Sort the blackberries, wash, drain and cut into smaller pieces. Put in a tall saucepan and mix with the preserving sugar and lemon juice. Bring to a boil over high heat and stir constantly. If you like, you can also use a potato masher or hand blender to chop the blackberries a little more finely. When the food is bubbly, let it cook for another four minutes at the same temperature while stirring. Then immediately pour the jam into jars that have been rinsed with hot water. Close this and place on the lid for ten minutes. Then let it cool down and store the jam in a cool place until it is opened (e.g. in the cellar). Makes about three small glasses.