Occasionally, I will come across a used chip that has some gray colored gunk on it, in one or several little pitches. It is always gray. The closest thing it reminds me of is modeling clay. It is sticky, but will come off from the surface fairly easily with a fingernail or a toothbrush. When in the mold, it is very difficult to get out. Does anyone know what this stuff is? I suspect that the chip got wet and sticky from a spilt drink and then picked up some dirt or ash or something.