The mineral component of bone is comprised mostly of the calcium phosphate mineral hydroxyapatite which is embedded in the organic component type I collagen. When bone is exposed to a mildly acid environment the mineral component leaches out leaving only the pliable organic component. Vinegar is the classic medium to use because it works slowly enough to release the calcium and not destroy the protein scaffolding that gives bone it’s characteristic shape. If left in vinegar long enough the collagen would become destroyed as well. This is a human fibula tied in a knot.