Step 1Combine the berries, sugar and lemon juice in a large nonaluminum bowl. With a potato masher or large metal spoon, press gently on the berries to bruise and lightly crush them. This allows the juices to start exuding freely. Allow the fruit to macerate at room temperature for at least 4 hours, stirring occasionally, and as long as overnight (covered and refrigerated).
Step 2Pour the contents of the bowl into a wide, shallow saucepan (not unlined aluminum or iron) and bring to a boil over high heat. With a metal spoon or fine mesh skimmer, skim off any foam that collects on top and reduce the heat to moderate. Continue cooking the fruit mixture for 25 to 35 minutes, stirring constantly the last 10 minutes to prevent the jam from sticking to the pan. When the bubbles begin to change from large intermittent ones to very small all-over ones, the jam is ready. The mixture should be reduced by half and will look like bubbling tar. The jam will thicken more while cooling.