Peel, quarter and core the quinces and throw them straight into acidulated water (water with the juice of a lemon squeezed in). When they're all done, take them out of the water briefly and weigh them. Put them in a large preserving pan with 1 pint/568 ml of water to each pound/450g of fruit. Simmer gently until pulpy and soft, like apple sauce, but don't let the juice redden - it should be pale. Turn into a scalded jelly bag suspended over a large bowl or jug and allow to drain overnight without pressing the fruit (or you'll get a cloudy jelly). Set aside the pulp in the jelly bag to make membrillo. Weigh the juice, pour it into the cleaned preserving pan, and boil quickly for 20 minutes. This helps preserve the colour. Take it from the heat and stir in, until completely dissolved, 12 0z/340g of sugar to every 1lb/450g of juice. If the juice is very sour, increase the quantity of sugar to 14oz/400g of sugar to every pound of juice.