The black or brown marks on the back of pans result from accumulated grease and food residues that splatter on the outside of the pan while cooking. Over time, these build up and harden. The good news is that they're only cosmetic and don't affect the pan's performance. But, if you want to remove them, use a cut lemon dipped into Kosher salt or a damp sponge dipped into baking soda and rub in a circular motion. Wash and dry the pan when you're done.