Stir 2 cups of white flour with the yeast in a large bowl.
In a saucepan, heat the water, sugar and salt until it is warm to the touch.
Add the liquid to the flour and yeast with the shortening and beat with an electric mixer at high speed for 3 minutes. The mixture (called a sponge) should be smooth.
Add the whole wheat flour and enough additional white flour to form a rough mass that cleans the sides of the bowl. Turn the dough out onto a floured
surface and knead it until smooth, about 12 to 15 minutes.
Cover the dough with a bowl and let it rise until doubled in bulk, about 45 to 55 minutes.
Punch down the dough and divide it into three parts of equal size. Form each part into a loaf about 13 inches long.
Sprinkle a baking pan with cornmeal. Place the loaves in the pan and make several diagonal slices on the top surface with a very sharp knife or razor.
Cover them with a towel. Place the pan in a cold oven on the middle rack and place a pan of boiling water on the lower rack. Let the loaves rise for 20 minutes. Remove the towel.
Turn the oven to 400 F. and bake the loaves for 45 to 50 minutes, or until they are browned and sound hollow when rapped on the bottom.