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1
Put the blackberries in a wide, 6- to a-quart preserving pan and crush them with your hands.
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2
Add 1/2 cup water and bring to a boil; boil until the berries are tender and have released their juices, about 5 minutes.
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3
Set a large, very-fine-mesh sieve (or jelly bag) over a deep bowl or pot.
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4
Pour the blackberries into the sieve and let drain for 30 minutes, stirring occasionally but not pressing down too hard on the solids; discard the solids.
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5
Measure the juice; you should have about 4 cups.
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6
Prepare for water-bath canning: Sterilize the jars and keep them hot in the canning pot, put a small plate in the freezer, and put the flat lids in a heatproof bowl (see Notes for details).
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7
Rinse the preserving pan and pour in the blackberry juice.
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8
Add 3/4 cup sugar for each cup of juice, then add the lemon juice.
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9
Bring to a boil over high heat and cook, stirring occasionally, until the mixture registers about 220F on a candy thermometer or a small dab of it spooned onto the chilled plate and returned to the freezer for a minute wrinkles when you nudge it, about 20 minutes.
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10
Ladle boiling water from the canning pot into the bowl with the lids.
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11
Using a jar lifter, remove the sterilized jars from the canning pot, carefully pouring the water from each one back into the pot, and place them upright on a folded towel.
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12
Drain the water off the jar lids.
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13
Ladle the hot jelly into the jars, leaving 1/4 inch headspace at the top.
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14
Use a damp paper towel to wipe the rims of the jars, then put a flat lid and ring on each jar, adjusting the ring so that it's just finger-tight.
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15
Return the jars to the water in the canning pot, making sure the water covers the jars by at least 1 inch.
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16
Bring to a boil, and boil for 5 minutes to process.
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17
Remove the jars to a folded towel and do not disturb for 12 hours.
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18
After 1 hour, check that the lids have sealed by pressing down on the center of each; if it can be pushed down, it hasn't sealed, and the jar should be refrigerated immediately.
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19
Label the sealed jars and store.
