Combine apple juice and 4 tablespoons lemon juice in heavy large deep pot. Peel, core, and cut pears, 1 at a time, into 1/2- to 3/4-inch pieces; mix pears into juice mixture in pot as soon as pears are cut, to prevent browning. Cook over medium heat until pears release enough juice for mixture to boil, stirring frequently, about 16 minutes. Reduce heat to medium-low; cover and simmer until pears are very tender, stirring frequently, about 20 minutes (mixture will splatter). Remove pot from heat. Press pear mixture through fine plate of food mill into large bowl. Return pear puree to same pot. Add 2 tablespoons lemon juice, brown sugar, nutmeg, and 3/4 teaspoon coarse salt. Bring to boil over medium heat, stirring until sugar dissolves. Reduce heat to medium-low and simmer uncovered until pear butter thickens and is reduced to 8 cups, stirring every 5 minutes to prevent scorching, about 1 hour.

Ladle pear butter into 8 hot clean 1/2-pint glass canning jars, leaving 1/4-inch space at top of jars. Remove air bubbles. Wipe jar threads and rims with clean damp cloth. Cover with hot lids; apply screw bands. Process jars in pot of boiling water 10 minutes. Cool completely. Store in cool dark place up to 1 year.