Wipe plums dry, pack into a jar, and set in the oven after baking bread. Pour off the juice the next day and cook it over a low fire, constantly stirring. Test with a spoon over ice: if the juice begins to congeal like jelly, remove from the fire and immediately pour into a wide, glazed earthenware pot or into a jar. When it has cooled, tie it up and store in a cold, dry place. Slice like cheese and serve with vodka.