Ceramic Art Trends, Tools, and Techniques for Potters Worldwide

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Firing

Understanding "Cones"

If you've often wondered about the difference between small "junior" cones and large "senior" cones, take a look at Orton Firing Foundation's "Cone Equivalents" chart. You'll see that cone 6 translates into 2291 degrees F (1255 degrees C), if you're using small junior cones in your kiln sitter.

But wait a minute! If you're shutting off the kiln manually by viewing the large, senior cones in a cone plaque, your kiln peep will reveal senior cone 6 bending at somewhere between 2194 degrees and 2232 degrees F, depending on how fast your fire.

The reason why Orton junior (small) cones melt at a greater temperature than corresponding senior witness cones is because junior cones are designed to bend under the pressure of the kiln sitter rod. Therefore, junior cones should not be used as witness cones. If you use the junior cone as a witness cone, you will overfire.

Because everything in the kiln -- pots, shelves, posts, bricks -- must heat to the desired firing temperature, you will obtain more accurate results if you fire slowly. (If you raise the temperature too quickly, the thin-walled pots might reach cone 7 or cone 8 before thick shelves and kiln sitters or witness cones reach cone 6.)

What is Raku?

"Raku" refers to an ancient Japanese technique used for quick-firing pots to temperatures of around 1800 degrees F, removing them with tongs from the kiln when red hot, and quick-cooling in the open air.

Over the past 40 years, the ancient firing method has been expanded to include post-firing reduction by placing the pot into a combustible material such as sawdust or shredded paper, at which point the pot bursts into flame and is covered with a metal container to allow it to smoke. The burning of the combustible material produces enough smoke to color unglazed clay completely black. The pot is often quick-cooled by submerging into a bucket of water. Sometimes a "crackle" glaze is used, which is extremely temperature-sensitive and cracks when cooling. During the smoking, these cracks are colored black by the smoke, while the glazed area retains its proper color. The most popular crackle effect is usually a white surface with black crackle, generally achieved by using a white clay covered with a clear or white crackle glaze.

Other raku glazes are wide and varied in texture and color. Often seen are copper lusters. These can range in color from an "oxidized" bluish green (where more oxygen is present because the pot comes into contact with less burning material) to a "reduced" red or shiny copper color (achieved when so much burning is taking place that most of the oxygen is pulled from the glaze).

Copper matte glazes are also commonly used for raku, as they offer intriguing, rough textures and usually feature a full spectrum of highlights like magenta, blue, and gold on a copper surface.

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