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Ceramics and pottery are general terms for wares or utensils made with clay. Earthenware is a category of clay. Earthenware is a clay fired at low temperatures where it does not become vitreous. Glazes are usually very bright colored and if the glazes are properly chosen, earthenware can be quite strong and functional.
Earthenware is usually best for conventional and microwave oven use. If glazed, it becomes non-porous and can also be used to cook liquids. Washing by hand is generally recommended.
Terracotta refers to a type of earthenware that contains red burning clay. Majolica is terracotta with an opaque white glaze, usually decorated with a colored overglaze, and is stronger than terracotta.
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Glazes
Glazes are often applied to their surface to seal, smoothen and color earthenware.
Glazes are liquids applied to clays that, after hardening (firing), they seal, smoothen and color their surface. Many compounds are used to make glazes, such as silicates, aluminates, oxides, tin, sodium, potassium, lead, iron, copper, and many more. The recipes made from such compounds usually take in consideration the utensils intended use, matching thermal expansion properties between the clay and the glaze ingredients for longer useful life, and color. Myriad factors, both natural and controllable ones, can alter a glaze suitability and behavior.
Some glazes that pass the government's tests are still unsuitable for dinnerware use. Matt finishes and textured glazes do not wash easily and are undesirable because they can trap food particles.
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Unglazed Spots
Unglazed spots are common to all ceramics, and are found in areas that do not affect the usability of the pottery. The "foot" of a pot or bowl is the area that rests on the surface or shelf of the kiln, is normally unglazed, because otherwise the glaze would bond to the kiln shelf during the firing process. It is not uncommon for imported ceramics to be "dry footed". By using this method it reduces the cost of producing ceramics, more items can be placed in the kiln and it takes less time to complete the piece. An unglazed foot or "dry foot" piece will absorb water from washing and can leave a water ring on furniture if not completely dry. In time the glaze may flake or peel or even crack. It is not fit for food or beverages.
Unglazed spots or bubbles can sometimes appear in other areas of the pottery, caused by improper glazing, contamination to the earthenware (grease or oil), or by gas bubbles in the clay or glaze. Ceramics with such unglazed spots should be avoided for food contact, as the spots can harbor colonies of bacteria. It is recommended not to purchase ceramics with this type of defect.
The most expensive ceramics sit on the points of little stands or "stilts" in a kiln, so that more of the surface will take the glaze; the spots are evident if you look closely or run your hand along the bottom. By using stilts your piece is competely waterproof which is most desireable. Even though this method is more expensive it is worth the difference in price.
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Crazing
Crazes are small cracks in the glaze of ceramics. They are caused by many factors, such as a different thermal expansion rate between the glaze and the clay, glaze ingredients, and the firing process. Whenever possible, crazed (and cracked surface) ceramics should be generally avoided for food contact, as the cracks can harbor colonies of bacteria; using a bit of chlorine bleach or lemon to clean the cracks will help to rid bacteria, however it is recommended to avoid using these items for food or beverages.
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Specks
Usually found on stoneware & ceramic earthenware pottery, dark specks in the glaze can be iron or other minerals that are inherent parts of the clay. This is normal and will not usually affect the usability or longevity of the pottery.
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Use and Care
Caring for ceramics is easy. The glazes that cover the clay protect it from discoloration. Glazes that are approved for use with food do not react with acidic foods, and can be used to store any food safely. Though earthenware pottery can easily take the temperature and harsh detergents of a dishwasher, we recommend washing all ceramic wares by hand, especially to prevent accidental damage from other utensils beating against them in the dishwasher.
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Health Concerns
There are no known adverse health effects from using unglazed clay in cooking, primarily because of the limited contact of food with the clay, and the fact that it is the clay that does most of the absorbing. Most glazed clay products produced into the US and Canada are deemed safe through a series of tests that manufacturers and importers are required to submit to the government, proving the quantities of cadmium and lead to be within acceptable levels. Beware of clay/ceramic cooking or tableware products you bring in from other countries, especially countries that have little or no regulatory standards; better to use them as flower pots instead.
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Lead in Ceramics
Lead can be found all around us in dishes, fine crystal, painted walls and woodwork, toys, furniture, antique varnishes, solder, dust and soil. The effects of lead poisoning are cumulative throughout our lifetime, therefore it is important to limit our exposure to it.
In the ceramics industry, some lead glazes are still used to color or decorate and to smooth the surface of ceramic products. As long as the clay and glazes are compatible, and these glazes are properly fired (at a high enough temperature and for an appropriate amount of time), the lead is not likely to leach through the surface.
With constant use and scrubbing, ceramic products can wear down over time, and may allow lead to leach through. Hot and highly acidic foods, and prolonged time of contact, will increase lead leaching from such damaged surfaces. Antique, highly decorated ceramics are the most likely to leach lead.
You might be able to visually detect lead leaching if ceramic items show a dusty or chalky gray residue on the glaze after they are washed. When testing for lead content, be sure to test the surface that comes in contact with the food. If you see any signs of a dusty or chalky residue on any piece used for food immediately discontinue using it.
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