Wash, peel, core, and finely chop the ripe pears. Measure out 4 cups. Wash the canning jars and lids with hot, soapy water. Sterilize them by placing the jars in a boiling water bath for 10 minutes. Leave them in hot water until ready to use.
Fill a water bath canner with water and bring it to a simmer. Place a small heatproof plate in the freezer for testing the jam’s consistency later.
In a large, non-reactive pot, combine the chopped pears and lemon juice. Cook over medium heat until the pears are soft, stirring occasionally.
Stir in the fruit pectin and bring the mixture to a rolling boil, stirring constantly. Once boiling, add the granulated sugar all at once. Stir well to dissolve the sugar.
Bring the mixture back to a rolling boil and cook for 1-2 minutes, or until the jam reaches the desired consistency. If there is foam forming on the top, add the butter and skim off any foam with a spoon.
To check if the jam is ready, place a small amount on the frozen plate and let it sit for a minute. Run your finger through the jam; it should wrinkle and hold its shape. If not, continue boiling and testing until it reaches the desired consistency.
Remove the hot jars from the water bath canner and ladle the hot pear jam into the jars, leaving about 1/4 inch headspace. Wipe the jar rims with a clean, damp cloth.
Place sterilized lids on the jars and screw on the metal bands until fingertip-tight. Process the jars in the boiling water bath for 10 minutes to ensure they are properly sealed.
Remove the jars from the water bath and let them cool on a clean towel or cooling rack. As they cool, you should hear the lids pop, indicating a proper seal. Once cooled, store the jars in a cool, dark place.