In blender or food processor, combine 12 ounces grapes with lemon juice and puree. Set aside. Coarsely chop remaining grapes.

In medium saucepan, combine sugar and water then bring to boil, stirring a few times. Simmer without stirring, brushing down sides of pot with brush dipped in water if crystals start forming. When mixture has turned deep, golden brown, remove from heat and whisk in pureed grapes until smooth sauce has formed. Turn on heat and simmer until mixture has reduced by one-third and forms smooth caramel sauce. Whisk in cream and salt; stir in chopped grapes and serve over ice cream.