Baking in colonial America was done in a brick bake oven built into the back wall of the hearth, because they did not have a flue and gasses had to be vented up the main chimney. In time, the oven was mounted on the side wall of the hearth opening and eventually relocated next to the hearth after masons learned how to build ovens with their own flue. Because of the lack of iron in the colonies, the oven opening was sealed with a door made from wood, some with a tree limb attached for a handle. Because of the high heat, the wooden door had to be frequently replaced. Tin-faced wood doors and cast-iron doors mounted on hinges were two important improvements.