This jam has no added pectin (most jams often do). Adding pectin helps the jam set up, but necessitates more sugar, which dilutes the natural flavor of the fruit. Making jam without added pectin requires more careful cooking, but the extra effort pays off in a deliciously old-fashioned, fruity product. Straining out the seeds takes a bit of time, but is very worthwhile in the final product.
Place sugar in an ovenproof shallow pan, and warm in a 250°F (120°C) oven for 15 minutes. Warming the sugar before hand keeps the jam boiling evenly and ensures success.
Place berries in a blender of food processor until pureed. Strain the puree through a sieve pushing the puree through with the back of a wooden spoon to strain out all the seeds. Measure four cups of the puree, and place in a large saucepan, and bring to a full boil over high heat. Boil hard for 1 minute, stirring constantly. Add warm sugar, return to a boil, and boil until mixture forms a gel about 5 minutes.
To determine when the mixture gels, use the spoon test: Dip a cool metal spoon into the hot fruit. Immediately lift it out and away from the steam and turn it horizontally. At the beginning of the cooking process, the liquid will drip off in light, syrupy drops. Try again a minute or two later, and the drops will be heavier. The jam is done when the drops are very thick and two run together before falling off the spoon.