An interesting flaw

[Robert S]

 

 

This photo is an extreme close up of the edge of a tanto.  It appears possibly to show two types of flaws.  The area to the left, the slightly wider crack, appears to be a very small burst fukure - about 2 mm long.  The hairline cracks on the right I only discovered using a powerful macro lens and blowing up the resulting image - it's not visible to the naked eye, even with an ordinary magnifying glass.  It appears only on this side of the blade, in what is probably an extension of the small area of poorly forged steel layer which caused the fukure, and appears to possibly be the result of a blow against a slightly softer object - possibly the corner of a piece of hardwood, or?  The crack has not consistently run along the lamination line in the steel.  I'd guess that this is a result of a combination of a small area of poor forging, a blow, and possibly a slight over-hardening or insufficient tempering.

 

It's also interesting that there is a slight color difference on the steel on the other side of the blade around this area, although I haven't been able to catch it in a photo yet.  I'm wondering if that might result from a slight change in the steel surface resulting from a moment of distortion in the slightly more ductile steel on that side of the blade, as a result of the blow.

 

There's also a slightly larger fukure farther down the blade on the other side, which also appears to possibly have resulted in part from a blow, as well as poor lamination... and again, discoloration on the opposite side of the blade.

 

Overall it is still quite a lovely blade, albeit with what is at a microscopic level a fatal flaw.  I fear it may not have been treated entirely kindly at some time in the past.

 

 

[ROKUJURO]

In my opinion, and as there is no distortion, these are all forging flaws without external impact influence. These can easily happen at any stage of forge-welding without being noticed by the smith, and unfortunately, they appear only as late as in the TOGI process.

Colour changes could possibly be caused by locally different reactions of the steel to polishing. Uneven hardness of the cutting-edgel, caused by a poor YAKIMODOSHI (= tempering), can be felt by the polisher when working with UCHIGUMORI (= finest finishing stone). Maybe this results even in a colour change, but that is just a guess.

[Robert S]

18 hours ago, ROKUJURO said:

In my opinion, and as there is no distortion, these are all forging flaws without external impact influence. These can easily happen at any stage of forge-welding without being noticed by the smith, and unfortunately, they appear only as late as in the TOGI process.

Colour changes could possibly be caused by locally different reactions of the steel to polishing. Uneven hardness of the cutting-edgel, caused by a poor YAKIMODOSHI (= tempering), can be felt by the polisher when working with UCHIGUMORI (= finest finishing stone). Maybe this results even in a colour change, but that is just a guess.

My initial thought was the same as yours.  But then I realized that there were two (or possibly three) cracks emanating from the same point on the edge.  Normally I would expect that one crack would start, and the stress would then move to the end of the crack, resulting in the crack extending, but reducing the likelihood of another crack starting from the same place.  That's what got me thinking about there having been some blow or transient force which started both cracks at the same time... but it is just a hypothesis.

[ROKUJURO]

Robert,

I would not exclude other impacts as you describe, but I am only looking at photos, and even with the blade in hand, it might be very difficult to guess.

[Tohagi]

That's the issue with fukure, when you have bubble early in the folding process, it's spread anywhere when foldind... it's like folding to make bread...

 

I have a sword with the same flaw at the same place... a pity.

 

Best

Eric