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Tannic acid forms a complex with the ferrous atoms and take them out. That's what gives the dark color. Chesnut is very good at it too.

Thanks for the light Kyle. I thought it has to stay in the elastic zone to sustain the use. Makes sens. Cory, on your first pic, the bolt and the tap don't seem to have the same pitch, like fine and coarse (standard) threads. Edit, I just had a look in the Machinery Handbook. That takes a lot...