Proof the yeast and sugar in the ¼ cup warm potato water. In a large mixing bowl combine the butter, salt, and liquids. Stir to melt the butter. Add the potatoes, stir in the yeast mixture, and add enough flour to make a stiff dough. Turn out on a floured board and knead until the dough is no longer sticky, 5 to 10 minutes. Place in a buttered bowl, and roll the dough around so that it becomes coated with butter. Cover, and allow to rise until doubled in bulk, 1½ to 2 hours. Punch down the dough and knead briefly. Cover, and allow to rise again about 30 minutes. Place on a floured board and divide into two parts. Mold into balls. Cover the dough and let it rest 15 minutes. Shape into loaves and place in well-buttered 8½ × 4½-inch bread pans. Cover, and allow to rise in a warm place till the dough appears above the top of the pans. Dust the loaves with flour. Bake at 400 degrees 20 minutes, then reduce the heat to 350. Bake another 20 to 30 minutes, or until brown and hollow-sounding when tapped with the knuckles. Immediately transfer to a rack to cool.