Begin by pitting the plums, chopping them, and coating them with sugar. For every pound of medium sized chopped plums you need 1/2 cup of granulated sugar and 1 teaspoon of lemon juice. If the plums are ripe and sweet, you may want to use less sugar than you would if the plums are not fully ripe and firm. The lemon juice will enhance the flavor of the plums though it is optional.

If making the jam at a later date, place the plum and sugar mixture in a sealed container and allow them to soften for 24 hours. You can do this for up to two days (place in the fridge if keeping it for longer than a day).

If you are choosing to make the jam the same day, run the plums through a food processor first, then add the sugar and mix well.

While the plum mixture could be cooked on the stovetop, (this will require your undivided attention since it can easily burn) I prefer to place it in a casserole dish and bake it in the oven at 325° for approximately 2 hours, checking it and stirring it occasionally. Once it has begun to thicken (liquid has evaporated through cooking), you will be ready to place the jam in sterile jars, making sure that the lids have been sterilized right before filling the jars. (You can bake for longer, if you would like the jam to have an even thicker consistency.) Cover the filled jars with the hot lids and screw the rings on tightly.

Processing the Jam

The National Center for Home Food Preservation has current canning guidelines here.

Personal Experience with Processing the Jars of Jam

I have made jam using this method for many years without the additional water-bath method after the jars are filled with the boiling jam. Instead, after sealing the jars, I have placed them on my counter and covered them with a blanket to maintain the temperature of the boiling jam for as long as possible, and then left them undisturbed for 12 to 24 hours. This has proved successful in rendering sealed jars of jam (they pop within this time when they are sealed). If a jar does not properly seal, we use that jam right away.

According to the National Center for Home Food Preservation, "the high sugar content of fruit jams, jellies and preserves add an extra measure of safety and barriers to even spoilage." Though this method has worked personally, we recommend you use your own personal discretion in choosing the method that best protects spoilage and the safety of your family.