Fill a water bath canner or large pot with enough water so that jelly jars are covered with 2" of water. Place a canning rack for jelly jars in the bottom of the pot and bring to a boil while you prepare the jars and jam.
Thoroughly wash six half-pint jelly jars and screw bands. Cover with hot water while you prepare the jam for canning. Prepare the canning lids according to the manufacturer's directions.
Peel, core, and chop firm, ripe pears, and toss with the lemon juice to prevent browning. Measure the ingredients.
Place the pears with lemon juice and the fruit juice concentrate in a large stainless steel saucepan. Slowly stir in the pectin to prevent lumps. Add the butter or margarine, if desired, to reduce foaming.
Bring pear mixture to a full, rolling boil on a medium-high burner. Boil hard for 1 minute, then ladle into clean, hot jars using a canning funnel to reduce spills. Leave 1/4" headspace.
Wipe rims of jars, place canning lids on jars, then screw on bands until finger tight.
Use a jar lifter to place jars into the boiling water bath. The water level should be 2" higher than the tops of the jelly jars for proper canning. Add boiling water if needed. Place the lid on the canner and return the water to a steady boil.
Begin processing time when the water has come to a boil. Process jam for 10 minutes at an elevation of up to 1000 ft. Increase processing time to 15 minutes for 1001 - 3000 ft, 20 minutes for 3001 to 6000 ft, 25 minutes for 6001 to 8000 ft, and 30 minutes for 8001+ ft.
When the processing time is up, turn the heat off, remove the lid, and allow the canner to cool for 5 minutes. Remove the jars from the canner with a jar lifter and set on a dishtowel on the counter to cool.
After 24 hours, check the lids for a proper seal. The canning lids should not flex up and down when pressed in the center. If a jar has not sealed properly, refrigerate the jam and use up in 2 weeks.
Remove the screw bands and wash the jars to remove any jam that may have leaked during processing. Lable the jars and store in a cool dry place. Use within 1 to 2 years for best results.