if you are going to try to burn those aluminum foil bags, please try to do it this way:   don't just throw the bag in the fire pit - put it on the end of a stick and hold it over the fire; the food inside will boil and burn out without having to be in the fire that long - before the bag can fall apart, pull it out of the fire and set it onto a rock or bare ground - it will cool quickly if you have burned out all the food - then just crumple it up.  I really really hate finding foil in my campfire rings, which happens all too often - a campfire won't get hot enough to actually burn the foil, but it will disintegrate leaving the residue.  If you time it right when trying to burn out the food from the foil bag, no mess - no residue in the fire ring.