Prepare jars as described in General Rules.
Combine plums, sugar, lemon
juice, and salt in a stockpot over
high heat. Bring to a boil,
mashing with a potato masher.
Skim scum off top.
Boil, skimming and stirring frequently,
until bubbles slow, you
can see chunks of fruit showing
through top, and mixture
clings to a spoon, falling off
in languid clumps, 10 to 12 minutes.
Pick up and discard skins
with a fork, if desired.
Lift jars out of hot water one
at a time, draining water. (See
General Rules for how-to's.) Ladle
hot jam into hot jars using a
wide-mouth funnel, leaving
1/2-inch headspace. Slide a nonmetallic
spatula or chopstick
between jam and jar to release
trapped air bubbles.
Wipe rim and threads of jar
with a clean, damp cloth. Remove
lid from hot water. Place
lid on jar. Screw band down
evenly and firmly, just until resistance
is met. Place in canner
and lower into water.
Put lid on canner. Bring water
to a boil. Start keeping time after
water comes to a rolling boil.
Process 1/2 pints 10 minutes at a
gentle but steady boil.
Turn off heat and remove canner
lid. Let canner cool 5 minutes,
then remove jars and set them
upright on a dry towel or rack.
Do not retighten bands. Let jars
cool 12 to 24 hours.
After jars have cooled, check
lids for a good vacuum seal by pressing on center of each. If center is pulled down and does not flex, remove band and gently try to lift lid off with your fingertips. If lid does not flex and you cannot lift it off, seal is set. Wipe lid and jar
surface with a clean, damp cloth
to remove food particles and
residue. Label. Store jars in a
cool, dry, dark place.