Just fruit and sugar - no artificial pectin
Be sure to use a few under-ripe plums so that the jam will gel.
Wash the plums and remove pits (no need to remove the skin), discard any bruised areas. Cut the fruit into pieces. Put the pieces through an electric meat grinder – or – chop one batch at a time in food processor until pureed. Measure 4 cups of pureed fruit.
Combine fruit and sugar in large pot. Stir over low heat to liquefy. Bring to boil, stirring constantly. Boil until mixture begins to “sheet” from the edge of spoon. This can take anywhere from 10 to 20 minutes depending on the fruit.
Another way to test for doneness: Put a small plate in the freezer. After 10 minutes of boiling put 2-3 drops of hot jam mixture on the cold plate. If it runs when turned sideways it is not ready yet (return plate to freezer). Do this every 1-2 minutes. When it is sticky and does not run, then it is done.
Turn off heat and remove the pot. Skim the foam and discard. Ladle the hot jam into clean jars, wipe the top edge with a clean, damp cloth or paper towel. Let the jam cool, then put lids on.
Store in the refrigerator or the freezer.
Yield about four 12-ounce jars.
If your jam comes out too thick, just warm it up and stir in some water. If it is too thin, just serve it on top of ice cream or waffles...!