The heat of the kiln can have a dramatic effect on a glaze. In my own studio I usually have several kilns at my disposal, so I only obtain results pertaining to these specific kilns. If it were possible to farm out glaze tests to kilns of all types and sizes, people would be surprised at the varied results obtainable from a single glaze recipe. You may use a small test kiln for developing glazes, and use your large kiln for firing ware. But why don’t the final glazes on the ware look the same as the test results? This is because of the differing heat work on glaze materials during the melt and then the cooling of the glaze.
Glaze Three Ways
I once saw three stoneware pots glazed in the same feldspathic iron glaze, and fired in the same kiln. When the pots were removed from the kiln, the first was a honey color, the second a black, and the third an opaque, satin olive green known as a tea dust. While they had all been coated in the same glaze, each had been fired in a different chamber of a three-chamber climbing kiln. The first pot was in chamber one, which was fired on a normal firing cycle but was crash-cooled (air was pulled through the first chamber into the next). As chamber two reached temperature, hot air was pulled through to help the third chamber reach temperature. The black pot in chamber two thus experienced a slower firing but also a slower cooling than the first. The tea-dust pot in the third chamber cooled slowly, allowing crystalline growth in the glaze and the development of a satin green.
If a potter shifts from using a brick kiln to a fiber-insulated kiln, his/her glazes on the whole will shift as well. Firings will be quicker, both in the time taken to reach the required temperature and in the cooling. Glazes may lack the same depth and the surfaces will often be glossier. By firing a little higher or by increasing the flux content, the finished quality can be adjusted to be closer to the original.
Glazes developed for fiber kilns will also take on a different character when fired in a brick kiln. In figure 1a and 1b, the two tiles are glazed with two variations of the same base glaze recipe. The glaze on the upper half uses nepheline syenite (a lower melting point than potash feldspar), while the glaze on the lower half uses potash feldspar. The left tile (1a) was fired in a brick kiln, so cooled more slowly than the right tile (1b), which was fired in a fiber kiln. The slower cooling prompted iron crystals to form.
The effect of heat on glaze materials can easily be seen through the use of pyrometric cones. While a pyrometer measures air temperature within a kiln, cones measure the work done by heat energy. Not surprisingly, this is called heat work, a term denoting the combined effect of time and temperature on ceramic material. For example, prolonged heating of a ceramic body at a lower temperature may result in the same degree of vitrification as a shorter period at a higher temperature. On most cone boxes there are two or more columns informing the user of the temperature at which the cones will bend, based on degrees per hour of heat input. The table in figure 2 refers to cone-bending temperatures for Orton cones: Cone 9 at 270°F per hour, the pyrometer would read 2332°F when the cone bends. Cone 9 at 108°F per hour, the pyrometer would read 2295°F when the cone bends. Cone 03 at 270°F per hour, the pyrometer would read 2014°F when the cone bends. Cone 03 at 108°F per hour, the pyrometer would read 1987°F when the cone bends.
The faster you fire the kiln, the higher in temperature you will need to go. You just have to fire to a higher temperature to get the same amount of heat work acting on the ceramic material. Note: Materials melting in a reduction atmosphere will have a lower melting point than those fired in oxidation.
Time Effects on Cones
This brings us to how important cones are in every firing; it’s the cones, not the pyrometer, by which you should judge whether or not the firing is completed. Be aware of the heat work in firing. Potters who reach cone 9 may soak their kilns for 30 minutes. The reading on the pyrometer may stay at 2336°F, not increasing or decreasing in its temperature, but they will see cone 10 bend. Soaking for a period of time allows the heat to continue working on the glazes and cones and this extra heat work causes the next cone to bend. If you don’t have cones in the firing, you may not realize what exact cone you have fired to, and this will affect the glaze. Pyrometers are excellent at recording the temperature’s rate of climb in a firing, but should not be the final guide.
All these factors go into altering the fired glaze recipe we started with. Some glazes, such as high-feldspar glazes, respond to a slower firing in the melting of the glaze.
The cooling of the kiln can be just as critical, as in the case of crystalline glazes, which require slow cooling through the temperature zones in which the crystals develop; the temperature needs to be held at the critical point. It’s worth putting the glazes you use most frequently into kilns of different sizes and types to see how important the firing cycle is to them.
Another way to look at a glaze melt is to examine an oil-spot glaze. This is a high-iron glaze (fired in oxidation) which, as the glaze begins to melt, creates bubbles, similar to sugar melting. On the tile in figure 3a, you can see the craters in the glaze that occur from bubbling at around 2264°F. As the temperature is increased, the surface heals over, leaving the iron dots of concentrate that give the glaze its name, shown in figure 3b. The glaze was fired to cone 7–8 in oxidation. If fired to a higher temperature, the iron is taken back into the glaze melt, and the result is black.
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