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Wash jars, lids, and rings.
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Keep jars hot until ready to fill.
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Prepare boiling water canner and begin heating water.
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Wash blueberries and remove stems.
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Mash berries in batches with a potato masher.
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Measure 6 cups crushed blueberries into a large saucepan.
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Add lemon juice, cinnamon, allspice, cloves, sugar, and butter if using.
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Bring mixture to a full rolling boil over high heat, stirring constantly.
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Stir in liquid pectin.
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Return to a full rolling boil and boil hard for 1 minute, stirring constantly.
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Remove from heat and skim foam if needed.
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Ladle hot jam into hot jars.
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Leave 1/8 inch headspace.
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Wipe jar rims clean.
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Apply lids and rings finger tight.
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Process in boiling water bath for 10 minutes.
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Remove jars and cool undisturbed for 12 hours.
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Check seals before storing.