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FREQUENTLY ASKED QUESTIONS


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CERAMICS

What is pottery clay?

Pottery clays and ceramic clays, also called moist clays, are virtually the same; they are all made with a water base and need to be fired in a kiln to vitrification to become permanent. The primary types of clay bodies are earthenware and stoneware which turn red, brown or white when fired. The earthenware clays generally vitrify at a lower temperature than the stoneware or porcelain clays. (Vitrification is the process whereby a clay body is heated until it becomes non-porous.) Stoneware clays fire to a higher temperature and is, therefore, a harder material than earthenware.

What are clay firings?

Simply put, there are two types of firing: bisque firing and glaze firing, both done in kilns at temperatures ranging from cone 014 to cone 10. Bisque firing is the initial of first firing of a dried piece of pottery or ceramic, also known as greenware, performed before glaze firing. Glaze firing is the second firing; glaze is a colored liquid material that can be matte, gloss, or speckled. It is a silica that when heated fuses together and forms a glass-like coating.

What does ceramic firing 06-7 mean?

It stands for the cone firing ranges of the clay. The “06” is the bisque firing temperature of a ceramic clay body (1841°F) and the “7” is the vitrification or maturing temperature of the clay (2280ºF). The vitrification and glazing temperatures are not necessarily the same. A clay that has a cone 7 vitrification can be glazed fired at cone 06 with no adverse effects. At the vitrification temperature the clay becomes nonporous.

If the clay is dry what can I do?

Sometimes when water-base moist clay has not aged properly or does not contain enough water it will seem dry. Add a little water and knead it into the clay body (25% to 28% of water to clay flour mix is a good ratio), then let the clay sit in the closed bag for a few weeks.

If the clay is too wet when removed from the bag what can I do?

This occurs naturally with normal condensation of water-base clay kept in a container, more often with porcelain and stoneware than with earthenware clays. Roll the clay in several layers of newspaper until it dries up a little.

Why does the clay crack when I bend it?

This is usually indicative of what is known as short clay. Add a little water and age it a few days.

The color changes between separate orders of the same clay; why is this?

clay is mined from the earth in different areas of the country. The clay may vary in color but not generally in texture, composition, or workability. As long as the chemical composition remains the same, the clay should work and fire the same.

When cooled why does the glaze have cracks like a spider web?

This is not uncommon and can be caused by several things. First, the clay and glaze may not be compatible. For future firings make test tiles trying different glazes with different clays to determine compatibility. Second, the glaze may have cooled too fast and constricted. This occurs when the kiln is opened too soon before the pieces are ready. Third, the glaze may have been applied too thinly or all coats may have been applied in the same direction rather than altering the direction with each coat.

Why are there glaze bubbles on the clay?

This can be caused by incompatible clay and glaze or the glaze may have been fired at too high a temperature. Too much glaze may have applied which will happen when the piece is dipped rather than applying the glaze by brush.

Why does glaze drip off the piece and fall on the shelf or bottom of the kiln?

This is a result of over-glazing. The dried glaze can be sanded off before firing if there is too much of a build up. The glaze on the shelf presents a small problem. It can sometimes be chipped off, using a wood carving or stone carving tool and hammer without causing damage, but not always.

What causes crazing, running, bubbling and flaking?

These mishaps, faced by all ceramists, can be caused by firing with incompatible clay and glaze, by cooling the kiln too rapidly, by firing the kiln too high, or by over-applying the glaze. Don’t be discouraged; just follow the instructions and try again!

What tools can I use in Ceramics?

Most of the tools that are used for clay modeling can be used for ceramics and pottery, but there are also ceramic tools specifically designed for ceramics and pottery. They include: loop tools, sharpened steel tools used during the trimming and footing process; the needle tool used to level the excess clay from the top of a pot; fettling knives, great for scraping, cutting and removing dried clay from seams and other areas; toggle clay cutters, a 16-inch wire attached to a wooden handle at both ends, essential to cut sections of clay in the wedging process and for removing thrown pots from the wheel. Other essential ceramic tools are the elephant ear sponge and potter ribs.
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