For the stock, cover quince peels and cores in a saucepan with the same mass of water (e.g. 1 kg of fruit cuttings + 1 l of water, perhaps add fruit if the ratio is not right), bring to the boil and simmer gently over a low heat for a few hours. Drain the contents of the pot and collect the stock. Gently squeeze the fruit waste.
For the thick juice: simmer the quince stock without sugar over low heat, stirring occasionally, for several hours until the juice is of a honey-like consistency. Immediately pour into preheated storage jars.