I just found out that the black on the bottoms of my old pottery is from a coal or wood stove
OLDZ06, I don't think that I will agree with that assessment.
Coal and wood stoves are black because they put Stove Blacking on them and it doesn't "rub off" that easy after it has been polished. And it will "burn off" the cooking surface. But now if one placed one over an open fire, then yes, it would probably get black on the bottom.
Old stoneware has a "habit" of getting "black" and/or "black" splotches on the bottom after years n' years of use and sitting around. Molds and mildews are the likely problem causers and besides, one rarely evers "scrubs the bottom" of such utility vessels.
I just checked 4 pieces of my stoneware and they all have discolorations on the bottom.