The chief virtue of plain terra cotta (the orange clay that looks like red bricks) is its porosity, a fancy way of saying that terra cotta roasting and baking pans allow excess steam to escape while holding in just enough moisture to make bread so moist and chicken such tender pickings.
(particles) flaking off the surface bind with and stabilize the foam. (Aluminum ions stabilize but darken the whites.)
But wait. Isn't copper toxic? (See Chapter 11.) Yes, but the amount you get in an occasional batch of whites is so small it's insignificant, safetywise.
Decorated ceramic vessels are another matter. For one thing, the glaze makes the pot much less porous, so that meat or poultry cooked in a covered painted ceramic pan steams instead of roasts. The practical result: a soggy surface rather than a crisp one.
More importantly, some pigments used to paint or glaze the pots contain lead. To seal the decoration and prevent lead from leaching into food, the painted pots are fired (baked in an oven). If the pots are fired in an oven that isn't hot enough or if they aren't fired for a long enough period of time, lead will leach from ceramics when in contact with acidic foods, such as fruit juices or foods marinated in wine or vinegar.
Ceramics made in the United States, Japan, and Great Britain generally are considered safe, but for maximum protection, hedge your bets. Unless the pot comes with a tag or brochure that specifically says it's acid-safe, don't use it for cooking or storing foods. And always wash decorated ceramics by hand; repeated passes through the dishwasher can wear down the surface.
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