Wash grapes; remove from stems. Measure 17 cups grapes. In a 6-quart kettle or pot combine grapes and 2 1/2 cups water; cover. Bring just to a simmer. Cover; simmer for 10 to 15 minutes or until grapes are softened, stirring gently occasionally.
Remove from heat; strain through a jelly bag or colander lined with several layers of cheesecloth, pressing on grapes gently to extract juice. Transfer juice to a clean large pitcher. Cover; let juice stand for 24 hours in refrigerator. Without stirring, pour off and save clear liquid; discard sediment that collects at the bottom of the pitcher. If desired, for a clearer juice, pour juice through a sieve lined with a coffee filter.
In a large pot or kettle combine grape juice and sugar, if desired. Heat just to simmering, stirring to dissolve sugar.
Skim any foam that collects on surface of juice. Ladle hot juice into hot, clean quart canning jars, leaving a 1/4-inch headspace. Wipe jar rims; adjust lids. Process in a boiling water canner for 5 minutes (start timing when water returns to boil). Remove jars from canner; cool on wire racks. Chill thoroughly before serving.