If conventional jam-making isn't working out for you, or if you want to get away from the huge quantities of sugar needed to make ordinary jam set, gelatin might be your best friend. The mysterious balance of raspberries, sugar, acidity and pectin required to make good raspberry jam can be frustratingly difficult to achieve, especially for a tart jam. Unflavored gelatin sets the jam more reliably, and using it enables you to make a jam that's less sweet and has a fresher berry flavor.
Mash the berries lightly and bring them to a gentle simmer. Sprinkling them with up to 1/4 cup of sugar helps bring out their juices more quickly.
Remove 1/2 cup of the berry juice and chill it in your refrigerator until it's cold to the touch. Sprinkle an envelope of unflavored gelatin over the cold juice, and wait for a few minutes until it has absorbed the liquid and softened. Use one envelope of gelatin, or 2 tablespoons, for every quart of berries and juice.
Bring the raspberries to a boil and stir the gelatin mixture. Stir gently for 1 minute, breaking up the raspberries as little as possible, then remove the mixture from the heat.
Stir in half the sugar called for in your original recipe, or the equivalent in agave nectar or artificial sweetener. Taste the jam and add more sweetening if necessary.
Pour the hot jam into sterilized jars, and seal them with fresh, unused lids. Refrigerate the jam immediately.