Understanding firing schedules and firing ramps are essential for good results when using a kiln. Firing pottery is not like turning on an oven and baking cookies or bread. It needs to be considered as a fairly complex process which has specific stages.
The firing schedule is the set of pre-determined stages the kiln moves through from the time it is turned on to the time it has either turned off or has cooled enough to unload. These changes are defined by changes in the kiln atmosphere or the firing ramp.
The ramp is the rate at which the kiln's temperature is changing. This can include decreasing (cooling) as well as increasing temperatures.
Just as a car's speed on the highway is measured in miles per hour (mph) or kilometers per hour (kph), the ramp rate is measured in degrees per hour (F/hr or C/hr).
Heat Is Harder to Gain the Higher the Temperature
There is something to note when you are firing your clay, and especially when you are considering what clay body to use. That is the fact that the hotter the kiln gets, the harder it is to raise the temperature further.
For example, let's say an electric kiln takes 50 kilowatts to raise its temperature by ten degrees when the kiln is at 150 degrees. However, when the same kiln is at 1,000 degrees, it will now take 300 kilowatts to raise the temperature by ten degrees. The same rule applies to all heating sources: the hotter the interior of the kiln is, the more energy it will take to raise its temperature further.
Not only does this affect energy consumption, but also the ramp. A kiln that can ramp at 500 degrees F/hr when starting at 200 degrees F may only be able to reach a ramp of 120 degrees F/hr when the kiln is at 2,000 degrees F.
A Typical Bisque Schedule With Ramps
As an example, let's look at a typical bisque firing schedule. The written schedule would be:
- Overnight warm up with the temperature holding at 150 degrees F
- Two hours with the ramp at 200 degrees F/hr
- Two hours at 300 degrees F/hr
- Ramp at 400 F or best rate until the temperature reached
Two more stages could be written out but are generally understood. These are the kiln shut-off when the desired temperature is reached, and a normal cool down in which the closed kiln is allowed to cool at its own rate. Generally, normal cooling will take about the same amount of time as the main firing.
A Cone 04 Glaze Firing Schedule With Down-Firing
Because the bisque firing has already changed the clay into the ceramic material, glaze firings can increase temperatures faster at the beginning and middle of the firing. For this example, we'll also see that the kiln will be down-fired. The heat will be applied just enough to slow the cooling.
- Two hours ramping at 150 degrees F/hr
- Three hours with a ramp at 400 degrees F/hr
- Ramp at 120 degrees F until the temperature reached
- Ramp at -200 degrees F/hr until kiln cools to 500 degrees F
A Cone 9 Reduction Glaze Firing Schedule
This is an example of a typical cone 9 reduction firing, with the reduction taking place during a soaking period at the very end of the firing.
- Two hours with the ramp at 150 degrees F/hr
- Ramp at 400 degrees F/hr until cone 8 reached (about 2240 degrees F)
- Soak with a ramp of 0-40 degrees F/hr until cone 9 reached (about one-half hour)