4-1/2 cups prepared fruit (buy about 3 lb. fully ripe plums)
1/2 cup water
7-1/2 cups sugar, measured into separate bowl
1/2 tsp. butter or margarine
1 pouch CERTO Fruit Pectin
Bring boiling-water canner, half full with water, to simmer. Wash jars and screw bands in hot soapy water; rinse with warm water. Pour boiling water over flat lids in saucepan off the heat. Let stand in hot water until ready to use. Drain well before filling.
Pit unpeeled plums. Finely chop or grind fruit. Place fruit in saucepan; add water. Bring to boil. Reduce heat to low; cover and simmer 5 min. Measure exactly 4-1/2 cups prepared fruit into 6- or 8-qt. saucepot.
Stir sugar into prepared fruit in saucepot. Add butter to reduce foaming. Bring mixture to full rolling boil (a boil that doesn't stop bubbling when stirred) on high heat, stirring constantly. Stir in pectin. Return to full rolling boil and boil exactly 1 min., stirring constantly. Remove from heat. Skim off any foam with metal spoon.
Ladle immediately into prepared jars, filling to within 1/4 inch of tops. Wipe jar rims and threads. Cover with two-piece lids. Screw bands tightly. Place jars on elevated rack in canner. Lower rack into canner. (Water must cover jars by 1 to 2 inches. Add boiling water, if necessary.) Cover; bring water to gentle boil. Process 10 min. Remove jars and place upright on towel to cool completely. After jars cool, check seals by pressing middles of lids with finger. (If lids spring back, lids are not sealed and refrigeration is necessary.)
How to Measure Precisely
To get exact level cup measures of sugar, spoon sugar into dry metal or plastic measuring cup, then level by scraping excess sugar from top of cup with a straight-edged knife.
At altitudes above 1,000 feet, increase processing time as indicated: 1,001 to 3,000 feet-increase processing time by 5 min.; 3,001 to 6,000 feet-increase processing time by 10 min.; 6,001 to 8,000 feet-increase processing time by 15 min.; 8,001 to 10,000 feet-increase processing time by 20 min.