If you are going to preserve the jam, prepare the jars and lids: place six half-pint jars on a rack in a large pot. Add enough water to cover the jars, and bring to boil over high heat. Boil for 10 minutes, then turn off heat and allow the jars to rest in the hot water. Meanwhile, put bands and lids in a small saucepan and cover with water. Heat over medium heat until the water is simmering, then remove the pan from heat and allow the bands and lids to rest in hot water until ready to use.
Put a small plate in the freezer for gel-testing later.
Rinse and de-stem the grapes. Separate the grape skins from the pulp by squeezing the grapes between your fingers. Put the skins in the work bowl of a food processor and pulse until they are coarsely chopped. Transfer them to a large heavy-bottomed pot and add 1/4 cup of water. Bring to a simmer and cook until the skins have softened a bit, about 10 minutes.
