Put the plums, lemon juice and sugar into a preserving pan over the heat. Bring to a boil then turn down to a moderate heat to simmer gently while evaporating. Stir frequently, skim. Generally, when no more froth forms, the jam is cooked. Check by taking the skimmer out vertically. The juice must fall in ribbons. Remove from the heat, leave to cool for 10 minutes. Put into jars up to the edges. Clean the edges with a moistened cloth, cover immediately or the next day as you wish. I prefer to cover while still hot. Thus, a vacuum is created while cooling. The less air there is in the jar, the longer the jam will keep.
