Put the pan on the stove, bring the contents to a boil, and then carefully remove all the foam from the surface with a slotted spoon. Reduce the temperature, preventing further rapid boiling. Beef and lamb bones should be cooked for about 4-4 and a half hours, and veal and pork bones – 2-3 hours. After an hour, put the onion and continue cooking over low heat. 40 minutes before the broth is ready, put carrots, potatoes and salt in it.