To create plum jam, boil plums in water, and then mix in sugar, powdered fruit pectin and some butter if desired. After adding the cooked jam to individual jars, place the closed jars into a large stockpot with water, and boil. The jars can then be stored.
Eight half-pint jars of jam require 4 1/2 cups of pitted, chopped plums. First, mix the plums with 1/2 a cup of water and bring to a boil. Reduce the heat to a simmer, and after 5 minutes, stir in 7 1/2 cups of sugar, and add 1/2 a teaspoon of butter to reduce foaming if needed. Returning to a boil, mix in one package of powdered fruit pectin, and continue to stir.
After boiling for 1 minute, remove the jam from heat, pack it into sterilized jars and attach the lids. Fill a stockpot halfway with water and bring to a boil. Add the jars of jam into the pot, leaving 2 inches of space in between. Make sure the jars are entirely submersed, and add more water if needed. Boil the jars for about 10 minutes, and then remove them from the pot, and place them on a safe surface until cool. Once cool, ensure that the lid seals are tight, and store or refrigerate the jam.