Prepare hot half-pint or pint jars and lids. Make anti-darkening soak with 1 teaspoon ascorbic acid granules in 4 quarts water.
Wash and peel peaches, using a 30-to-60-second boiling-water dip if needed to loosen skins. Halve, pit, slice, and hold in anti-darkening solution.
Drain slices and mash with a potato masher. Do not use a blender or food processor because they incorporate unwanted air.
Measure 8 cups mashed peaches into a Dutch oven. Stir in sugar and bottled lemon juice. Bring to a full boil, reduce heat, and simmer 5 minutes while stirring.
Fill hot jars, leaving 1/2-inch headspace. Process half-pints or pints 15 minutes at 0–1,000 ft, 20 minutes at 1,001–6,000 ft, or 25 minutes above 6,000 ft.