Working directly over a small nonreactive saucepan, use your fingers to gently squeeze the flesh from each grape, being careful to catch all the grape juices in the pan. Set the skins aside in a large mixing bowl.
Over medium heat, bring the grape innards and juices to a simmer, cover, and cook until soft, 3 to 5 minutes. Immediately force as much of the pulp as possible through a fine-mesh strainer or chinois. Discard the seeds.
Add the sieved grape pulp, sugar, lemon juice, orange zest, and orange juice to the grape skins, stirring well. Transfer the mixture to an 11- or 12-quart copper preserving pan or a wide nonreactive kettle. Bring to a boil over high heat. Continue to cook until done, 20 to 30 minutes. Stir very frequently during the cooking with a heatproof rubber spatula; if the jam starts sticking, lower the heat slightly. To avoid overcooking the jam, test it for doneness after 20 minutes of cooking. When the jam is done, it will acquire a glossier sheen and will have a thicker, more luxurious look than it did initially.
To test, remove the jam from the heat and carefully transfer a small representative half-spoonful to one of your frozen spoons. Replace the cold spoon in the freezer for 3 to 4 minutes, then remove and carefully feel the underside of the spoon. It should be neither warm nor cold; if still warm, return it to the freezer for a moment. Tilt the spoon vertically to see how quickly the jam runs; if it is reluctant to run, and if it has thickened to a spreadable consistency, it is done. If it runs quickly, cook it for another minute or two, stirring, and test again as needed.
When the jam is ready, skim any white foam from its surface with a stainless-steel spoon.
Recipe from The Blue Chair Jam Cookbook by Rachel Saunders/Andrews McMeel Publishing, 2010.