TO BECOME HARD AND ROCK-LIKE, CLAY MUST BE "FIRED." THIS MEANS THAT IT MUST BE BAKED IN A SPECIAL FURNACE, CALLED A KILN, TO A MINIMUM TEMPERATURE OF ABOUT 700O C.
The temperature may be measured in three ways:
1. The color of the kiln atmosphere. We look through the peepholes in the wall of the kiln and simply note the color inside. The atmostphere in the kiln glows red at about 500 oC, then turns orange to yellow to white as it gets hotter. By 1300 oC the atmosphere is white hot and damaging to the naked eye.
2. Pyrometer. This is a thermometer capable of registering high temperatures.
3. Cones. Cones are the most accurate method because they measure the effects temperature has on the clay over time, the "working temperature." Often clays and glazes are referred to by the cone number to which they are best fired, ie. a "D9 glaze," a "D8 porcelain," etc. The cones themselves are small pieces of specially formulated clay which begin to melt at specific temperatures. They are placed inside the kiln so that we can see them when we look through the peepholes while the kiln is being fired. When the cone begins to melt and bend over we know that the specific temperature has been reached.
WHAT HAPPENS AS CLAYS AND GLAZES ARE FIRED
100oC Water boils.
100-200oC Clays lose "mechanical water."
200oC Typical kitchen oven baking temperature.
374oC "Critical" temperature of water. Chemically combined water leaves clay.
500oC Red glow in kiln.
573oC "Quartz inversion" A relatively sudden change in the clay causes quick expansion.
SLOW, CAREFUL FIRING UNTIL 600oC
800oC D015 All organic matter in clay burns out by this temperature.
800-1000oC D015-07 Orange color in kiln. Low-fire earthenwares and lowfire lead glazes mature. Normal firing temperature for red bricks, flower pots.
1000-1160oC D07-1 Yellow color in kiln. High-fire earthenwares mature. Feldspars begin to melt.
1170-1190oC D3-4 Bright yellow-white color in kiln. Mid-range clays and low-fire stonewares mature. High iron-content clays begin to melt.
1250-1285oC D7-9 White color in kiln. Stoneware clays vitrify, feldspathic glazes mature.
1285-1350oC D9-13 High-fire stonewares, porcelains vitrify.
MOST STUDIO POTTERY IS FIRED TO D9 OR LOWER.
1712oC Silica melts
2050oC Alumina melts