Choose plums that are ripe but not over ripe for the best butter.
Wash plums carefully and put into a large preserving kettle with the water.
Cover and cook over low heat just to simmering point; simmer gently until plums burst and juice flows freely.
Rub plums and juice through a puree sieve, there will be about 9 cups puree.
Return puree to preserving kettle, heat to simmering, and add sugar; stir until well mixed.
Bring to boil and boil vigorously, stirring almost continuously until mixture gives jelly test (two drops sheeting from edge of metal spoon).
This will require about 20 minutes’ cooking.
Pour into hot sterilized glasses.
Seal at once with melted paraffin.