Wild Blackberry Jam

Wild blackberries make better jam than the cultivated because their flavor is more intense.

Wash the blackberries well in cool water and drain thoroughly. Place the berries in a large bowl and crush with a potato masher. You will need exactly 9 cups of crushed berries.
Place the 9 cups crushed blackberries in a large, heavy nonreactive kettle; add the sugar and lemon juice, and stir well. Insert a candy thermometer.
Set the uncovered kettle over moderate heat and bring the berry mixture slowly to a boil, stirring until the sugar dissolves completely. Now cook slowly, still uncovered, stirring as needed to keep the jam from sticking to the bottom of the kettle, until the jelling point is reached (218° to 220° F.).
Meanwhile, wash and rinse 8 half-pint preserving jars and their closures and submerge in a large kettle of boiling water.
Ladle the boiling jam into the hot jars, filling each to within ¼ inch of the top. Tip: To avoid spills, use a wide-mouth canning funnel. Wipe the jar rims with a damp cloth and screw on the closures.
Process the jars for 15 minutes in a hot water bath (185° F.). Lift from the water bath; complete the seals, if necessary, by tightening the lids, then cool to room temperature.
Date and label each jar, then store on a cool, dark shelf for about a month before serving.