Step by step recipe:

1. 3 min.
   Wash and brush 800 g quinces, but do not peel, as the skin add to the flavour.
2. 8 min.
   Cut the quinces into quarters, then remove and discard the core. Cut each quarter into small pieces.
   
   Put the quince pieces into a saucepan, add ½ lemon cut into 4 and just enough water to cover.
   
   Note: From 800g of quinces, you should be left with 500 g of cut fruit.
3. 20 min.
   Put the pan uncovered on medium heat and cook until the fruit is soft enough for the point of a knife to go through it easily.
4. 5 min.
   After cooking, discard the lemon pieces and pass the quinces through a vegetable mill on "fine" setting.
   
   Do not throw away the cooking liquid.
5. 3 min.
   Transfer the quince pulp into a saucepan and weigh it (from 500 g cut fruit, you should be left with about 350 g of cooked pulp).
   
   Weigh 30% of this weight in cooking liquid (100 g here), then mix this with the quince pulp.
   
   You can add the zest of 1 orange at this point.
6. 3 min.
   Weigh 450 g granulated sugar (this should be the same wieght as the mixture of fruit pulp and cooking liquid).
   
   Take out 50 g of this sugar at mix it separately with the 30 g jam sugar.
7. 4 min.
   Put the saucepan on high heat and bring to the boil, then tip in the 50 g of sugar with its gelling agent.
8. 3 min.
   Mix well, bring back to the boil and cook for 3 minutes, stirring constantly.
9. 1 min.
   Add the rest of the sugar all at once.
10. 3 min.
    Mix well, bring back to the boil and cook for a further 3 minutes, stirring constantly.
11. 1 hour
    Pour into a mould or tin, lined with a sheet of cooking parchment.
    
    Leave to cool for at least 1 hour.
12. 10 min.
    Turn out and cut into pieces about the size of a sugar cube.
13. 
    You can make the paste look more attractive by rolling the pieces in a little granulated sugar, but this is not essential.