we have built up a nice little set of copper tri ply pans, because apparently they are great to cook with. however I have noticed that after cooking and cooling down or before and even after cleaning there always appears to be white powder leached out from the rim of the pan. do we know what this powder is? does this make them unsafe to use? we clean it off, and cook with them, but it always comes back, after or sometimes during each use. does anyone have any knowledge on this? thanks any thoughts gratefully received.