Wash the grapes and drain in a colander. Pick off and use only the good ripe grapes from the stems. In a large kettle, just barely cover the grapes with cold water and bring gently to a boil. Simmer until the grapes become very soft and have lost their vivid blue color. This will take a few hours. Cool and strain through a sieve. This juice can be canned for later use in making jelly, etc.