Wash and crush grapes with potato masher, one layer at a time. Reduce heat, cover and simmer 10 minutes. Drip through multiple layers of dampened cheesecloth in refrigerator overnight. Do not squeeze bag of grapes. Carefully pour off grape juice to avoid any crystals.
Crystals tend to form in grape juices.
Measure 1 cup grape juice in large, heavy enamel-coated sauce pot. Measure well-strained lemon juice and add to grape juice. Measure 3 cups of raspberry juice and add to pot. Add sugar and stir to dissolve. Over high heat, bring mixture to rolling boil.