In a large saucepan combine grape juice, lemon juice, butter, and pectin. Bring mixture to a rolling boil over high heat, stirring constantly. Boil for 1 minute. Add sugar and stir until dissolved. Keep stirring and bring mixture to a rolling boil and boil hard for no longer than 1 minute, stirring constantly. Remove from heat and skim foam if necessary. Ladle hot jelly into hot sterilized jars leaving 1/4-inch head space. Adjust 2-piece caps. Process 5 minutes in hot boiling water canner.