4 cups prepared fruit (buy about 1-1/2 qt. fully ripe blueberries)
1 box SURE-JELL Fruit Pectin
1/2 tsp. butter or margarine
4 cups sugar, measured into separate bowl
Bring boiling-water canner, half full with water, to simmer. Wash jars and screw bands in hot soapy 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.
Stem and crush blueberries thoroughly, one layer at a time. Measure exactly 4 cups prepared fruit into 6- or 8-qt. saucepot. Stir pectin 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 sugar. Return to a 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 cups, 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.