Thread: Broken razors and grain structure

i cannot remember the correct Japanese name, but i have read that the little "fingers" of clay that reach down toward the edge of a Japanese sword from the thicker application of clay up on the shinogi are done specifically for that purpose. If you get a crack on the edge, it will not propagate because that little "cell" created by the slightly softer area. Thats is just what I read somewhere. Not sure if it is accurate as really top drawer nihonto rely as much if not more on the fact that they are made out of at least two or sometimes three different types of steel each having a different carbon content.
Bruno, have you seen the Sheffield videos where the makers were using that evil molten lead and mercury brew to austenize pocket knife blades much like some folks use molten salt today?

Originally Posted by Bruno

I have seen that too. However, on any razor that is still thick enough to be a razor, a crack should propagate all the way through unless there is a hardness gradient breaking the crystalline surface.

Actually, I had a razor that I water quenched by partially dunking it in water, and then pulled out. The differential hardening was clearly visible as a hamon afterwards. I pulled out the blade and let the heat from the thick spine seep back into the blade, tempering it on the go. I saw this trick from an old sheffield knifemaker. I think he was 82 or such, and the video was from the eithies iirc. Sadly I cannot find the video anymore.

Anyway, as it was tempering, I noticed the straw blueish color crawl back to the edge. When the edge was straw, I dunked it whole. Unfortunately, there was still too much heat in the spine, and the edge had lost all flex. It cracked in 2 spots. However, the cracks both stopped at the line where straw became blue. It is now a 6/8 razor

This leads me to believe that indeed, a crack will run propagate until it stops at a hardness gradient.

Sometimes these things are not easy to figure out. You have to wonder whether certain "properties" are the result of steel choice, grind, geometry, something gone very right or slightly wrong with heat treatment or all of the above. I remember one master smith telling me that with steel like W2, if you could get a very long, floppy wire edge/burr when initially setting the edge on the belt grinder (sacrilege, i know!! LOL) that could be pushed back and forth with your finger but was very easy to remove with a few strokes on a fine stone, etc, you had VERY fine grain structure in the steel. On the other hand, I have heard more recently that a persistent, hard to remove burr/wire edge may be a sign of too much retained austenite. We know that a properly HT'ed carbon steel blade can be hard and flexible, sometimes to amazing degrees, but when you get as thin as a razor edge, it may not matter as much. Have you guys noticed difference not in whether a blade will chip but HOW it chips? That might be interesting to look at.

Originally Posted by JDM61

i cannot remember the correct Japanese name, but i have read that the little "fingers" of clay that reach down toward the edge of a Japanese sword from the thicker application of clay up on the shinogi are done specifically for that purpose. If you get a crack on the edge, it will not propagate because that little "cell" created by the slightly softer area. Thats is just what I read somewhere. Not sure if it is accurate as really top drawer nihonto rely as much if not more on the fact that they are made out of at least two or sometimes three different types of steel each having a different carbon content.

Close, but no cigar. Not even close actually.

1) The thing you mentioned are called ashi, and indeed they are little thin lines that are less hard , and they serve the purpose of stopping chips on the edge side. They have nothing to do with the sword itself breaking or not, at all. If a sword is going to break, the ashi will not stop that in any way.
2) ashi are needed on both high end nihonto and low end.
3) all nihonto have the 2 types of steel design.
4) the ashi are unrelated to the fact that there is a different type of steel at the core.
5) nihonto are not inherently superior to modenr monosteel katana. The 2 different types of steel are used because that was the best way of doing things at the time, with the primitive materials and understanding they had. Today, things are still being done that way because that is the only way a licensed smith is allowed to make a sword in Japan, by law. Any non traditional sword is not a cultural artefact but an illegla weapon.
6) A modern mono steel katana will outperform the best nihonto

Look I don't mind discussing things, that's what we are here for, but you really seem to have a tendency to talk about things you heard of but don't understand, and then make conclusions that don't make sense at all. Please, you mentioned wanting to learn to make razors, then stick to actually making razors and learning things, instead of only talking about things without learning anything.

The only things we care about here is what people do, learn, and see themselves.
So I suggest you start doing things and stop the trivia time because it is serving no purpose.

Make something and tell us how it went and why it worked or not. THAT is useful discussion.

Reread my post. We essentially said the same thing about the ashi. The little "cells" that i was referring to were the spaces of hardened martensitic structure formed between the ashi. I will leave it there.

Originally Posted by Bruno

Close, but no cigar. Not even close actually.

1) The thing you mentioned are called ashi, and indeed they are little thin lines that are less hard , and they serve the purpose of stopping chips on the edge side. They have nothing to do with the sword itself breaking or not, at all. If a sword is going to break, the ashi will not stop that in any way.
2) ashi are needed on both high end nihonto and low end.
3) all nihonto have the 2 types of steel design.
4) the ashi are unrelated to the fact that there is a different type of steel at the core.
5) nihonto are not inherently superior to modenr monosteel katana. The 2 different types of steel are used because that was the best way of doing things at the time, with the primitive materials and understanding they had. Today, things are still being done that way because that is the only way a licensed smith is allowed to make a sword in Japan, by law. Any non traditional sword is not a cultural artefact but an illegla weapon.
6) A modern mono steel katana will outperform the best nihonto

Look I don't mind discussing things, that's what we are here for, but you really seem to have a tendency to talk about things you heard of but don't understand, and then make conclusions that don't make sense at all. Please, you mentioned wanting to learn to make razors, then stick to actually making razors and learning things, instead of only talking about things without learning anything.

The only things we care about here is what people do, learn, and see themselves.
So I suggest you start doing things and stop the trivia time because it is serving no purpose.

Make something and tell us how it went and why it worked or not. THAT is useful discussion.

If you mentioned ashi as only relevant for edge chips, then yes.
Your conclusions about 'top end nihonto' however, and whether they need ashi or not, or what they rely on in terms of quality, are still dead wrong.