Blackened frame corners

[Mary Lever]

Does anyone have a tip for stopping corners of oak frames sometimes becoming slightly blackened after joining. I’ve read about the cause but not a way to avoid or correct it? Thanks

[Justintime]

Can you tell us what the cause is? It might help work out a solution. At the moment I have no idea of what a blackened corner looks like.

[vintage frames]

Could it be something to do with the wet glue rusting the staple pins a bit and that reacting with the tannins in the oak to create a black stain?
Or not?

[prospero]

Oak and Ash can react to steel. That's why you never touch raw oak with steel wool.
Tiny fragments will lodge in the pores and it will go very spotty.

The only steel thing on the corner is the v-nails and I would have thought if any blackening shows they might be too
near the outside.