This is an excerpt from A Treatise of Portuguese Cuisine from the 15th Century
(Portugal, 15th c. - Fernanda Gomes, trans.)
The original source can be found at
Lochac Cooks' Guild Website
61 - Geléia de marmelo. Tomem partes iguais de suco de marmelo e açúcar, e levem tudo ao fogo, com um pouco de água-de-flor. Deixem o tacho em fogo brando, até a calda adquirir consistência de mel. Guardem em potes de louça ou de vidro.
Quince Jelly. Take equal prts of quince juice and sugar, and take everything to the fire, with a little flower water. Leave the pot on low heat, until the syrup reaches the consistency of honey. Store it in jars of clay or glass.
Other versions of this recipe:
GOOD MEMBRILLATE WHICH IS A POTTAGE OF QUINCES (Libre del Coch)
To make marmalade of quinces good and fantastic (Libro di cucina / Libro per cuoco)
TO MAKE QUINCE MARMALADE (Le Menagier de Paris)