Wash the plums well, cut them in half and remove the pits. Heat the oven to 392°F (200 °C).
Arrange the halves in a tray but not too densely. Sprinkle with sugar on top and stir well.
Pour in a little water and bake 30 min. Then take the tray out and pour out the water. Stir the plums and return to the oven.
Bake for another 2 - 2 1/2 hours, while stirring every 30 min. Once the jam begins to thicken, pour in the rum and bake another 10 min.
Distribute the mixture into pre-sterilized and still hot jars, close them with caps and cool.