Understanding firing schedules and firing ramps is 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 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).
Gaining Heat Gets more Difficult 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 the 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 with 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 effect energy consumption, but also the ramp. A kiln that can ramp at 500°F/hr when starting at 200°F may only be able to reach a ramp of 120°F/hr when the kiln is at 2,000°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 warmup with temperature holding at 150°F
- two hours with ramp at 200°F/hr
- two hours at 300°F/hr
- ramp at 400°F or best rate until 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 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. Heat will be applied just enough to slow the cooling.
- two hours ramping at 150°F/hr
- three hours with ramp at 400°F/hr
- ramp at 120°F until temperature reached
- ramp at - 200°F/hr until kiln cools to 500°F.
A Cone 9 Reduction Glaze Firing Schedule
- two hours with ramp at 150°F/hr
- ramp at 400°F/hr until cone 8 reached (about 2240°F)
- soak with a ramp of 0-40°F/hr until cone 9 reached (about one half hour).