Boil water. Bring a pot of water to a boil.
Stem and smash grapes. Remove the stem from the grapes and rinse the grapes under water. Place them in a separate large pot. Using a potato masher, smash grapes down.
Boil. Cover the grapes with the boiling water. Bring the mixture back to a boil over medium-high heat, stirring occasionally.
Simmer. Reduce the heat and simmer for 30 minutes.
Strain. Transfer to a dampened jelly bag or fine mesh strainer set over a deep bowl or large stock pot. Let the mixture drip, undisturbed, for at least 2 hours.
Chill. Cover the juice and refrigerate for 24-48 hours to allow the sediment to settle.
Prep for canning. Prepare your jars by washing them with hot, soapy water and place them in a water bath canner. Fill the canner with water so the jars are fully submerged. Place the canner over medium heat and cover. Place the rings and lids of the jars in a saucepan filled with water set over medium-low heat and simmer (do not boil).
Strain again. Strain juice again through a dampened jelly bag or cheesecloth-lined colander. Transfer to a clean saucepan.
Add sugar (optional). If adding sugar, add now according to your taste.
Heat juice. Heat the juice to 190℉ (88℃) over medium-high heat. Do not boil. Keep the juice at a consistent 190℉ for 5 minutes, adjusting the heat to maintain the temperature.
Fill jars. Ladle the hot juice into the hot jars, leaving a ¼ inch headspace.
Wipe rims and twist on lids. Wipe rims with a dampened cloth and place the lid and rings on the jars tightening them so they’re fingertip tight.
Process. Place the jars in the canner, making sure that they’re fully submerged in water with 2 inches above the jars. Bring to a boil. Once boiling, process for 15 minutes for quart jars or 10 minutes for pint jars.
Cool and check seals. Turn the heat off, remove the lid, and let the jars sit for 5 minutes. Remove the jars from the canner and transfer them to a heat-safe place to cool undisturbed for 24 hours. Once completely cooled, check the jars for a proper seal. Once a proper seal has been confirmed, remove the rings and store the jars in a cool dark place for up to 18 months or per your lids manufacturer’s recommendations.