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Page Contents:
-Porcelain
-Stoneware
-Earthenware
-Glazes
-Unglazed spots
-Crazing
-Specks
-Use and care
-Health concerns
-Lead in ceramics
Related Pages:
-Tutorials
-Fantes.com Main Page
What are Ceramics?
Ceramics and pottery are general terms
for utensils made with clay.
Porcelain, stoneware and earthenware
are categories of clays.
Glazes are often applied to their surface to seal, smoothen
and color them.
Firing is the process of heating the clays and glazes to
harden them and make them more functional and longer lasting.
Porcelain
Porcelain is a clay body; a combination of fine china clay, ballclay,
feldspars and the like. When fired, it becomes very hard and strong and
usually translucent. It is normally very white and has a very smooth surface
when glazed. Unglazed porcelain is referred to as bisque or biscuit.
Hard-paste porcelain is the standard, soft-paste porcelain is not as dense
nor as white or translucent, and bone china uses bone ash to increase its
translucence. Porcelain clays lack iron impurities and are ground to very
fine particle sizes, which contributes to their higher density.
Porcelain is not reactive, so virtually any food can be cooked or stored in
it. With ample liquid inside, it can usually be used on the stove, in
conventional and microwave ovens, and even under the broiler. Freezer safe.
Virtually non-stick, and dishwasher safe.
Stoneware
Stonewares are high fired ceramics (usually containing fireclay, which
adds to their strength) often made of clays that are not highly refined.
They can be brown, buff or white, and commonly have some specks and some
particulate material such as sand or fine grog. Stonewares are vitreous or
semi-vitreous, not translucent.
They can be used in conventional and microwave ovens. Some stoneware can
also be used over an open flame. Some is dishwasher safe; we recommend
washing by hand.
Earthenware
Earthenware is a clay fired at low temperatures where it does not become
vitreous. Earthenware is porous and therefore not as strong as stoneware and
porcelain. Glazes are usually very bright colored and if the glazes are
properly chosen, earthenware can be quite strong and functional. Earthenware
glazes will never be as hard as a porcelain or stoneware glaze surface.
Therefore an earthenware glazed pot or plate will scratch or chip more
easily than the harder surface of porcelain or stoneware. 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.
Earthenware is usually best for conventional and microwave oven use, though
some can also be used on top of the stove. If glazed, it becomes non-porous
and can also be used to cook liquids. Washing by hand is generally
recommended.
Glazes
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 utensil's 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 the glaze's suitability and behavior.
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, the
area that rests on the unglazed or stilted surface or shelf of the kiln, is
unglazed, because otherwise the glaze would bond to the kiln shelf during
the firing process. The most expensive ceramics sit on the points of little
stands 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. An
unglazed foot will absorb water from washing and can leave a water ring on
furniture if not completely dry.
Unglazed spots or bubbles can sometimes appear in other areas of the
pottery, caused by improper glazing, 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.
Crazing
Crazes are small cracks in the glazing 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.
Specks
Usually found on stoneware, earthenware, and low-quality porcelains, 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.
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
porcelain 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. Porcelain wares, with the exception of thin, fragile pieces,
take to dishwasher use in the same way as a glass. Because of the dense
nature of porcelain, discoloration of bare (unglazed) spots can usually be
cleaned with detergent and nylon scrubber.
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's the clay that does most of the absorbing. Glazed clay
products produced and imported 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 products you bring
in from other countries; better to use them as flower pots instead.
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.
 - This triangle is
required to be displayed by California law, on or next to pottery that has
been tested and found to leach lead into food above California's Proposition
65 warning levels. The pottery may have been tested to be within the safe
guidelines established by the Food & Drug Administration, however the
standards established by Proposition 65 are significantly stricter than
those of the FDA. Unfortunately, if the importer or manufacturer of the
pottery has less than 10 employees, they are exempt from displaying the
triangle warning, so be wary of where you shop. When you see the triangle
displayed, it is to help you make an informed choice.
You will find links for additional information on our Kitchen Info Links page under "Lead." We use
LeadCheck swabs to
check for suspected lead content. And we have also used Brandywine Science Center (610-444-9850) to
test suspect imported ceramics for lead and other leachates.
(Ceramics_Guide.PDF
99.2KB)
(Also see Ceramics
Library for more information.) |
