Stem about 3 pounds fully ripe grapes and crush. Add 1/2 cup water; bring to a boil and simmer, covered, 10 minutes. Place in cloth or bag and squeeze out juice. Measure 4 cups into a very large saucepan. (If wild grapes are used, use 3 1/2 cups juice and 1/4 cup strained lemon juice.)
To the juice in the saucepan, add the sugar and mix well. Place over high heat and bring to a boil, stirring constantly. At once, stir in Certo. Then bring to a full rolling boil and boil hard 1 minute, stirring constantly. Remove from heat; skim off foam with metal spoon. Pour quickly into glasses. Cover at once with 1/8-inch hot paraffin.