Preheat oven (electric cooker: 150 °C/circulating air: 125 °C/gas: level 1). Wash the plums, remove the stalks and remove stones. Place the plums, cloves and cinnamon sticks in a high, wide, heavy roasting pan. Stir in 100 g sugar.
Reduce the plum puree in the hot oven for about 4 1⁄2 hours. Sprinkle 75 g sugar every 30 minutes and stir well with a wooden spoon. Then reduce for another 3-4 hours, stirring several times.
Prepare the glasses and lids. Remove the cinnamon sticks. Stir in rum. Pour plum puree immediately into the glasses. Close them and put them on the lid for about 5 minutes. This sterilises the air still present in the jar and creates a vacuum.
Turn glasses over again, let plum jam cool down.