I use the bones from previous cooked meats. We eat most of the meat off the bone and then toss 'em in the freezer. I've never had an issue with "scum" or "froth".
I just made beef bone broth earlier this week. I tossed the bones into the crockpot (from the freezer) and covered with water and a heavy-handed splash of red wine vinegar for about an hour before turning on the crockpot to "high". Once it got hot, then I turned it down to "low". At some point that day, I used a slotted spoon to remove the then-twice-cooked meat (once it fell off the bones) and discarded it. (I learned the hard way that it turns really nasty in the broth if it is left in. If you start with uncooked meat bones, then the meat would probably be good this way.) That night, I added veggie scraps (parts we don't typically eat - red pepper top, carrot ends, etc) from the freezer (we keep a bag and add to it). Once the broth got hot again, I turned it down to "keep warm" overnight. Late the next morning, I turned it off and let it sit for an hour before straining it. I froze six half-cup portions for rice and used the rest of make a delicious beef soup. I added fresh carrots and potatoes from the Farmer's market and I pureed green beans from the FM, too. (DH & DD don't like them, so I disguise 'em.) I added stew meat from Costco and fresh and dried herbs and Celtic sea salt. YUM!
