Wash the grapes and pull them off the stems. Put them in a large pot, crush them well with a potato masher, and add the water.

Bring to a boil, then simmer, covered, for 10 minutes, stirring now and then.

Pour into a damp jelly bag or a colander lined with several layers of damp cheesecloth set over a bowl. Let it drip until you have at least 5 cups of juice. For the clearest jelly, don't squeeze. Optional: refrigerate the juice overnight, then pour it off carefully, leaving any sediment and crystals behind.

Prepare a boiling water canner. Wash jars and lids, and sterilize the jars by boiling them for 10 minutes. Keep them hot.

Measure exactly 5 cups of juice into a large, deep pot. Stir in the pectin and butter. Bring to a full rolling boil (a boil that doesn't stop when stirred) over high heat, stirring constantly.

Add all the sugar at once and stir. Return to a full rolling boil and boil for exactly 1 minute, stirring constantly. Remove from the heat and skim off any foam.

Ladle quickly into the hot jars, leaving 1/4 inch headspace. Wipe the rims, add lids and bands, and process in the boiling water bath for 5 minutes (adjust for altitude).

Let the jars cool undisturbed for 24 hours. Check the seals, then store in a cool, dark place.