Pop the skins off the grapes by pinching them at the end opposite the stem; set them aside.
Put the pulp (without water) into a heavy pan, bring it to a boil, and let it boil 5 to 6 minutes. Pass it through a colander or food mill to remove the seeds.
Pour the hot pulp over the skins and let the mixture sit for 5 hours. Add the sugar and tapioca, then pour the mixture into the pie crust and dot with butter. Put on the top crust, or a "floating" top crust— a circle of dough slightly smaller than the top of the pie. This is easier than crimping top and bottom together, and it also makes a pretty purple ring visible around the edge.
Bake at 400 degrees for 15 minutes. Lower the temperature to 350 degrees and cook 20 minutes more until the crust is browned and the juice begins to bubble up.