Boil jars and lids and swill both with rum.
With a cloth, rub the skin off the quinces until they are no longer furry.
Cut out the core and boil with the water for about 1 hour, then strain.
Cut fruit into small pieces, or coarsely grate with a food processor if possible, and place in a pot with the quince water.
Add water if necessary, the fruit should be covered.
Add sugar, lemon juice and zest, and salt and cook everything for about 10 minutes. Puree with a blender and boil again.
Pour hot into the jars, pour a little rum on top and close immediately.