In a large bowl, combine brown sugar, oats, lard, molasses, caraway seed, and salt. Pour boiling water over mixture. Stir until combined. When mixture cools to 105 degree F to 115 degree F, stir in yeast mixture. Beat in 2 cups of the all-purpose flour. Stir in the rye flour and as much of the remaining all-purpose flour as you can with a wooden spoon.