5 cups crushed blackberries
1 box (1.75 ounces) regular powdered fruit pectin
6 1/3 cups granulated sugar, divided
1/2 teaspoon unsalted butter (optional)
1. Prepare canning jars and lids and bring water in water bath canner to a boil.
2. Pour blackberries into an 8-quart stainless steel stockpot.
3. In a small bowl, combine pectin and 1/4 cup of the sugar. Gradually stir into the fruit. Add butter, if using.
4. Bring fruit mixture to a full rolling boil over medium-high heat, stirring constantly. Gradually stir in the remaining sugar. Return to a full rolling boil, stirring constantly, and boil for one minute.
5. Remove pot from heat and skim off any foam. Let jam cool in the pot for five minutes, stirring occasionally.
6. Ladle hot jam into hot jars, leaving 1/4 inch headspace. Remove any air bubbles. Wipe jar rims and threads with a clean, damp paper towel. Center hot lids on jars and screw on bands until finger-tip tight.
7. Place jars in canner, making sure they are covered by at least 1 inch of water. Cover and bring to a gentle boil. Process 4-oz jars and 8-oz jars for 10 minutes; process 1-pint jars for 15 minutes.
8. Remove jars from canner and place on a wire rack or cloth towel. Let cool for 24 hours, then check seals. Wash and dry jars and store in a cool, dry, dark location.