-About Unglazed spots
-About Health concerns
-About Lead in ceramics
-About Other Ceramics
Guide to Ceramics
What are Ceramics?
Defined as any non-metallic, inorganic material that is formed by the action
of heat. Clay is the longest known and most common of the ceramics.
- Ceramics, Pottery
General terms mostly used for utensils made with clay.
- Porcelain, stoneware and
Categories of clays, explained below.
Paint-like liquids applied to the clay surfaces, hardened under heat to
seal, smoothen and color. More below.
The process of heating the clays at high temperatures to harden them. The
process may be repeated, especially after glazes are applied.
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
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, under the broiler, and even in the freezer, though thermal shock is to be avoided. It is virtually non-stick, and dishwasher safe. Discoloration of bare
(unglazed) spots can usually be cleaned with detergent and a nylon scrubber.
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, but not translucent.
They can be used in conventional and microwave ovens. Some stoneware can
also be used over an open flame. Glazed stoneware is non-porous and can also
be used to store foods. Some stoneware is dishwasher safe; we recommend
washing by hand, both to avoid harsh detergents and shock.
Earthenware is a type of clay that is fired at low temperatures so as not to make
it vitreous. Its porosity makes it less strong than
stoneware and porcelain, however it makes it better at retaining and evenly distributing heat. The type of clay and firing temperature determine its usability in conventional and microwave ovens, and on gas stovetops.
Unglazed earthenware absorbs foods cooked on it, and is often seasoned before use to help seal the pores and help prevent absorption. The surface of glazed earthenware is sealed, and no absorption takes place, making it more stick-resistant.
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 over-glaze, and is stronger than
Washing by hand is generally recommended.
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 tensile strength and thermal expansion properties between the clay and the
Many factors, both natural and controllable, can alter the glaze's
suitability and behavior.
Glazed surfaces are apt to craze when used in hot or cold applications, and some glazes made for use on utensils intended for use on open flame are micro-crazed on purpose during the firing process.
About 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.
Crazes are small (usually microscopic) cracks in the glazes of ceramics. They are caused by
many factors, such as a different thermal expansion rate between the glaze
and the clay, incompatible ingredients in the clay or the glazing, or the firing process.
Glazed ceramics intended for cooking
on open flame, for example, will often craze, however the seasoning that is applied before use is intended to keep them safe for cooking. Some manufacturers intentionally craze the enamel during the firing process.
Whenever possible, ceramics with crazed, and especially cracked surfaces should be
generally avoided for food contact unless they were intended by the manufacturer. As the cracks can harbor colonies of
bacteria, under certain circumstances using a bit of chlorine bleach or lemon to clean the cracks will
help to rid bacteria.
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, unless pitted, the appearance of specks does
not affect the usability or longevity of the pottery.
About Health Concerns
There are no known adverse health effects from cooking in unglazed clay that is intended for food use.
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.
Be wary of ceramic cooking utensils you might bring in from your travels
to non-regulated countries; better to use them as flower pots instead.
About 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
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.
About Other Ceramics
Ceramics that don't use clay include borides, carbides, nitrides,
silicides, and oxides (alumina and zirconia). Most ceramic knives, for
example, are made of zirconium dioxide, a type of zirconia.
These types of ceramics have very different properties and uses from clay
ceramics, and each has its own specific use and care requirements.
(The above Ceramics
Guide in PDF
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.
Helpful External Links:
Library - Definitions and articles from Digitalfire.com
Lead - U.S.
Environmental Protection Agency
LeadCheck - Swab
test kit to detect Lead
for Parents - CDC Parents Guide to Childhood Immunizations
Lead in Tableware
- Great info from CA's Childhood Lead Poisoning Prevention Branch
Lead in the Home -
Alliance for Healthy Homes
Lead Poisoning -
Green & Healthy Homes Initiative
Lead Safe USA -
From the Nat'l Assn of the Remodeling Industry
Lead Testing - Brandywine
Science Lab tests ceramics for lead and other leachates