Preheat the oven to 425 degrees. Select a baking dish or rimmed sheet pan that's large sufficient to carry all the grapes in a single layer. Line it with parchment paper and put aside. If using muscadines, slice each grape in half and flick out any seeds.Place a rack within the bottom of a giant stockpot and fill midway with water. Bring to a boil and decrease jars 2 inches aside into the boiling water...