Pot metal was not made of anything lying around, or of chewing-gum
wrappers, or any of the other old-wives tales.  The problem was
impurities in the zinc which caused intergranular corrosion.
Wikipedia has some good articles under "Zamak" and "zinc pest"

Alan Douglas

 [ Ref. http://en.wikipedia.org/wiki/Zamak  and
 [ http://en.wikipedia.org/wiki/Zinc_pest  -- Robbie