Add quince pieces to a large pot and barely cover with water.
Bring to a boil and cook until fruit is soft but not mushy.
Drain overnight in a cheesecloth-lined colander, catching all the juice.
The next day, measure 1 l of quince juice and 750g of sugar.
Add to a large pot. While stirring until the sugar has dissolved, bring to a boil. Add citric acid, if using. Cook until the juice has turned red and will pass the gel test.
Transfer the quince jelly to hot glass jars and close the lids.