Tarnished/burned plane sole?

Maybe a shallow spot in the sole?

-- Shooting down the walls of heartache. Bang bang. I am. The warrior.

Maybe a shallow spot in the sole?

- TheFridge

That’s what I’m thinking, it was re-ground by a professional plane maker so it wouldn’t surprise me if he left that part shallow to avoid taking too much off the sole.

I’m just curious as to what is causing the blue colour.

Could it be the tannins in the oak changing the color of the steel? Kind of like removing a cut nail from oak and seeing the black mark left around the old nail hole. I believe that the tannins from the oak are reacting with the steel but its just a guess on my end.

-- Its not a crack, its a casting imperfection.

I suspect the blue color is “black oxide” Fe3O4. It’s actually a good thing. It’s hard, thin, doesn’t flake off. It’s the same coating that is intentionally created when “blueing” a gun.

-Paul

Looks to me the plane maker did very good job grinding the sole. Not an easy task as the soles are very thin. The discolored area looks like a depression to me from the photo, but it is very shallow.
I have ground some of my smaller planes in a 6X12” surface grinder. When going over a weak spot like the blade slot, the metal will heat up and expand a little, rise up. Then, end up a shallow area….even with flooding the work with coolant.
Candle wax in my experience with soles brought to such a fine finish doesn’t work very well. I found a very small amount of paste wax better.

Wait a minute! I know what it is! It’s the reflection of your head! :-)

OK. Sorry. That’s just what it looks like – a reflection.

[edit: Now I see that you already said that.]

-Paul