Information on Ceramic Glazing

A glazed ceramic pot

Ceramics are made of non-metallic, inorganic materials and are formed using heat. The most common forms of ceramics are made of clay; fine clay is used in porcelain, while unrefined clay is used in stoneware. Earthenware is fired at low temperatures. Any type of ceramic can be covered in a glaze. A glaze is a liquid that seals, colors and smooths the surface of the ceramic after it is fired.

History

The earliest pots known date to around 10,000 B.C. These earliest pots were earthenware and had no glaze on them. Egyptians were the first to produce ceramics with glazes around 5,000 B.C. The Chinese began developing high temperature ceramics around 1,400 B.C. and by 700 B.C. they had created a glaze. Modern techniques were introduced by Michael Cardew during the 1960s.

Types

There are three basic types of ceramic glazes: opaque, matte and transparent. Opaque glazes can cover the color of the piece to which it is applied, but if the amount of glaze applied to the piece is small enough, some of the quality of the ceramic will show through. Matte glazes are made up of crystals that form in the glass, giving it a less glossy finish. Transparent glazes are clear glass that sometimes has a small amount of color added to it to give the piece a transparent color.

Unglazed Spots

All ceramic pieces have unglazed spots. These spots can be large or small, depending on the quality of the piece. The bottom, or foot, of the pot is commonly left unglazed because it comes into contact with the kiln and if it were glazed, the glaze would adhere to the kiln shelf. Expensive ceramics sit on stands with just points touching the piece while they are being fired, so they have smaller unglazed spots, but they can be seen or felt.

Impurities

There can be different forms of impurities in ceramic glazes. Unglazed spots, other than those intended on the base of the piece, can be the result of gas bubbles or improper glazing. Crazing is another form of impurity and is characterized by small cracks in the glaze. Many things can cause crazing, including a different expansion rate between the glaze and the ceramic while firing and the ingredients of the glaze. Specks are iron or other materials in the clay and can show up after the glaze has been fired.

Considerations

There are a few considerations when glazed ceramics are used around food or drink. Ceramics with unglazed spots, other than those on the base of the piece, or crazing should be avoided because the spots or cracks can harbor bacteria and make the piece difficult to clean. Some glazes are made of lead or other toxic material, such as cadmium, and should be avoided for use around food.

References

• Fantes' Kitchen Wares Shop: Ceramics
• Ceramics Resource DePaw University: History of Ceramics
• Scarva Pottery Supplies: Glaze Types