Combine all ingredients in a large pot; bring to a boil, stirring occasionally until sugar dissolves. Using a candy thermometer, cook to the gelling point (See Table 1. below). Stir constantly to prevent sticking or burning.
Pour hot jam into hot, sterile jars, leaving ¼ inch headspace. Wipe rims of jars with a clean dampened paper towel; adjust two-piece metal canning lids. Process jars in a Boiling Water Canner according to National Center for Home Food Preservation (See Table 2. below).
Using a jar lifter, carefully remove the jars from the canner and place them on a flat, towel-lined surface, leaving at least 1-inch spaces between the jars during cooling. Keep away from air drafts and let the jars cool at room temperature. Check seals; store jam in a cool dark place.
Table 1. Gelling Point – Take the temperature of the jelly with a candy or jelly thermometer. The temperature of the jelly should be 220°F if you are at sea level. NOTE: For each 1000 feet of altitude above sea level, subtract 2 degrees F. For instance, at 1,000 feet of altitude, the jelly is done at 218°F; at 2,000 feet, 216°F, etc. (See below)
Temperature Test – Use a jelly or candy thermometer and boil until mixture reaches the following temperatures at altitudes of:
Table 2. Recommended Processing Time for Plum Jam in a Boiling Water Canner - Jar Sizes: Half-pints or Pints
Process Time at Altitudes of:
Sea Level - 1,000 ft 1,001 - 6,000 ft Above - 6,000 ft