I know when making chicken stock (from bones) that that foamy white scum that appears in the beginning should be skimmed off the top. But I also get another type of film that starts to appear after the stock has cooked for about an hour over low heat. It's clear, and has an almost papery texture to it. If I pick it off the pot, it comes off in a sheet, almost like a small piece of plastic wrap, so I guess it's solid? What is this film made of, and should I be skimming it? I get it every time I made stock and wasn't sure what to do with it.