MAKES EIGHT 8-OUNCE JARS
Be sure to keep some saucers or little plates in your freezer for testing the jam. I prefer the plate to be ceramic or porcelain but any will work.
2¼ pounds blackberries
2¼ pounds raspberries
6 cups sugar
⅛ cup plus 1 tablespoon lemon juice
Place the blackberries and raspberries in a food processor and pulse until they are chopped but still a little chunky. Alternatively, put the berries in a large bowl and crush them with your hands. Or, for a mix of textures, puree half in the food processor and squeeze the other half with your hands.
Transfer the berries to a large bowl and stir in the sugar and lemon juice. Let sit for at least a few minutes, or up to 30 minutes.
Pour the mixture into a stainless-steel jam pot. Put a few saucers in the freezer for a plate test.
Cook over high heat, stirring and scraping the bottom of the pot so the sugars don’t burn. Use a fine-mesh skimmer to skim off any scum. Take the temperature with a candy thermometer. Cook until the temperature reaches 217°F, about 10 to 12 minutes.
If the jam is bubbling more than you can manage, turn off the heat, skim, then turn the heat back on. As the jam cooks, the solids will separate from the liquids and then rejoin. The surface will look matte at the beginning and then suddenly look shiny and glossy. Sometimes with blackberry jam, the surface will form what look like dry patches. This indicates that the jam is nearly done.
Prepare the plate test. Spoon a little bit of jam onto your frozen saucer. Put the plate back in the freezer for 1 minute. Take the plate out and slide a finger through the jam. It’s done when it parts and you see a strip of clean saucer. If it doesn’t, return the pot to the heat, stir frequently, and test after another minute.
Let cool and pour into sterilized jars. Always follow USDA guidelines for home canning.