PPP: DRYING & FIRING<br>Questions & Answers
QuestionWhat is a firing cycle, and how can I use it to improve my products?
AnswerThe term "firing cycle" can refer to the time required to heat and cool ceramic products in a kiln, or it can specifically refer to the time vs. temperature curve used during firing.
In some small electric kilns, the only control is how much electricity to use and for how many hours. As a result, many potters who began their pottery production careers using these types of kilns tend to think of the beginning of the firing cycle as the time the kiln is turned on, and the end of the firing cycle as the time the kiln is turned off. However, this only describes the heating portion of the cycle. When firing any ceramic or glaze, it is also important to consider the cooling portion of the cycle. A complete firing cycle includes the entire temperature profile from "cold to cold."
A Sample Firing CurveFigure 1 shows a sample firing cycle to demonstrate the different parts of the cycle or "curve," as it is sometimes called.
Segment 1 is the "candling" or preheat that is sometimes used by potters with larger gas kilns. In this phase, the kiln is gradually heated to around 100-200 degrees F before the main firing begins. However, for potters who use well-designed dryers and kilns, this step typically isn't necessary.
Segment 2 is the "binder burnout" phase, which removes not only the binder materials, but also the organic materials that naturally occur in most clays. The temperature at which the organics burn off depends on both the nature of the organic and the speed of the temperature increase. This phase typically begins at 400 to 450 degrees F and lasts until about 900 degrees F, or until all of the binder and organic materials are removed from the interior of the ceramic body.
Complete burnout of the organic materials is one of the most important steps in firing ceramic ware. If the organic materials are not removed before the ware reaches 900 degrees F, these materials will "crack" and turn into elemental carbon, which is very difficult to remove and often creates a black core in the ware. If you notice black coring in your products, try slowing down this portion of your firing cycle to ensure that all organic materials are adequately removed. You can also try a different setting pattern to ensure that larger, thicker pieces experience a complete burnout of organic materials at this stage.
Segment 3 is the oxidation phase, in which the ceramic materials are oxidized to complete their chemical reactions. This phase typically occurs between 1400-1800 degrees F; however, the actual temperature range will vary depending on the type and combination of materials used in the ceramic body. Improperly oxidized ceramics might exhibit structural weakness, or a discolored or "white" core. If you notice these problems, try slowing the rate of heating in this stage.
Segment 4 is the final heating before the soak temperature. It is shown as a curve to illustrate the natural heating rate most often encountered when firing ceramic ware. It is often possible to heat a kiln at a faster rate in this stage, but this will require more energy and will therefore increase the cost of producing your products. Additionally, unless a sophisticated temperature controller is used, trying to ramp up to the soak temperature too quickly can cause the temperature to overshoot the final mark and overheat the ware.
Segment 5 is the "soak." In most industrial kilns, the temperature at the peak of the curve is held constant for a certain amount of time to allow the temperature to penetrate or "soak" into the cooler parts of the load. Typically, the larger the kiln, the more necessary this procedure becomes.
The ideal soak temperature can be determined by placing pyrometric cones in various parts of the kiln. Try firing a test load at a slightly lower temperature (about 10 degrees) and holding it at that temperature for an additional 15 minutes. Look at the results of the cones in this firing. The extra time at temperature should make the cones in the coldest part of the kiln drop a little farther, while the cones in the hottest part of the kiln remain about the same. Repeat the process until you are satisfied that the temperature spread (as evidenced by the cones) is appropriate.
Segment 6 is the rapid cooling part of the cycle. Most ceramics are still flexible at this stage due to the presence of a molten glass binder. Until this binder is sufficiently hardened (generally between 1200-1600 degrees F), the ceramic can be cooled very rapidly without being damaged.
Segment 7 is the annealing phase and is another very important part of the firing cycle. As the molten glass binder begins to harden, it often develops internal stresses. These stresses can be exacerbated in body formulations that experience quartz inversion (i.e., quartz crystals within the body that increase or decrease in size during cooling). In some cases the quartz itself cracks, causing the ware to crack. However, it is usually the stress placed on the glass binder that causes the biggest problems.
The purpose of the annealing phase is to slowly lower the temperature of the glass binder (and/or quartz) so that the stresses can gradually be relieved as the product cools. Annealing generally takes place when the glass bond is between 1400 and 1000 degrees F and the kiln temperature is between 1600 and 900 degrees F. If the cooling progresses too quickly at this stage, the internal stresses will cause the ware to crack.
Segment 8 is the final cooling phase and is drawn to show the natural cooling curve that can be seen in almost any kiln. Faster cooling can be done at this stage without damaging the ware, but large air supplies-and therefore higher energy-in the form of cooling fans will be required.
In some cases, potters line their kilns with heavy refractories to slow the cooling rate to achieve a desired body or glaze effect. However, heavy refractories consume a significant amount of energy. If slow cooling is needed, it is far better to use a kiln with lightweight refractories (such as ceramic fiber lining) that has been designed to enable the operator to control the cooling part of the cycle.