Combine apple juice and 4 tablespoons lemon juice in a large, heavy-bottomed pot.

Peel, core, and cut pears into 1/2 to 3/4-inch pieces, adding them to the pot immediately to prevent browning.

Cook over medium heat, stirring frequently, until the pears release enough juice to boil (about 16 minutes).

Reduce heat to medium-low, cover, and simmer until the pears are very tender (about 20 minutes), stirring frequently.

Remove the pot from the heat.

Press the pear mixture through a food mill into a large bowl.

Return the pear puree to the pot.

Add the remaining 2 tablespoons lemon juice, brown sugar, nutmeg, and salt.

Bring to a boil over medium heat, stirring until the sugar dissolves.

Reduce heat to medium-low and simmer uncovered until the pear butter thickens and is reduced to 8 cups (about 1 hour), stirring every 5 minutes to prevent scorching.

Ladle the pear butter into 8 hot, clean 1/2-pint glass canning jars, leaving 1/4-inch space at the top.

Remove any air bubbles.

Wipe the jar threads and rims with a clean, damp cloth.

Cover with hot lids and apply screw bands.

Process the jars in a pot of boiling water for 10 minutes.

Cool completely.

Store in a cool, dark place for up to 1 year.