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Sterilize 3 (1-pint) jars, along with fitting lids and rings.
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Slip the skins off the grapes. To do so, hold a grape between your thumb and index finger and gently squeeze - the inside should pop right out. Transfer the skins to a food processor and pulse a few times, but do not puree; set aside.
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Transfer the grape insides to a heavy-bottomed, 5-quart pot; cover the pot, bring to a simmer over medium heat, and simmer 10 minutes. Turn heat off and cool to room temperature (this will take a couple hours). Transfer the grape insides to a cheesecloth-lined sieve fitted over a bowl to catch the liquid; gently wring the cheesecloth to extract as much liquid as possible.
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Transfer the liquid back to the pot, along with the processed grape skins, the sugar, and the lemon juice. Bring to a boil over high heat (uncovered), then turn heat down and simmer (uncovered) until jam is done (see one of the tests below), about 35 minutes, stirring frequently.
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Transfer the jam to the sterile jars; the jars should be preserved through proper canning methods or if not, you should store them in the refrigerator.
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*Wash the grapes before you de-stem them, but weigh them after.
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There are three methods I know of for testing when jam is done:
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1) The Temperature Test: The jam should be at 220F (this is the magic number if you're at sea level, like me); a cheapie candy thermometer works fine here.
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2) The Sheet Test: Dip a wooden spoon into the jam and hold it sideways over the pot; the jam should fall off in a sheet, or the last jam to drip off should hang off the spoon in a bulging drop but not fall. (You can see what this should look like by clicking on the related blog post.)
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3) The Freezer Test: Freeze a plate, then put a little jam onto the frozen plate and put it back into the freezer for a few minutes; the jam should gel and not run.
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I like to do both the Temperature Test (because it's reliable) and the Sheet Test (because it's fun!).
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(Adapted from Epicurious.)
