I'm making beef stock for french onion soup from Thomas Keller's Bouchon cookbook. I roasted the beef bones, a mix of knuckle bones and marrow bones, and then covered with cold water. I brought to a light simmer about an hour ago and besides for some very slight foam, there are no impurities. This seems weird to me. I've made veal stock from his cookbook before and there was really gross looking scum that I had to skim off often, but here there is none. Any thoughts? Am I doing anything wrong?