Preheat the oven to 350 degrees with the rack placed in the lower third. Generously butter one 28-ounce can (or two 15 1/2-ounce cans). Set a kettle of water to a boil.
In a large bowl, whisk together graham and rye flours, cornmeal, baking powder, baking soda, and salt. Stir in sour cream, molasses, and water. Fold in raisins, if using. Pour batter into prepared can.
Place can in a deep pot with a tight-fitting lid (foil may also be used) and add enough boiling water to come halfway up the sides of the can. Cover the pot and bake until a cake tester inserted in the center comes out clean, about 1 1/4 hours for 28-ounce bread or about 55 minutes for 15 1/2-ounce breads.
Remove can from water and let cool slightly on a wire rack. Turn out bread, shaking can if necessary to loosen. Return to rack to cool completely.
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