In small bowl, dissolve yeast in 1 cup warm water (105 to 115°F.). In large bowl, combine warm milk (105 to 115°F.), molasses, shortening and salt; blend well. Stir in dissolved yeast. Add 2 cups all-purpose flour; blend at low speed until moistened. Beat 3 minutes at medium speed.
By hand, stir in 3 cups rye flour and an additional 3/4 to 1 cup all-purpose flour until dough pulls cleanly away from sides of bowl.
On floured surface, knead in remaining 1/4 to 1/2 cup all-purpose flour until dough is smooth and elastic, about 5 minutes.
Place dough in greased bowl; cover loosely with greased plastic wrap and cloth towel. Let rise in warm place (80 to 85°F.) until light and doubled in size, 45 to 60 minutes.
Grease 2 cookie sheets. Punch down dough several times to remove all air bubbles. Divide dough into 2 parts; shape into balls. Shape dough into two 12-inch oblong loaves; round ends. Place on greased cookie sheets.
With sharp knife, make four 1/4-inch deep diagonal slashes on top of each loaf. Cover; let rise in warm place until doubled in size, 20 to 30 minutes.
Heat oven to 350°F. Uncover dough. In small bowl, combine 1 tablespoon water and egg yolk; beat well. Brush over loaves. Bake at 350°F. for 35 to 45 minutes or until loaves sound hollow when lightly tapped. Immediately remove from cookie sheets; cool on wire racks.