Small, tart plums may be used

Stone your damsons or plums, and after this weigh them. Be careful to save any juice that comes from the fruit while stoning it. Allow a half pound of sugar to each pound of fruit, unless it is exceptionally tart, in which case increase the proportion to three quarters of a pound of sugar to a pound of fruit. Stew the fruit for half an hour, put in the sugar, and simmer gently for an hour. By this time the jam should have cooked down until it is quite thick and may be put into glasses or jars and sealed. Small, tart plums may be used for this jam.