The primary function of a kiln controller is to help users input (and successfully execute!) their kiln firing schedules…but what is a kiln firing schedule? Below, we’ll be helping you understand kiln firing schedules, as well as how firing schedules differ for materials such as glass, ceramic, pottery, and metal heat treat!
Definition of Kiln Firing Schedules
A kiln firing schedule is a progression of steps, made up of temperature changes over specific time intervals, that a kiln moves through during a firing. Each step of a kiln firing schedule is made up of four components:
- Step #: Also known as a ‘segment,’ step # represents the order in which the steps of the schedule occur.
- Ramp Rate: Measured in degrees per hour, the ramp rate is the speed at which the kiln is heated up or cooled down.
- Setpoint: Measured in degrees, the setpoint is the desired temperature the kiln reaches during each step.
- Hold Time: Also, known as a ‘soak,’ hold time is the length of time (defined in days, hours, or minutes) the kiln stays at a specific setpoint before advancing.
Each of these components determines the properties of the finished ware once the firing schedule reaches completion. Even extremely minor variances in adhering to kiln firing schedules can have a major impact on the finished result, so it’s important to accurately input firing schedules into your kiln controller and to utilize kiln controllers that are able to automatically execute kiln firing schedules with extreme precision.
Example of a Kiln Firing Schedule
Kiln firing schedules, sometimes colloquially referred to as programs or firing schedules, can best be described as the road map the controller uses to execute a firing. While kiln firing schedules can string together as many steps as necessary to achieve the desired firing result, below we’ll be looking at an example of a three-step firing schedule:
Assuming the kiln starts at room temperature, or 70° F, the example schedule shown above will result in a firing that takes 5 hours and 24 minutes to complete. Below is a visual graph representing the firing profile of this schedule:
In this graph, we can see that the kiln follows a 500 degree-per-hour ramp rate from time 0 (when the kiln was started) to 950 degrees (the first setpoint). Once the setpoint is achieved, the controller regulates the temperature to keep the kiln at 950° for 30 minutes.
Once the hold time from the first step is completed, the kiln advances at a rate of 1200 degrees-per-hour to a setpoint of 1425° and holds there for 20 minutes.
Finally, the kiln moves to step three, cooling at a rate of 300 degrees-per-hour down to a setpoint of 700°. Because the hold time at Step #3 is zero, the kiln firing schedule is now complete!
See our article on Alerts and Alarms so you can be notified when your kiln firing schedule reaches certain firing points!
Ramp/Hold vs Time-to Temp Schedules
Kiln firing schedules can also be expressed in different formats. The example above is the common Ramp/Hold format, which can also be described as a Ramp/Soak or Ramp/Dwell schedule. This is the most common kiln firing schedule format, and it is also the format that is supported by TAP Kiln Controllers.
However, kiln firing schedules can also be written in a Time-to-Temp format, which contains all of the same information but prioritizes the timing of the firing as opposed to the temperature of the firing.
When generating a Time-to-Temp schedule, you are, in effect, saying “I want to be at 950 degrees in 1 hour and 45 minutes.” At that point, the controller is responsible for converting the defined “Time-to-Temp” into a usable Ramp Rate. By saying we want to be at 950° in 1 hour and 45 minutes, and assuming we’re starting from 70°, we’ve essentially created a firing schedule with an implied ramp rate of 500 degrees-per-hour.
NOTE: Some controllers that use Time-to-Temp format do not report accurate ramp rate, which can affect outcomes of the firing schedule. For instance, a Time-to-Temp controller might report that your kiln went from 100° to 1250° in one minute, because that was what it was programmed to do, even though achieving that level of temperature change over that time interval simply isn’t possible.
Below is the exact same kiln firing schedule from before written in a Time-to-Temp format:
The firing graph for both formats would look exactly the same – and executing either format would yield the same outcome once the firing schedule reaches completion (assuming the controller was capable of converting the Time-to-Temp into an accurate ramp rate). The only difference is how the kiln firing schedule is expressed. What was defined in three steps in the Ramp/Hold format requires five steps in the Time-to-Temp format, despite yielding the same firing profile.
What Factors Does a Kiln Firing Schedule Depend On?
Kiln firing schedules are dependent on the material/media being fired, as well as the physical capabilities of the kiln. There is no one-size-fits-all approach to kiln firing schedules, as the material within the kiln will require its own unique schedule to achieve optimal results. Later in the article, we’ll be looking at examples of firing schedules for glasswork, firing ceramics, and metal heat treat.
