Rinse quince. Remove stem and flower and cut shell and core barrel into cubes.
Place in a saucepan and cover with 1 1/2 liters (approximately 6.3 cups) of water. Add lemon juice and boil for about 45 minutes, until soft.
Line a large colander with cheesecloth and set over a bowl or pot. Pour Quince mixture into colander and drain. Squeeze residual juice from fruit.
Let quince juice cool completely.
Combine 1 part juice and 1 part jam sugar in a large pot and bring to a boil. Cook for 4 minutes, stirring constantly.
Remove pan from heat. Pour mixture into jars and fill completely. Immediately close with a screw cap. Leave to cool and serve on fresh bread, as desired.