Peel 5 pounds of Concord grapes, then purée the skins with 1 cup sugar in a food processor and transfer to a heavy pot, and add 3 tablespoons lemon juice and 4 cups sugar. Over medium heat, bring to a boil and stir frequently while skimming foam off the top. After 20 minutes, remove from the heat and strain through a sieve. Return the strained liquid to the pot and bring to a rolling boil for about 35 minutes.
After the 35 minutes, test the jam for doneness. Remove the mixture from the heat and drop a teaspoon of jam onto a chilled plate. After about one minute, tilt the plate to check if the jam stays in a solid mound or runs off the plate. If it runs, return to heat and continue to boil the jam; retest the jam every five minutes, for up to 25 minutes, until it remains in a mound on the chilled plate.
The recipe yields 6 to 7 1/2-pint jars of jam. Seal the jam in jars and store in a cool dark place for up to six months. The total time for this recipe is one day, of which 2 3/4 hours is active. The additional resting time allows flavors to develop.