Question about "bubbles" in yakiba

[Nicolas Maestre]

Hi everyone, 

 

Very quick question I'm sure 😊.

I have been using this steel Iaïto for years and have always been wondering what these tiny bubble-like circles were in the yakiba.

 

Does anyone know? Is it due to oil quenching? Bad temperature control? I have no idea...

 

I don't want to take up too much of anyone's time, as it's not a Nihonto, but I'm sure there is something to learn here (for me and maybe someone else as well!!).

 

Any information will be greatly appreciated 👍 

[ROKUJURO]

Nicolas,

 

maybe it's my old eyes, but I don't see "bubbles". The photo is not really well focused, so perhaps a good one will show more details.

Generally, you can assume it is oil-quenched in case it is made of modern/industrial steel. It can show something that looks like fine NIOI.

[DoTanuki yokai]

I think I see what you mean and maybe it is the result of decarburization that was not grinded away.   
 

Could also be some failure while etching the surface like not cleaning the blade properly.

[Nicolas Maestre]

1 hour ago, DoTanuki yokai said:

I think I see what you mean and maybe it is the result of decarburization that was not grinded away.   
 

Could also be some failure while etching the surface like not cleaning the blade properly.

Yes, also visible here at the bottom of the picture:

 

The thing is that I have never seen these large dots grouped together like that anywhere else so... 🤔