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1
Make starter:.
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2
Peel potatoes and thinly slice. Put potatoes in a large bowl, then pour boiling-hot water over them. Stir in cornmeal, sugar, and salt until sugar and salt are dissolved. Set bowl into a larger bowl of hot tap water (about 130u00b0F) and let starter stand in a warm draft-free place (such as the inside of a turned-off gas oven), replacing hot water in larger bowl every 8 hours, for 24 hours. (Starter will be covered with a light, airy foam and have a pungent cheesy aroma.) Discard potatoes.
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3
Make sponge:.
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4
Add warm milk, baking soda, and flour to starter, whisking briskly until mixture is smooth. Cover bowl tightly with plastic wrap and set into a larger bowl of hot tap water (about 130u00b0F). Let sponge rise in a warm draft-free place (such as the inside of a turned-off gas oven) until doubled in bulk, about 3 hours. (Sponge will be covered in a thick layer of cappuccino-like foam.).
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5
Make bread:.
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6
Whisk together 4 cups flour and salt in a bowl. Blend in shortening with your fingertips or a pastry blender (or pulse in a food processor) just until mixture resembles coarse meal with some small (roughly pea-size) lumps.
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7
Add mixture to sponge and beat with a wooden spoon until combined well. Stir in enough of remaining flour to form a soft dough (it will be sticky).
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8
Turn dough out onto a well-floured surface and knead, adding more flour as needed to keep from sticking, 2 minutes. Let dough stand 10 minutes, then knead on well-floured surface, flouring your hands as needed, until smoother (it will not feel as elastic as a traditional yeast dough), about 10 minutes more.
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9
Divide dough into thirds and place each portion into a buttered loaf pan. Cover loaves loosely with plastic wrap and let rise in a draft-free place at warm room temperature until increased in bulk by one third (it will reach almost to rim of pan), about 3 hours.
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10
Put oven rack in middle position and preheat oven to 350u00b0F
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11
Bake until golden brown, 45 to 50 minutes. Turn loaves out onto a rack and cool completely.
