Mix the cherries with the lemon juice and puree with a hand blender (or put through a meat grinder).
Pour the puree into a saucepan with the preserving sugar and bring to a boil, stirring constantly. Boil the mixture for 3 minutes, continuing to stir. Remove from the heat and skim off the foam.
Immediately fill the hot jam to the brim in prepared jars, seal them and let cool upside down.