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.