Blueberry Jam

From the garden

• 2½ lb ripe blueberries, picked over and rinsed — to yield 4 1/2 cups crushed

From the pantry

• 7 cups granulated sugar — measured out in advance; the full amount is required for the set
• 1¾ oz powdered fruit pectin — one standard box
• 2 tbsp bottled lemon juice

1. Fill a boiling-water canner and bring it to a simmer. Wash 8 half-pint jars and keep them hot in simmering water; prepare the lids per the manufacturer's directions. Measure the sugar into a bowl.
2. Crush the blueberries one layer at a time with a potato masher — the skins need breaking or the pectin can't reach the juice. Measure exactly 4 1/2 cups crushed fruit into a tall, heavy pot and stir in the lemon juice and pectin.
3. Bring to a full rolling boil over high heat, stirring constantly. Add all the sugar at once, return to a full rolling boil that stirring can't knock down, and boil hard exactly 1 minute. Off the heat, skim any foam.
4. Ladle the hot jam into the hot jars, leaving 1/4 inch of headspace. Wipe the rims clean and screw on the lids to fingertip-tight.
5. Process in the boiling-water bath, covered by 1–2 inches of water, for 10 minutes at a steady boil. Altitude adjustment: add 5 minutes at 1,001–6,000 feet, 10 minutes above 6,000 feet. Rest 5 minutes off the heat, then lift the jars out.
6. Cool undisturbed 12–24 hours, check that the lids sealed concave, remove the bands, and label. Blueberry jam can take a week or two to reach its full set — resist opening a jar to check.