Wash and rinse the 4 jam jars and place upside down on an oven tray (baking sheet). Put into the oven at 120 ℃ (250 ℉). Place the jar lids in a jug and pour boiling water over them. Put two saucers or small flat plates into the fridge to test for set.
Wash 900 g (2 lb) plums, remove the stones (pits) and cut into quarters.
Put the cut plums into a large pan with 4 tbsp of water.
Cook gently until the plums are soft but not disintegrated.
Add 900 g (2 lb) sugar.
Heat gently, stirring with a wooden spoon, until the sugar has dissolved, then add 2 tbsp lemon juice.
Bring to the boil, and boil rapidly, (rolling boil) for 10 minutes.
Remove from the heat and plate a teaspoon of jam on a cold saucer then return the saucer to the fridge for 5 minutes. After 5 minutes push your finger across the jam and if a wrinkle forms the jam is ready. If not return the pan to the heat and bring back to the boil. Boil rapidly for a further 5 minutes and test again for set.
When setting point has been reached, us a metal spoon to skim off the most of the froth into a small bowl (you can still eat this, it's not dirty just frothy) then add the knob of butter which will disperse the remaining scum.
Add 1 tsp butter to the pan and stir. This will disperse any remaining froth. Let the jam cool in the pan for about 10 minutes so the fruit settles and doesn't sink to the bottom of the jars.
Remove the jars from the oven and use a ladle or a jug to pour the jam into sterilised jars.
Put the sterilised lids onto the jars and tighten. The lids will expand with the heat from the jam and the jars, so remember to return and tighten them again after the jam has cooled.
Serve Easy Plum Jam with any kind of bread, toast, scones or pastries.