You can buy chernushka "black caraway" in some health food stores or from mail-order. They are nothing like the familiar caraway seeds. This bread is a New York specialty, usually made in bakeries.
Combine 1/2 cup of the warm water, the yeast and sugar and let stand until doubled in bulk, about 10 minutes.
Dissolve the salt in the remaining warm water in a mixing bowl. Mix in the sourdough starter, then the yeast mixture, then the gluten flour and 2 cups of the all-purpose flour; make a soft dough.
Spread 1 1/2 cups flour on the kneading surface and turn the dough out onto it. Knead, adding more flour if necessary to make a soft dough that will hold its shape. Do not over knead. . the dough should be only slightly elastic, even a bit sticky.
Form the dough into a ball and put in an ungreased bowl; cover with plastic, and let rise until doubled in bulk, about 1 1/2 hours.
Turn the dough out onto a lightly flour surface and knead it a few strokes to expel air. Cover with a towel and let rest for 15 minutes.
Divide the dough in half. Form each half into a loaf by flattening in to an inch-thick oval about 12 inches long, then rolling it up from one long side. Pinch the seam closed. Shape the ends of the oval loaves neatly and place them on two cornmeal-dusted baking sheets, seam down. Cover with towels and let rise until "three-quarters proofed, " or not fully doubled.
While the loaves are rising, set a large roasting pan containing 2 inches of boiling water on the oven floor (or the lowest shelf of an electric oven) and preheat the oven to 400 degrees.
Brush the loaves with egg-white glaze, being careful not to let the glaze drip onto the pan, as it sticks and burns. With a single-edged razor blade or a small, sharp knife, cut three diagonal slashes, holding the blade almost parallel to the surface, about 1/4 inch deep on each loaf. Sprinkle the loaves with "black caraway" or caraway seeds, if you like.
Bake for 30 minutes on the middle and upper shelves of the oven, then brush again with glaze, exchange shelf positions, and remove the water pan. Bake for 20 to 30 minutes longer, or until there is a hollow sound when you rap the bottom of the loaf.
Cool the bread on racks.
There is no historical or etymological theory for the term, "corn" in this bread. Ah-Choo in Md.