1 (1,000-mg) vitamin C tablet (to prevent discoloration), crushed to a powder
6 1/2 cups cold water
24 firm-ripe small peaches (6 to 7 lb)
2 1/2 cups sugar
1 1/4 cups distilled white vinegar
4 teaspoons pickling spice
1/4 teaspoon kosher salt
6 (1-pt) canning jars with lids and screw bands; a boiling-water canner, or a deep 10- to 12-qt pot
plus a flat metal rack; an instant-read thermometer
                    

Dissolve vitamin C powder in 6 cups water in a large bowl
(to acidulate water). Cut a shallow X in bottom of each
peach with a sharp paring knife and blanch in 4 batches in a
5- to 6-quart pot of boiling water 10 to 15 seconds.
Transfer with a slotted spoon to a large bowl of ice and
cold water and let stand until cool enough to handle. Peel
peaches, then halve lengthwise and pit. Add peaches to
acidulated water and let stand 10 minutes, then drain well
in a colander. Toss peaches with sugar in a 6-quart wide
heavy pot and chill, covered, at least 8 and up to 12 hours.
Wash jars, lids, and screw bands in hot soapy water, then
rinse well. Dry screw bands. Put jars on rack in canner and
add enough water to cover jars by 2 inches. Bring to a boil,
covered, then boil 10 minutes. Cover lids with water in a
small saucepan and heat until thermometer registers 180°F
(do not let boil). Keep jars and lids submerged in hot
water, covered, until ready to use. Add vinegar, spice,
salt, and remaining 1/2 cup water to peaches (sugar will
have dissolved and will have drawn out peach juices) and
bring to a boil over moderate heat, skimming off foam.
Reduce heat and simmer until peaches are barely tender,
about 3 minutes. Remove jars and lids from water, reserving
water in canner, and transfer to a clean kitchen towel, then
divide peaches among jars using a slotted spoon. Return
peach-cooking liquid to a boil, then pour into jars, leaving
1/4 inch of space at top. Run a thin knife between peaches
and sides of jars to eliminate air bubbles. Wipe off rims of
filled jars with a dampened kitchen towel, then firmly screw
on lids with screw bands. Put sealed jars on rack in canner
and, if necessary, add enough hot water to cover jars by 2
inches. Bring to a boil, covered. Boil jars 20 minutes, then
transfer with tongs to a towel-lined surface to cool. Jars
will seal as they cool (if you hear a ping, that signals
that the vacuum formed at the top of the jar has made the
lid concave). After jars have cooled 12 to 24 hours, press
center of each lid to check that it's concave, then remove
screw band and try to lift off lid with your fingertips. If
you can't, the lid has a good seal. Store in a cool dry
place up to 6 months. Promptly put any jars that haven't
sealed in the refrigerator and use them first.