In a 8-qt pan, combine 4 1/2 cups of peeled, pitted and crushed plums with 7 1/2 cups of sugar. Cover and let stand for 20 minutes.
Remove the cover and over medium-low heat, stirring constantly, heat the mixture until the sugar completely dissolves. Increase the heat to medium-high and bring to a boil. Boil for 2 minutes, stirring gently. Remove the pan from the heat and skim off the foam.
Return the pan to the heat and bring to a boil; boil for 1 minute stirring constantly. Remove the pan from the heat and skim off the foam.
Stir in 1/2 teaspoon unsalted butter and once again over medium-high heat bring the mixture to a full rolling boil, stirring constantly. Stir in an entire 3 ounce pouch of liquid pectin; return to a full boil, stirring constantly. Boil, stirring for 1 minute. Remove the pan from the heat and skim off foam. It is important to skim off the foam all three times since the plums and stirring make a lot of foam and you do not want that in your jars of jam.
To prevent the jam separating in the jars allow the jam to cool 5 minutes before filling your hot jars. Gently stir the jam every minute or so to distribute the fruit. Ladle the hot jam into hot jars, leaving 1.4-inch headspace. Wipe the jar rims and threads with a clean, damp cloth. Cover with hot lids and apply screw rings.
Process 1/2 pint jars in a 200 degree F water bath for 10 minutes, pint jars for 15 minutes. Cool jars on a wire rack, leaving at least 1-inch space jars for air to circulate to cool.