Wild grapes or underripe Concord grapes make the best jelly. Wash and stem the grapes. Put into the kettle, mash down and cook slowly for about 10 minutes, until the juice flows freely. Water may be added a little at a time if necessary to prevent scorching or sticking. Strain through a jelly bag and leave the strained juice overnight in the refrigerator to allow the white crystals that sometimes form to settle. In the morning, pour off the juice carefully, discarding the sediment, if any, at the bottom.
Measure out 4 cups (leave the rest for another batch). Heat the sugar in a double boiler; with 4 cups of juice, you'll need 3 cups of sugar. Bring the juice to a full rolling boil, then add the heated sugar and bring the mixture back to a full rolling boil. Boil quickly, stirring, until the candy thermometer reads 8 degrees above the boiling point or until the jelly sheets off the spoon. Skim and pour into jelly glasses and seal.