Wash the canning jars, lids, bands, and tools in hot, soapy water and rinse well. Check the jars for cracks or chips. Fill a boiling-water canner about halfway with water and begin heating it. Keep the clean jars hot until you are ready to fill them.
Combine 1 gallon of water and 1 level teaspoon of ascorbic acid powder in a large bowl. Stir until the powder dissolves. This solution will help keep the peeled pears from turning brown.
Wash and peel the pears. Cut each pear lengthwise into halves, then remove the core and blossom end with a melon baller or small spoon.
Place each prepared pear half into the ascorbic acid solution as you work. Combine 8 cups of water and 4 cups of granulated sugar in a large stockpot. Bring the mixture to a boil, stirring occasionally until the sugar has completely dissolved.
Drain the pears and carefully add them to the boiling syrup. Return the syrup to a boil, then gently boil the pears for 4 to 5 minutes.
Use a slotted spoon to pack the hot pears into one hot jar at a time. Ladle the boiling syrup over the pears, leaving ½ inch of headspace. Remove trapped air bubbles, check the headspace again, and add more syrup if needed. Wipe the jar rim, center the lid, and tighten the band until fingertip tight.
Place the filled jars on the rack in the boiling-water canner. The water should cover the jars by 1 to 2 inches. Cover the canner and return the water to a full rolling boil before beginning the processing time. Process pint jars for 20 minutes and quart jars for 25 minutes at elevations up to 1,000 feet, adding time as directed in the elevation chart.
Turn off the heat, remove the canner lid, and let the jars rest in the water for 5 minutes. Lift the jars straight up with a jar lifter and place them upright on the counter. Let them cool undisturbed for 12 to 24 hours. Check the seals, remove the bands, wash and label the jars, and store them in a cool, dark, dry place.