acetone, pure acetone 100%, and salt, its safe dont worry, but it will probably dissolve whatever the residue maybe, it could just be minerals from the water at that persons house, use distilled water next time. And if it continues to remain there, then maybe something on the outside of the chamber, without realizing it you could have had some salt left on your hands that got on it and made it seem like it was on the inside. But if it wont come out, then i dont imagine it ever will, so go ahead and smoke out of it, it probably wont come up with the smoke, and if it does, well then you have a nice clean bong.