Shows apply plate after maching flat and the discoloration is a primitive but functional heat treating I do when donw cutting, This is done by heating plate red hot then a fast splash through cool water (real fast) this hardens the outter layer of the surface to help with rigidity but leave the flexability inside. I know the process works because i have played with recuting on the lathe and after this process is done the outter .005 or so cuts hard while the inner remain soft to cut.