1. Cut 4 lbs of plums in half, pit them and place in a large mixing bowl. Drizzle well with 1 cups sugar. Using a large spoon, stir plums with sugar until all are coated. You can add more sugar to taste while its cooking. Let plums sit at room temp with the sugar for about 1 hour, or until sugar is somewhat dissolved.
2. Transfer plums/sugar mixture in to a large cooking pot. Place it on the stove uncovered and bring to a boil, stirring occasionally. Make sure if you see a light boil to stir because the whole pot may not be boiling, just the center. If it stops boiling after you stir it, continue boiling until everything is uniformly bubbling, then simmer for 10 minutes and turn off the heat. Let the pot stand uncovered until it is just warm to the touch or reaches room temp.
3. As soon as it cools, repeat step 2 the same way – simmering 10 minutes. You will bring it to a simmer a total of 4 times, stirring to prevent scorching the bottom. This is why it takes 2 days to make. It’s really easy though and so worthwhile! There’s no “set” waiting time between boilings. If 2 days doesn’t work for you, by all means, take 3 days. Preserves have plenty of sugar so they won’t spoil at room temp if you leave it on the counter overnight. If you want the preserves to have an even thicker consistency, you can boil it 5-6 times if you wish.
4. The last time you bring it to a boil you will want to transfer it to sterilized jars while it’s boiling hot.