Note that with this recipe I use less-sugar than traditional jam so it will not keep for months and months. I would imagine these proportions would work well with other sweet plums, like Reine Claudes (greengage) and prune plums, but for tart plums, like Santa Rosas, you might want to add more sugar to counterbalance the tartness. To preserve (can) the jam, you can follow the guidelines here. The recipe can easily be doubled or scaled up. Because this is a jam that isn’t mean for long-conservation, I just eyeball it when it’s ready (in Step 3). However if you’re unsure, I’ve given a couple of methods for checking for doneness. If you do the “wrinkle test,” put the plate in the freezer when you start making the jam.