Wash canning jars in hot, soapy water and rinse thoroughly. Keep jars hot by submerging in hot water or running through dishwasher cycle. Prepare new lids according to manufacturer's instructions.
Wash grapes thoroughly under cold running water, removing any damaged fruit and large stems. You do not need to remove every small stem as these will be strained out later.
Place prepared grapes in large, heavy-bottomed saucepan with 1/2 cup water. Heat over medium heat, crushing grapes gently with a potato masher or wooden spoon as they begin to soften and release juice.
Simmer grapes for 10-15 minutes, stirring occasionally, until grapes are completely soft and have released maximum juice. Do not boil vigorously as this can make juice cloudy.
Strain hot grape mixture through fine mesh strainer or food mill, pressing gently to extract juice. For clearer juice, strain a second time through cheesecloth-lined strainer.
Return strained juice to clean saucepan and heat to 190°F (do not boil). Add sugar to taste if desired, stirring until completely dissolved. Maintain temperature while filling jars.
Fill hot, sterilized jars with hot grape juice using canning funnel, leaving exactly 1/4 inch headspace. Wipe jar rims clean with damp cloth to ensure proper seal.
Apply new lids and rings, tightening rings to fingertip tight (snug but not over-tightened). Do not retighten rings after processing.
Process filled jars in boiling water bath for 15 minutes, ensuring water covers jar tops by 1-2 inches. Adjust processing time for altitude: add 5 minutes above 3,000 feet, 10 minutes above 6,000 feet.
Remove jars with jar lifter and place on towel-covered counter to cool completely for 12-24 hours. Check seals by pressing center of lid - properly sealed lids will not flex. Store properly sealed jars in cool, dark place for up to 2 years.
