For the quince jam, first wash the quinces and dry them well. Then, using a large knife, divide the very hard fruit into quarters and possibly eighths.
Cut away the core, the flower buds and the stems, put the rest in a little water and simmer the quinces until you can easily pierce them with a toothpick.
Put the mixture through the liquid, add the preserving sugar and cook according to the instructions on the preserving sugar package.
Pour into clean jars, making sure to create a vacuum.