A very old French recipe
Four lbs of of pitted, quartered Italian or French Prune plums
2- 3 cups of sugar only
The juice of two lemons (freshly squeezed)
One cinnamon stick
Place the plums in a large glass bowl, add the sugar, fresh lemon juice, and cinnamon stick. Cover with saran and place in the fridge over night. (this is where the magic happens). You will be surprised at the amount of natural juice the plums will produce in the maceration process. No need for water.
The next day place the plums in a jam pot and cook for about a half hour then ( remove the cinnamon stick), stirring constantly over low to medium heat. You will see a low ‘bubble’. Make sure all of the plums turn to mush. Then place a candy thermometer on the side of the pot turn up the heat so it boils med-high, and wait for the jam to reach 210-220 degrees. I stop cooking at 210 degrees (Its a bit runny but not much, its better than a tough clump from over cooking). All of the plums should be cooked through, but you should still see the fibers of the fruit. The skins will make the jam a beautiful deep reddish purple.
Place in sterilized jars with sterilzed lids and rings. Process for 10 minutes in a hot water bath. Done and delicious!