the left is just after hardening, the centre illustrates the part that ground off, the right is the completion of shape.

Even in this stage, we can see that the wild hamon pattern on the obverse side has changed to calm.


Now, the blade is going to polishing work.
We have some reports by the polisher.

-On the stage of Koma-nagura stone-
I will refer to the different sides as new and old to reflect the reason for the experiment, new being the side that is original and old being the ground down side - if that is ok with you.
The new side has a crisp and tight nioi guchi with prominant choji jumping up into the ji.  While the old side has a more subdued and calm hamon now with a very wide nioi guchi, and the temper seems to be at about the same height as on the new side - but with no visible choji.
For polishing remarks, the new side and old side polish from stone to stone in about the same amount of time, but so far the new side is whiter with nagura stones with deeper marks, the old side steel is darker but polishes more satisfactoraly with less deep marks - if you understand my meaning.  There might seem to be more hada and activity being revealed on the old side, but perhaps the new side will produce more as the polishing progresses.
Incidently, when I polish swords I sometimes smell a nice smell, seems like carbon or something, like electrical contact between a motor brush and armature or the motor, not sure if I am explaining myself correctly.  On older swords the smell happens more often and usually on the uchigumori with harder pressure.  I noticed this smell on the new side of your tanto - not the old side.
-On the stage of Uchigumori Hato stone-
I have just about finished the uchigumori hato, so now is a good time for pictures. The difference in the 2 sides is very distinct. The new side has a harder temper than the old side it seems, the steel works quicker on the stone than the new side. Harder steel always takes a little more effort to whiten with the uchigumori. Incidently, the smudges of the koma nagura came out quickly on both sides, but when working the koma nagura it was very easy to leave scratches.
So, now you can see 2 very different sides, the old side has a wider nioi guchi than the new side.  I can also see some nice inazuma and other activities appearing easily on the old side.
-On the stage of Uchigumori Jito stone-
I am just about finished with the uchigumori jito. The result is interesting. On the old side we can see nice lively activity and a coarser hada as compared to the newer side. The old side hamon is more subtle but the activity is more lively, the new side hamon is more wild but the activity is less in my opinion. I worked each side about the same amount of time, but your forging method is usually tight (and this tanto suggests tight grain) so I tried not to overwork the new side - and the result is a dense grain pattern. I worked the old side until I felt the proper hada and activity were revealed.
The old side steel seems easier to work so it might suggest a softer structure.

Finishing work with finger stones
-On the stage of Hazuya-
I have just about finished the hazuya stone on the tanto, a good result was obtained quickly on both sides. The old side hardened area seems a little more whitish and not as hard as the new side to the eye.
-On the stage of Jizuya-
I have to clean up some small scratches and make the finish even over the whole surface. I will send pictures now but please keep in mind that I am not finished.
I am amazed at what has be presented on the old side compared to the new side, it is a little confusing for me though. When I see all the activity and hada/structure on the old side it makes me wonder if I have worked the new side enough to bring out the detail - so it is confusing if you know what I mean since I worked it just about as much. You might be able to see in the old side pictures where there is some inner steel perhaps near the machi, and there seems to be an area of darker steel near the mune (about 1cm wide) that runs the length of the tanto - this area has less activity. The whole old side looks to be covered in ko nie. I am concerned to work that area harder as that piece of rough jigane may come out and create some damage. But I will make a determination tomorrow.
I do notice that to bring out the hada and activity still requires a little more work than a very old sword, but perhaps that is because of a different hada.


It is polished up. Let's study the difference between the obverse side and the reverse side.


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