Four days ahead of breadmaking, prepare the "starter". Combine 1 package of the yeast, 2 cups warm water, and 2 cups all-purpose flour in a plastic bowl or container. Cover tightly and let stand for 2 days at room temperature. Then refrigerate for at least another day (see note below).
The day before preparing the dough, combine 1 cup of starter, the rye flour, and 1 cup warm water in a bowl. Cover with plastic wrap and let stand overnight at room temperature. The next day stir down the dough and add the second package of yeast, dissolved in 1/4 cup warm water, salt, caraway seeds, poppy seeds, butter, and sugar.
Then add up to 4 cups all-purpose flour, 1 cup at a time, to make a stiff but workable dough. Knead for 10 to 12 minutes, then shape into a ball. Place in a buttered bowl, turning to coat the dough with the butter. Cover and let rise in a warm, draft-free place until doubled in bulk, about two hours.
Punch down and divide the dough in half. Shape into two round loaves and place on buttered baking sheets generously sprinkled with cornmeal.
Cover and let rise again until doubled in bulk, about 1 hour. Brush with the egg wash, and bake in a preheated 375 deg F. oven for 30 minutes, or until lightly browned and the loaves sound hollow when rapped with the knuckles. Cool, covered with towels to prevent the crust from hardening.
NOTE: This will provide more starter than you need for this recipe. To keep it going, replenish with equal parts of warm water and flour, let stand again at room temperature, then refrigerate. Continue the process each time you use some of it.