6 heaping cups fresh Marionberries or other caneberries such as wild blackberries, loganberries and boysenberries
4 1/2 cups sugar
1/3 to 1/2 cup strained, fresh lemon juice
1 teaspoon butter
Sort the berries, removing any leaves or twigs. Rinse them, drain well and transfer to a bowl. Using a rubber spatula, gently stir the sugar and lemon juice into berries (if berries seem extremely ripe, use 1/2 cup lemon juice; if at least 1/5 of berries seem firm or even slightly underripe, then you can get away with 1/3 cup lemon juice because underripe berries contain more natural pectin); let mixture stand, stirring gently once or twice, until sugar has dissolved, about 2 hours (I often let it sit all day in the refrigerator, or overnight).
Wash 7 half-pint jars. Keep hot until needed. Prepare lids as manufacturer directs.
Scrape berry mixture into a large, wide, heavy-bottomed nonaluminum pan (such as a 12-inch cast-iron skillet). Add the butter and bring mixture to a boil. Boil rapidly, stirring occasionally at first, and then continuously toward end, until mixture reaches jelling point on your thermometer (220 F from sea level up to 1000 feet; 216 F at 2,000 feet; 214 F at 3,000 feet; 212 F at 4,000 feet; 211 F at 5,000 feet; 209 F at 6,000 feet; 207 F at 7,000 feet; 205 F at 8,000 feet), which usually takes only about 7 to 10 minutes. Remove from heat.
Butter helps reduce foam, but if some foam remains after removing pot from burner and letting jam settle for about 2 to 3 minutes, skim it off. Ladle hot preserves into 1 hot jar at a time, leaving 1/4 inch head space. Wipe jar rim with a clean, damp cloth. Attach lid. Fill and close remaining jars.
At this point, if you want to store jars at room temperature, place them in a large pot with enough hot water to cover jars by 2 inches, bring water to a boil and process for 10 minutes (15 minutes at 1,000 to 3,000 feet; 20 minutes at 3,000 to 6,000 feet; 25 minutes above 6,000 feet).
If you plan to store jars in refrigerator, simply let them cool on counter and refrigerate. Quality will be good for at least 12 months. Makes 6 to 7 half-pints.