Fill a large pot with the water.
Wash the quinces and cut off the stem ends. Leave the peels on. Core the fruit by chopping around the cores. Compost or discard the stems and cores. Chop the fruit into large chunks, 6 – 8 pieces per quince.
Place the quince chunks in the large pot of water. Bring to a boil over high heat. Reduce the heat and simmer until the fruit is mushy-soft, about 1 hour.
Mash the cooked quince with a potato masher. If the mashed fruit is on the dry side, add a little more water. You want a consistency like a soupy applesauce.
Place a colander lined with a double layer of cheesecloth (I use new J Cloths) or a very finely meshed strainer, over a large bowl or pot. Ladle the runny quince mash into the strainer or cheesecloth-lined colander. Leave it for 2 hours. Save the mash to make quince paste.
You should end up with at least 8 cups of juice. If you aren’t getting much juice, stir a little more water into the mash in the cheesecloth-lined colander or the strainer (do not add the water directly to the strained juice or it will be too diluted).
Sterilise the glass jars in boiling water then dry in a 150°C/300°F/gas mark 2 oven for 10 minutes.
While the jars are sterilising, measure the juice. Measure out equal quantities of sugar and pour into a large pot.
Bring to a boil over high heat. Stir constantly at first until the sugar is completely dissolved. Continue cooking at it reaches 105°C/220°F. It should be turning a gorgeous rosy colour. I cook it a little over to ensure a deep colour.
Skim off the white foam on top.
Pour into the sterilised jars and close immediately.