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1
Clean and wash the grapes. In a heavy stock pot, stir together grapes, lemon juice, and sugar and boil over moderate heat, stirring frequently and skimming foam, until pulp is broken down, about 20 minutes.
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2
Force jam through food mill set over a large bowl to remove skins and seeds. Return jam to pot and cook at a slow boil, skimming foam occasionally and stirring frequently as mixture thickens to prevent scorching, 35 minutes. Test for doneness. You can test for doneness by dropping a teaspoon of jam onto a chilled plate and then tilt it. Jam should not run but remain thick. If it hasn't set, continue to cook in a slow boil and test every 5 minutes or so until set.
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3
Pour the jam into hot, sterilized jars leaving 1/4 inch of air space, and then top with lids (also sterilized). Put sealed jars on rack in canner or pot and add enough water to cover by 2 inches. Bring to a boil, covered, then boil 10 minutes. With tongs, transfer jars to a towel-lined surface to cool. As they cool, you may or not hear a ping but the important thing is that the top of the jars remain concave. Store in a cool, dark place. The jam will thicken as it cools.
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4
Let jam stand in jars at least 1 day for flavor to develop. This jam will keep in sealed jars in a cool dark place for 6 months.
