I acquired a bargain by chance last week with 7lb of Greengage plums and I have gone on to process and have 10 jars of Greengage Jam for my effots.
Start off with 1 kg/2 1/4lb greengages. cutting them into quarters and removing and discarding all of the stones.
Put the fruit and 200ml/7 fl ozs water in the preserving pan. Place the pan over a low heat and gently cook until the plums are soft and pulpy - recipe says 10 minutes, in reality it often takes longer than this. Stir from time to time until the greengages start to deliver up some of their juices ( as this stops the mixture from sticking and burning on the bottom of the pan).
Only add the sugar when the plums are pulpy enough for your liking, as putting in the sugar too soon can make the plums go hard. You will need 750g/1 lb 10 oz of sugar. Continue simmering and stirring until all the sugar is dissolved. Increase the heat bringing the mixture to a full rolling boil and cook rapidly for about 5 minutes skimming off any scum that rises to the top (I put a nob of butter in each pan of jam as it helps stop the scum forming. Start to test for a set by putting a spot of jam on a cold chilled plate and then putting into fridge, for a minute or too then bringing out and pushing with your finger gently if the mixture "wrinkles"when you do this it is ready to pot. Once setting point is achieved pot into hot sterilised jars, seal and label.