Wash and rub the fuzz from the quinces, then peel and core, reserving the seeds and the parings. Cover them with water, simmer thirty minutes, then strain through a fine sieve or drip bag.
Put the small pieces of sliced quince in this, adding a little more water if necessary. Cook gently until nearly soft, then add about the same amount of good cooking apples, peeled and cored.
Their cores and parings can be cooked with those of the quinces.
When the quinces and apples are sufficiently soft, force them through a fruit press, adding a little water if too stiff to mash through easily. Cook this mixture, stirring frequently with a wooden spoon, until quite thick, then add half its bulk in granulated sugar, and cook slowly for one and one-half hours.
Turn into jars and seal.