Separate pulp from skins of grapes.
If desired, chop skins.
Cook skins gently 15 to 20 minutes, adding only enough water to prevent sticking (about 1/2 cup).
Cook pulp without water until soft; press through a sieve or food mill to remove seeds.
Combine pulp, skins and sugar in a large sauce pot.
Bring slowly to a boil, stirring until sugar dissolves.
Cook rapidly almost to jellying point, about 10 minutes.
As mixture thickens, stir frequently to prevent sticking.
Pour hot into hot jars, leaving 1/4-inch head space.
Process 15 minutes in boiling water bath.