The Clay
Clay has been traditionally used in Mexican cooking because the clay imparts a specific flavor to the foods that can't be reproduced in a metal pot. Due to the documented health hazards, lead glazed clay pots are not recommended for use. Only pots stamped "safe for food use" should be used for cooking.
Product of "lead free" pottery represents a very small percentage of pottery production in Mexico. Look for products marked "lead free" or "minimal levels of lead, food safe". These products are approved by the U.S.D.A. and must not exceed the allowable limits for lead. Pots marked "lead free" may actually contain small, allowable amounts of lead.
Prepare Clay Pots For Use
You should use a simple seasoning process before you use the pot. Fill the pot with water and place it over a direct flame. If you have an electric stove then you should use a diffuser or buy a small portable gas burner. Allow the water to boil for a few minutes then remove from the heat and discard.
How To Use Your Clay Pot
In Mexico these pots are used over a direct flame. As we mentioned above if you have an electric stove then you should use a diffuser or buy a small portable gas burner.
Avoid rapid temperature changes. As with all clay cookery you should not move a cold pot to a hot flame or vice versa. Allow the pot to warm to room temperature before use. The pots are fragile and should be handled carefully.
Lead Glazes
Although Mexican peoples have been using these pots for centuries you should be aware that the glazes most commonly contain lead. The FDA recommends that you never
cook, OR store foods in these pots as foods can absorb the lead when exposed to lead in the glaze. This is particularly true with acidic foods such as orange, tomato and other fruit juices, tomato sauces, wines, and vinegar.
**Important: The FDA Requires that any pots that contain lead be marked "for decorative use only". 
photo by gourmetsleuth: chirmolera - pottery grinding bowl
cazuela - for stews and sauces
Clay Pot Care
Always allow the pot to cool before you wash it or you may crack the glaze and spoil the pot. After use soak the pot in warm soapy water then scrub as needed with a non-abrasive pad. Make sure to rinse the pot well and allow to air dry. If you don't use your pot very often then you may want to use the "seasoning" process each time before use.
Rick Bayless mentions that some cooks suggest rubbing the unglazed pot exterior with a clove of garlic to eliminate some of the "earthy" taste. Many cooks would agree however that the reason to use the pot is for that very reason.
Never, ever put your clay pot in the dishwasher!