Pit and quarter the plums and put them in a heavy 4-quart pot. Add the sugar, the cinnamon stick, and the cloves. Stir well and let sit overnight or four 8 hours.

The next day, heat the oven to 350 degrees. Put the pot, unlidded, into the oven and cook for 2 hours, stirring the mixture occasionally.

Sterilize the glass jar and lids in boiling water.

When the plums have broken down and the liquid has reduced to a thick jam, remove pot from the oven and fish out the cinnamon stick (if you can find the cloves, fish them out too).

Puree the jam with an immersion blender until it resembles a fruit butter, and then fill the sterilized jars with the hot puree, screw on tops and immediately turn the jars upside down. If you prefer a jam with discernible chunks of fruit, however, don't puree the jam; simply ladle the hot jam into the sterilized jars.

Let the jars cool complete before turning the right side up again and labeling them. The jam will keep for at least a year.