Limitations of Kiln Firing Schedules
Now that you know the components of a kiln firing schedule, you should also understand the limitations. The physical capabilities of the kiln dictate certain physical boundaries that cannot be overcome. The material of the kiln, chamber size, power rating, and thermocouple gauge all contribute to the kiln’s demonstrated performance.
As kilns approach higher temperatures, their ability to heat at defined ramp rates begins to fall off. A kiln that can heat at a ramp rate of 3600 degrees-per-hour while at 200° will likely be unable to generate the same ramp rate at 1500°. This is a result of the kiln material and power rating.
Thermocouples are used to read the temperature inside a kiln chamber and communicate that temperature to the kiln controller. A kiln with an 8-gauge thermocouple will respond much slower to temperature input than a 20-gauge thermocouple. This can result in overshoot at low setpoints as the thermocouple needs time to “catch-up” to the heat that has been applied to the kiln.
Kiln Firing Schedules for Glass
While the kiln firing schedule example above was hypothetical, in this section we’ll explore actual kiln firing schedules for different types of glasswork techniques.
Please Note: Each of these schedules is for 90 COE glass. Additionally, each firing schedule will have to be adjusted according to your specific kiln, the size of your project, as well as the type of glass you’re using – some experimentation will be required, so please just use these as a general guideline.
For additional in-depth technical information about using your kiln to fire glass, please visit https://www.bullseyeglass.com/index-of-articles/.
Full Fuse Firing Schedule
A full fuse is when you use heat and time to combine two or more layers of glass to form one single solid piece of glass. The layers of glass fuse together – hence the name! Below is a full fuse firing schedule for projects that are smaller than 12”.
- 400°F/Hr to 1250°F – hold 30 minutes.
- 600°F/Hr to 1490°F – hold 10 minutes.
- AFAP°F/Hr to 900°F – hold 30 minutes.
- 150°F/Hr to 700°F – hold 0 minutes.
- AFAP°F/Hr to 70°F – hold 0 minutes.
You can find temperature guidelines for additional glasswork processes here.
Glass Casting Firing Schedule
Glass casting is when you melt glass until it is soft and malleable enough to conform to a mold. The glass then hardens to create a glass object in the shape of the mold. Below is a glass casting firing schedule for a small open face mold cast:
- 100°F/Hr to 200°F – hold 6 hours.
- 100°F/Hr to 1250°F – hold 2 hours.
- 600°F/Hr to 1525°F – hold 3 hours.
- AFAP °F/Hr to 1200°F – hold 4 hours.
- 50°F/Hr to 900°F – hold 6 hours.
- 12°F/Hr to 800°F – hold 1 minute.
- 20°F/Hr to 700°F – hold 1 minute.
- 72°F/Hr to 70°F – hold 1 minute.
Additional details about casting firing schedules can be found here.
Annealing Firing Schedule
Annealing glass is the process of stabilizing glass during the cooling process by holding it at a steady temperature to give it time to strengthen. COE 96 glass is typically annealed at a setpoint of 960°F. However, the size of the glass, its thickness, as well as the number of layers being used determines how long the anneal hold needs to be.
From the example of the Full Fuse Firing Schedule above, we highlighted the steps that involved annealing in green:
Notice that Step #3 has the kiln hold at the annealing setpoint 900°F for 30 minutes in order to give the fuse time to stabilize, and then Step #4 and Step #5 have the kiln slowly cooling down from the setpoint to the final temperature.
See our article Benefits of Using a Digital Controller for Glass Kilns for more information about using your kiln for glasswork!
Kiln Firing Schedules for Ceramics
Before getting into kiln firing schedules for ceramics, it’s important to know what Cone # the material you’re firing is rated for. This represents the setpoint at which the type of material you’re using is properly fired. So, for example, Cone 04 clay would need to reach a setpoint of at least 1945°F whereas Cone 6 Porcelain would need to reach a setpoint of 2232°F.
Please Note: All of these kiln firing schedules are for 04 Cone clay. Just like with glasswork, each firing schedule will have to be adjusted according to your specific kiln, the size of your project, as well as the type of clay, stoneware, or porcelain you’re using – some experimentation will be required, so please use these as a general guideline.
Candling Firing Schedule
Candling is the process of allowing clay to fully dry prior to high temperature ceramic firings. This involves heating your kilns to a low temperature for a prolonged period of time. Below is an example of a kiln firing schedule for candling your clay:
- 150°F/Hr to 150°F – hold 12 hours.
