Some clays can be dried, glazed, decorated and fired in a single sequence but many quality factories prefer to first apply and fire a transparent glaze (glost) prior to decorations for easier application of patterns. During a second firing, a hand-painted or decal decoration penetrates the glaze to become very durable.
If lead oxides are part of a glaze formula, they will leach
into food unless the firing temperature is sufficiently
high to burn out traces of lead.
The composition and firing temperature of each glaze must ‘fit’ each clay body because clay shrinks as it dries to the greenware state —and shrinks again an average of 12% or more when fired in a kiln. Glazes must shrink at a comparable rate.
Different colors or applied ornamentation may require firing at different temperatures so each piece may be fired several times before completion. Glazes seal the porosity of clay and protect decals or painted designs from wearing away. Labor costs increase every time a ceramic body is handled for decoration or firing.
Opaque tin-oxide glazes, usually white, were applied to earthenware bisque to cover over the color of the clay —and then hand painted with colorful designs over the glaze. Well known indigenous names for opaque tin-oxide glazes include:
Tin glazed earthenware has been made since 1100 in Faenza, Italy where even the street signs are ceramic. When the French began to make similar wares, they named it faience. It is still made in the oldest pottery factory in St. Clement, France.
Historians trace tin glazes and design influences on ceramics to the Moors who brought these techniques to Spain. By the 15th Century, tin-glazed pottery from the island of Majorca, made in Arabic tradition, inspired Italian ceramic wares named maiolica.
The name continues for soft-fired Italian earthenware covered with opaque glazes containing tin oxides which masque the color of the clay and allow for strong colors in a range of decorative motifs including original Renaissance pictorials painted by by hand in an earthy, robust style.
Originated by Minton in the 1860’s, English earthenware with high relief designs and covered with brilliant colored shiny lead glazes is majolica (with a j)— borrowed from—but not to be confused with maiolica.
Tin glazed earthenware made by Italian potters in the Netherlands mid-16th Century is delft. In the production of Delft, an opaque white glaze is applied to earthenware and overglazed with a hand painted design, commonly monochromatic blue, inspired by imports of blue and white Ming porcelain.
These Dutch ceramics remain popular.
Dishes used for preparing or eating food must be LEAD FREE.
For centuries, lead has been used in ceramic glazes to help glaze particles melt blemish free. Glazes must be formulated and fired at temperatures sufficiently high to destroy traces of lead.
Improperly fired, lead can leach into food. We’ve known for decades that severe health problems have arisen from lead in paint, the same problem that arises from dishes that leach lead into our food.
Most domestic manufacturers have stopped using lead in their glazes. Dishes made before 1971, but not all, are more likely to contain dangerous levels of lead although it is rarely found in plain white dishes.
The FDA recommends home test kits to determine if dishes are leaching lead. They’re available in most hardware stores–and detect only the presence of lead—not the amount.
Before buying ceramic dinnerware,
ask about lead content and require proof the dishes are safe.
Pat Breen: EYEWITNESS TO QUALITY
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