Place the plums and water into a large pot, and bring to a boil. Reduce heat to medium-low, cover, and simmer for 5 minutes. Stir in the sugar, and add butter to reduce foaming, if needed. Bring the mixture to a full, rolling boil over high heat, stirring constantly, and then mix in the pectin quickly. Return the jam to a full boil, and boil for 1 minute, stirring constantly.
Remove from heat, and skim off and discard any foam.
Sterilize the jars and lids in boiling water for at least 5 minutes. Pack the plum jam into the hot, sterilized jars, filling the jars to within 1/8 inch of the top. Run a knife or a thin spatula around the insides of the jars after they have been filled to remove any air bubbles. Wipe the rims of the jars with a moist paper towel to remove any food residue. Top with lids, and screw on rings.
Place a rack in the bottom of a large stockpot and fill halfway with water. Bring to a boil over high heat, then carefully lower the jars into the pot using a holder. Leave a 2 inch space between the jars.
Pour in more boiling water if necessary until the water level is at least 1 inch above the tops of the jars. Bring the water to a full boil, cover the pot, and process for 10 minutes.
Remove the jars from the stockpot and place onto a cloth-covered or wood surface, several inches apart, until cool. Once cool, press the top of each lid with a finger, ensuring that the seal is tight (lid does not move up or down at all). Store in a cool, dark area. Refrigerate opened jars for up to 3 weeks.
