Classic Strawberry Jam

From the garden

• 3 lb ripe strawberries, hulled · about 2 quarts, to yield 5 cups crushed

From the pantry

• 7 cups granulated sugar · measured out in advance: it goes in all at once; don't reduce it
• 1¾ oz powdered fruit pectin · one standard box
• ¼ cup 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 according to the manufacturer's directions. Measure the sugar into a bowl so it's ready to add in one go.
2. Crush the hulled berries one layer at a time with a potato masher and measure exactly 5 cups into a tall, heavy pot. Jam foams high, so the pot should be no more than a third full. Stir in the lemon juice and the pectin.
3. Bring to a full rolling boil over high heat, stirring constantly: a boil that keeps rolling while you stir. Add all the sugar at once, return to a full rolling boil, and boil hard exactly 1 minute, still stirring. Off the heat, skim the foam.
4. Ladle the hot jam into the hot jars, leaving 1/4 inch of headspace. Wipe the rims with a damp cloth and screw on the lids to fingertip-tight.
5. Process in the boiling-water bath, jars 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. Turn off the heat, rest 5 minutes, then lift the jars out.
6. Cool undisturbed 12–24 hours, then check the seals. Lids should be concave and not flex. Remove the bands, label and date, and refrigerate any jar that didn't seal. The set finishes as the jars cool; don't tip them to check.