You will need seven 8 oz mason jars for this recipe.
Chop each plum into quarters and discard the seed. Weigh the plums with a kitchen scale to get three pounds.
Place ⅓ of the plums in a Dutch oven. Sprinkle with one cup of sugar. Repeat layering until all the sugar and plums are added. Sprinkle with the lemon and orange zest.
Heat over low flame until the sugar begins to dissolve, stirring occasionally.
Juice enough oranges to get half a cup of orange juice. In my case, it took two oranges to get a half a cup of juice.
Stir the orange juice into the plums and bring to a rolling boil. Lower the flame and set a timer for 30 minutes.
Eventually, the liquid will begin to rise and could boil over if you're not paying attention.
When you notice the liquid rising to the top of the pot, stir vigorously with a wooden spoon until the liquid recedes.
After cooking for 30 minutes, your preserves should reach the right consistency or gelling point. When that happens, ladle into sterilized jars, leaving ¼ inch head-space and process for 15 minutes in boiling water.