Rinse the grapes well and remove the stems. Pierce each grape with a sharp knife and put them in 6-quart pot. Add water just until the grapes start to float. Cover with the lid and bring to a boil over medium-high heat. Reduce heat to medium and cook until the grapes are soft, about 30 to 40 minutes. Mash the grapes using the potato masher.
Press the mixture through a sieve or use a food mill to separate the pulp from the skins and seeds. Use a spatula to scrape excess pulp from underneath the sieve into the bowl. You should end up with about 6 cups of juice. Transfer the pulp and juice into the 4-quart pot.
Mix together the pectin and ¼ cup of the sugar, then add to the grape pulp and bring to a rolling boil over medium-high heat. Add the remaining sugar and bring back to a full boil, then reduce heat to a simmer. The mixture should have small bubbles constantly breaking the surface. Continue cooking for 10 minutes, then test the mixture by spooning a little out onto a plate. Wait a few minutes and check the jam, it should start to set as it cools. If it’s too runny, continue cooking and checking every 5 minutes.
Prepare the Jars and Lids
While the mixture cooks, place the jars in the canning pot and add water until it’s 1 inch over the top of the jars. Bring to a boil and then turn off the heat, leaving the jars in for at least 10 minutes. In the small pot, heat water to 180° F. Add the lids and leave them in for at least 10 minutes to soften the sealing compound and sterilize them. Do not boil the lids to avoid seal failure.
Process the Jam
When the mixture is ready, drain and remove the jars from the pot. Turn on the heat for the canning pot and bring the water temperature up to 180° F. Ladle the jam mixture into the jars using the wide mouth funnel, leaving about ¼-inch headspace. Run a thin spatula around the inside edge of the jar to remove bubbles. Wipe the top of the jars clean with a clean damp cloth and place the hot lids on top. Add the bands and tighten just until finger tight.
Use the jar lifter to gently lower the jars into the hot water. Cover with the lid and bring back to a rolling boil. Process for 10 minutes, then use the jar lifter to remove the jars from the pot.
Cool and Store the Jars
Place hot jars on a wooden board and leave them for 12 to 24 hours, until they cool completely. Check for a seal after they have cooled. Store sealed jars in a cool dark place for up to 12 months. Store any unsealed jars in the refrigerator.