Wash the beans and remove those that float in the wash water. Soak beans overnight in enough water to cover them by several inches. Drain. In a large kettle, place 3 quarts liters of cold water. Add the drained beans and the ham bone, and bring to a boil. Reduce heat and simmer, partially covered, for 2 hours. If scum forms on the surface, skim it off. Chop the onions, garlic, celery, and parsley. Add to the pot. Add the mashed potato to the pot. Simmer 1 more hour. Check the beans. If they are not yet tender, then simmer some more. Add salt and pepper. Remove the ham bone and pull the meat from it. Cut the meat into small chunks, and toss them back into the soup. Serve piping hot into preheated bowls, and sprinkle chopped fresh chives atop each.