Simple, right? However, this is just to get the clay ‘bone-dry’ before firing it, since the natural moisture of the clay, if fired too quickly, can cause your project to crack and fissure!
Bisque Firing Schedule for Cone 04 Ceramics
A bisque firing is the process of turning clay into ceramics! Below is a slow bisque firing schedule for Cone 04 clay:
- 80°F/Hr to 250°F.
- 200°F/Hr to 1000°F.
- 100°F/Hr to 1100°F.
- 180°F/Hr to 1695°F.
- 80°F/Hr to 1945°F.
You’ll notice that this firing schedule doesn’t include any hold times. However, the total firing time is 13 hours and 26 minutes. So how does that work? In this case, the firing time is dictated by the ramp rate – or the amount of time it takes for your kiln to reach each setpoint in the firing schedule.
Glaze Firing Schedule for Cone 04 Ceramic
When firing pottery, it’s important to match the Cone # of your glaze to the Cone # of your clay. In this case, we’re using Cone 04 clay, which is a “low-fire” clay. Therefore, we’d want to use a glaze that’s in the Cone 06-04 range. In other words, the temperature of the glaze firing schedule shouldn’t exceed the temperature of the bisque firing schedule.
- 150°F/Hr to 250°F.
- 400°F/Hr to 1695°F.
- 100°F/Hr to 1945°F.
See our article on How to Use a Pottery Kiln Temperature Controller for more information on how to fire ceramics!
Firing Schedules for Heat Treating Metals
Just like with glasswork and pottery, kiln firing schedules for metal heat treat is extremely dependent on the type of material you’re using. But, additionally, it’s dependent on the qualities you want the finished metal to have. For heat treat, the rate at which you cool the metal has a significant impact on the molecular structure of the metal. For these examples, we’ll be working with 1095 steel.
Please Note: All of these kiln firing schedules are for 1095 steel. Just like with Each firing schedule will have to be adjusted according to your specific kiln or heat treat oven, the type of metal you’re using, its thickness, as well as the desired properties – some experimentation will be required, so please just use these as a general guideline.
You can find more information about setpoints and cooling rates for different effects on different types of metal here.
Normalizing Firing Schedule for 1095 Steel
Normalizing is a process where metal is heated to an extremely high temperature for a defined period of time and then either air-cooled or furnace cooled at a controlled ramp rate. Normalizing relieves internal stress and ensures uniformity, resulting in harder, stronger metals. Below is a normalizing firing schedule for 1095 steel:
- AFAP°F/Hr to 1600°F – hold for 15 minutes.
- Remove knife or blade from the oven and allow to air-cool.
Quench Hardening Firing Schedule for 1095 Steel
Quenching is the process where metal is heated and then cooled rapidly by dipping it into an oil, polymer, or water, resulting in very hard, very brittle metal. This increases the hardening of the metal (but also its brittleness). Below is a quench firing schedule for 1095 steel:
- AFAP°F/Hr to 1600°F – hold for 15 minutes.
- Remove knife or blade from the oven and quench in fast oil to 150°F.
Tempering Firing Schedule for 1095 Steel
After hardening, the metal is heated to a lower temperature to reduce excessive hardness and relieve internal stress. Tempering makes metals less brittle – it should be done within two hours after the steel cools from the quench hardening process. Below is a tempering firing schedule for 1095 steel:
- AFAP°F/Hr to 400°F – hold for 2 hours.
- Allow knife or blade to slowly cool – either air-cooled or within the oven.
You’ll notice that most heat treat applications have simple kiln firing schedules that only involve a single setpoint and aren’t dependent on ramp rate. For this reason, it might make sense to use a single setpoint controller for heat treat applications like the TAP & Go by SDS Industries.
Check out Guide to Choosing Heat Treating Controllers for more information about different types of heat treatments!
The Easiest Way to Precisely Execute Kiln Firing Schedules
The TAP and TAP II Controllers by SDS Industries are the most advanced, precise, and easy-to-use digital kiln controllers on the market today. With responsive touchscreen controls, an intuitive graphical UI, and integration with the TAP Kiln Controller Mobile App, TAP Kiln Controllers can pair with any relay-controlled kiln or oven to allow you to easily manage and execute your kiln firing schedules.
We invite you to explore our selection of programmable kiln controllers, standalones, and conversion kits on our online store. You can also purchase TAP Digital Controllers or TAP-Controlled Kilns and Heat Treat Ovens through one of the following distributors:
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