I use them often; sometimes I mix bones. But I make the stock first. BOnes, water, carrots, onions, celery, garlic, seasonings. Then strain (as already mentioned, in a colander), THEN cook the beans, lentils, etc in the strained stock. At this point you can refrigerate the stock over night, skim the fat, then freeze. I will often reserve the pork bones, some T-bones, chicken bones, none of which are enough in themselves, and make a mixed meat stock.