Peel the quinces and cut out the core. Cut the flesh into pieces.
Boil the “waste” (i.e. the peels and the core) covered with water for about 1 hour until soft. Then filter the mixture through a cloth and collect the juice in a container.
Boil the quince pieces covered with water for about 30 minutes until soft and then strain.
Mix juice and quince puree and boil with 1 kg sugar for 20 minutes. Pour the puree hot into prepared jam jars and seal immediately.
Due to the gelling agent quittin contained in the core, you do not need extra jelling sugar for this jam, but can boil it down with normal sugar. The quince jam then gels in the jar all by itself.