Adjust oven rack to lower postion. Preheat oven to 325°F. Grease two 28-ounce cans or one 9- by 5-inch loaf pan. Fill a Dutch oven or roasting pan large enough to fir the cans with enough water to reach halfway up the sides of the cans, about two inches. (If using a 9- by 5-inch loaf pan, heat about four cups of water.) Bring water to a simmer.

While water heats, whisk together cornmeal, sorghum flour, brown rice flour, baking powder, baking soda, salt, and xanthan gum. Add buttermilk and molasses. Whisk until smooth. If using raisins, gently stir into the batter. Divide batter evenly among the two cans or spread batter evenly into loaf pan. Cover cans or loaf pan with a piece of parchment paper. Secure parchment with a large rubber pan. If you don't have parchment paper, use a piece of greased foil.

Carefully transfer the cans to the simmering water. Cover the Dutch oven with the lid. If using loaf pan, nest the loaf pan in a 13- by 9-inch pan. Carefully pour simmering water into the 13- by 9-inch pan. Water should reach halfway up the loaf pan. Cover 13- by 9-inch pan with foil. Transfer to oven and bake until a skewer inserted into the middle of the loaf comes out clean, about 90 minutes.

Remove the pan from the oven. Carefully remove lid or foil (steam will escape as you do this.) Remove cans or pan from the water. Allow bread to cool in the can or pan for about five minutes. Turn bread out onto a wire rack to cool completely.