Stem and crush the grapes, add to the water in a large nonreactive pot, bring to a boil, and simmer for 15 minutes. Strain all of the juice out of the mixture and allow to sit in a cool place overnight.
The next day, strain the juice again to remove the tartrate crystals that should have settled out during the night, clinging to the bottom and sides of the container. Measure the juice and add half the volume of juice in sugar. In a heavy kettle, boil rapidly until the jelly sheets from a cold metal spoon or until the mixture registers 220° on a candy thermometer. Pour the jelly into hot sterilized jars and seal. Process for 5 minutes in a boiling water bath.
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