Prep the jars and canner: Wash jars, lids, and bands in hot, soapy water. Rinse well. Place jars in a canner or large pot, cover with water, and bring to a simmer to keep hot. Keep lids handy (follow the manufacturer’s prep instructions).

Mash the berries: In a wide, heavy pot, lightly mash the strawberries to release juices. You should have about 8 cups of mashed fruit. Stir in the bottled lemon juice.

Mix in pectin: Whisk the powdered pectin into the fruit until fully dissolved. Add the optional butter to help reduce foam.

Bring to a rolling boil: Heat over medium-high, stirring often, until the mixture reaches a full rolling boil that doesn’t stop when stirred.

Add sugar: Pour in all the sugar at once. Stir constantly to dissolve. Return to a full rolling boil and boil hard for 1 minute. Skim any foam if needed.

Test the set (optional): Spoon a bit on a chilled plate. Wait 30 seconds and nudge it. If it wrinkles slightly, it’s set. If not, boil another 30–60 seconds.

Fill the jars: Remove hot jars from the canner and place on a towel. Using a funnel, ladle hot jam into jars, leaving 1/4 inch headspace. Wipe rims clean with a damp cloth.

Lid and band: Place lids on jars and screw bands to fingertip tight (snug, not cranked down).

Process in water bath: Return filled jars to the canner, ensuring they’re covered by at least 1–2 inches of water. Bring to a boil and process for 10 minutes (adjust for altitude if needed: add 5 minutes for 1,001–6,000 ft; add 10 minutes above 6,000 ft).

Cool and check seals: Turn off heat, let jars rest in the water 5 minutes. Remove to a towel-lined counter. Let sit undisturbed 12–24 hours. Check lids—they should be concave and firm when pressed.

Label: Wipe jars, remove bands if you like, and label with the date and contents.