Contrary to popular belief aluminum foil (tin foil)... doesn't burn completely when placed into a fire.
Why doesn't aluminum foil burn?
Probably because it is a metal that has a melting point over 1200 degrees F. which is a much hotter temperature than most campfires or backpacking stoves ever get. What actually happens when trying to burn aluminum or tin foil packaging is that the waxy paper or plastic coating on top of the foil burns away leaving you charred aluminum foil fragments. These burnt aluminum or tin foil pieces are messy to handle, are hard to dig out of fire ashes and are definitely not biodegradable because foil has excellent corrosion resistance. So please be careful cooking outdoors with aluminum foil and make sure to pack out foil trash or repackage your food before hitting the trail.