Pit the plums, then chop them into small bits to measure approximately 4 cups. Put the chopped fruit into a large stainless-steel or other non-reactive saucepan and stir in the sugar and lemon juice. Let the mixture stand for 1 hour or so, until the plums release some of their juices.

Over medium heat, slowly bring the mixture to a boil, stirring often. Continue to cook rapidly, stirring often, for about 15 minutes. Insert a candy thermometer into the mixture. When it reads 220°F, the jam is ready.

Remove from the heat. Skim off and discard any surface foam. Ladle the jam into hot, dry, sterilized jars, filling to within 1 inch of the rim. Using a damp cloth, wipe the rims clean. Cover with lids. Set aside on a kitchen towel to cool to room temperature. Store in the refrigerator for up to a year.