Step 1Prepare the canner, jars and lids by washing and rinsing the jars and lids, placing a rack in the bottom of a boiling-water canner and placing the jars in the rack, adding water to fill and cover the jars. Cover the canner and bring the water to a simmer (boiling the jars or pre-sterilization is unnecessary); keep the jars hot until ready to use.
Step 2While the water in the canner is heating, pit and halve or roughly chop the plums, adding them to a large, deep stainless steel pot as you go. Add the sugar and 1 cup water. Cover and bring to a boil over medium heat, about 15 minutes, stirring occasionally to dissolve the sugar. Place a few small saucers in the freezer to chill.
Step 3Increase the heat to high and boil hard, stirring frequently, until the mixture thickens, about 45 to 50 minutes. Remove from the heat and test the gel by placing a teaspoonful of the preserves on the saucer and chilling in the freezer for 1 minute. Remove the saucer from the freezer and push the edge of the preserves with your finger. A mixture that has reached the gel stage will be set and the surface will wrinkle when the edge is pushed. If the mixture has not reached the gel stage, return it to the heat and continue to cook, testing every few minutes until the gel stage is reached. Skim off the foam.
Step 4Ladle hot preserves into hot jars, leaving one-fourth-inch headspace. Remove air bubbles and adjust the headspace if necessary by adding hot preserves. Wipe the rim. Center the lid on the jar. Screw the band down until resistance is met, then increase to fingertip-tight.
Step 5Place the filled jars in the canner, ensuring they are completely covered with water. Cover and bring to a boil, then process for 15 minutes. Remove from the heat. Remove the canner lid. Wait 5 minutes, then remove the jars. Cool and store.