Carefully remove the grape juice from the refrigerator without disturbing the sediment. Gently pour off the clear liquid, and discard the sediment.
Add the strained grape juice to a large saucepot. Taste the juice and add sugar if you want your juice sweetened. Start with 1/4 cup and add more if desired.
Attach the candy thermometer to the pot, and heat the juice to 190˚F over medium-high heat. Do not boil, but adjust the heat to keep the juice at 190˚F for at least 5 minutes, and then fill your jars.
Spread a dry towel on the counter. Use your jar lifter to remove a jar from the canner, drain, and place on the towel. Keep the remaining jars in the canner so they stay hot.
Use your canning funnel and ladle to add the hot juice to the warm jar leaving 1/4-inch headspace. Wipe the rim of the jar with a damp towel.
Center a lid on the jar, and screw on the band until it is fingertip tight. Use the jar lifter to place the jar back into the canner, and repeat with the remaining jars.
Once all the jars are in canner, adjust the water level to two inches above the jar tops.
Cover the canner and bring to boil over high heat. Once water boils vigorously, continue boiling for 10 minutes at altitudes of less than 1,000 ft. Adjust processing time for your altitude if necessary. (See notes below).
When processing time is complete, turn off the heat, remove the cover, and allow the canner to cool down and settle for about 5 minutes.
Lay a dry towel on the counter. Use a jar lifter to remove the jars one at a time from the canner. Keep the jars upright, and place them on the towel a few inches apart. Do not tighten ring bands or test the seals yet. Let the jars sit undisturbed for 12 to 24-hours to cool.
After the jars cool for at least 12 hours, check to be sure jar lids have sealed by pushing on the center of the lid. The lid should not pop up. If the lid flexes up and down, it did not seal. Refrigerate the jar and use up within a week.
Remove the screw on bands and wash the jars. Label and date the jars. Store your jars in a cool, dark place and use within 12 to 18 months. Refrigerate your juice once the jar is opened and consume within a week. Yields 7 quart jars.