Black Blemish on Blade

[CoastieMike]

I have a 1960s shinsakuto katana that has a small black blemish on side of blade at shinogi above hamon. It is not rough or pitted, but relatively smooth. I am curious what causes these and the best advice for safely removing them. 

[16k]

I guess active rust that was removed. Or could be some sort of bagage against a rough surface, like a rock that has suffered oxidation over time. You picture makes it hard to know if it’s just on the surface or indented in the ji.

[Bruce Pennington]

Mike,

I've wondered about those stains too.  I have a number of gunto with them.  One guy felt it was tannins coming out of the wooden liner, into the steel.  Another thought it was the other direction, something in the steel going into the wood.  Either way, it's permanent.  I haven't had any success getting it out.

[Babu]

If I understand this correctly I think this may be rust following the path of least resistance by Chemical or galvanic corrosion.

Galvanic corrosion occurs when two dissimilar elements are subjected to a conductive solution and are electrically connected. One metal (the cathode) is protected, whilst the other (the anode) is corroded. The rate of attack on the anode is accelerated, compared to the rate when the metal is uncoupled.

For example, if you had oil or water or even a salty fingerprint on a blade you added the solution, something in the saya will create either the anode or cathode and the blade may react in this way.

Moisture is if course one of the problems so damp ,something on the blade before oiling or indeed the process started on a microscopic level them exacerbated during storage.

So acids or alkaline can be a part of the problem, but not exclusively.

If a grain of aluminium makes contact with steel you have a galvanic reaction. Are some military saya not aluminium?

Just a theory, but I've seen it happen and the steel ”eaten” in the same way on a very large scale. The cause was alkaline in this case and it corroded the liner of a liquid waste road haulage tanker lined with stainless steel.

It did one load then was washed and stood up for a month. Looked like a sieve when we opened it up.

This will start on a microscopic level.

[Brian]

It's called spider rust

[16k]

Yes. Spider rust is supposed to be rust from blood. But I guess it could be produced by other stuff too.