Original: "Grape Sauce: Take good black grapes and crush them very well into abowl, breaking in a bread or half a bread depending on the quantity you wish to prepare; and add a little good verguice or vinegar so that the grapes will not be so sweet. And boil these things over the fire for half an hour, adding cinnamon and ginger and other good spices. (Maestro Martino, Libro de arte coquinaria, 155, translated in The Medieval Kitchen, Redon et al.)"
Buy seedless black grapes. Strip them from the bunches and wash them. In a food processor, process until you get a thick mash. Pour into a pot, add breadcrumbs and vinegar (depending on how sweet the grapes are, you may need more or less vinegar). Bring to a boil and add spices. Boil for half an hour: it will be thick and dark purple/magenta. Cool and serve. Keeps for at least a week refrigerated.