Mystery blue shade on plane mouth

[Owd Jockey]

I remember the first time I turned English Oak, beside the unique and not unpleasant smell I was surprised to see the wood 'bleeding' a dark blue/purple liquid which I later learned was a mineral stain. It discoloured all of the my turning tools, which were easy to clean up with a bit of 00 steel wool.

 

[CaptainBudget]

And I do mean acetic acid, like what's in that stuff you put on your chips and also eats wire wool. Oak is full of it and kiln dried oak is even more acidic than air dried.

Edit.

Putting wire wool in vinegar also makes a nice blue/black stain for wood.

I believe this is why traditional furniture hardware on Oak tends to be brass/copper alloy (non-ferrous), as this doesn't stain the wood over time. The literature I've read tends to state it's the tannins that does this, but acetic acid makes far more sense...