Kiln Firing Schedules for Glass, Ceramics, and Heat Treatment

Ever wish your kiln came with a simple roadmap?

A kiln firing schedule is just that: a programmed temperature progression with set ramp rates, setpoints, and hold times you choose on purpose, not by guesswork. Industry tunnel kilns and fast-firing lines rely on tight schedules; hobby and pottery kilns use slower, periodic cycles, but the principles are the same.

For glass, ceramics, and metal heat treatment, precision means strength, stability, and fewer failures. Ready to fire with confidence? Keep reading.

Understanding the Four Components of Firing Schedules

Kiln firing schedules are composed of four components, all of which are important to understand.

Step Number and Segments

Step numbers are the spine of a firing schedule. Each step is a segment with its own job: how fast to change the temperature, the target temperature to hit, and how long to hold there.

One step might be a slow climb to drive off moisture, the next a faster climb toward maturity, and another a long soak to equalize heat through the whole load.

These steps run in order, one after another, like scenes in a movie. Change the order, and you change the story. Skip the gentle first step, and you risk steam explosions in damp clay.

Rush the final approach to peak temperature, and your glaze may under-melt, over-melt, or trap bubbles. The sequence is what turns random heat into a controlled process.

Ramp Rate (Heating/Cooling Speed)

Ramp rate is how fast the kiln changes temperature, usually measured in degrees per hour, like 300°F/hour (149°C/hour) or 150°C/hour (302°F/hour). It is the speed limit your material can safely handle.

Thin, even pieces can usually tolerate faster ramps, while thick or complex forms need slower changes so the heat can move through them without creating big temperature differences from inside to outside.

To pick a ramp rate, start with the material and its thickness. Thin glass sheets can usually handle fast early ramps but need slow changes through the annealing range. Thick cast glass may need very gentle ramps, often under 100°F/hour (38°C/hour), so the core can catch up.

Clay that still contains moisture needs slow warming so steam can escape before it cracks the walls. Even metal can benefit from controlled ramps in heat treatment processes, so the whole cross-section reaches the same temperature at the same time.

Setpoint (Process Temperature)

Setpoints are the Process temperatures you tell the kiln to reach, like "stop at 1480°F (804°C) and hold."

In ceramics, you often translate between cones and degrees: cone 04 for bisque or cone 6 for midrange glaze, for example. The controller thinks in degrees, but your clay and glaze recipe often speaks in cones, so you translate between the two.

Different materials have their own "big deal" temperatures.

Clay bodies care about quartz inversion, sintering, and vitrification ranges. Glass cares about the softening range, the annealing point, and devitrification temperatures. Steel has critical transformation ranges where its internal structure changes. Each setpoint is a gate your material has to pass through safely.

Hold Time (Soak Periods)

Hold time is the pause at a set temperature where you tell the kiln to sit still and do its job. During a hold, the load catches up with the readout.

The controller might say 1480°F (804°C), but the center of a thick piece could still be behind. The hold gives that heat time to soak all the way through. This is the underlying concept of “Heatwork.” A prolonged hold time can multiply the effects required at the process temperature, which can be a good thing or bad thing depending upon the effect desired.

Proper soaking is one of the quiet ways you prevent ugly defects. A short hold near glaze maturity helps pinholes heal, and bubbles burst. A longer hold above glass softening lets trapped bubbles rise and pop before you move into fusing.

In heat treatment, a soak at austenitizing temperature ensures the steel transforms fully before quenching. You are not just waiting around; you are giving the material enough time for the material to finish what the heat started.

Ramp/Hold vs Time-to-Temp Formats

Ramp/Hold programming tells the kiln exactly what to do in each step: how fast to move, where to stop, and how long to wait.

A step might read "300°F/hour (149°C/hour) to 1100°F (593°C), hold 20 minutes," followed by another that says "500°F/hour (260°C/hour) to 1480°F (804°C), hold 10 minutes." That is how most published schedules are written, and it maps directly to how modern controllers are designed.

Time-to-Temp works differently. You tell the kiln, "reach 1500°F (816°C) in 4 hours" and let it handle the in-between. In reality, the kiln may climb quickly at the start, then slow as it approaches peak temperature. That means the load sees an uneven, curved ramp rather than a clean, predictable slope.

When you are trying to troubleshoot or repeat results, that uneven curve can make things confusing. Ramp/Hold is the normal language for most modern kiln controllers because it gives you clearer control and a clearer record of what happened.

Glass Firing Schedules

Glass firing schedules change depending on the type of glass and the project requirements, as glass is very specific.

Matching Schedules to COE Ratings

Glass needs a firing schedule that fits its COE, or coefficient of expansion. COE tells you how much the glass grows and shrinks when it heats and cools. If you mix glasses with different COEs, they build up stress and can crack later on.

For most COE 90 sheet glass, a typical annealing temperature is around 900°F (482°C), so many fusing schedules park and soak right there to relax that built-up stress.

Full Fuse Firing Schedule Example

A simple full fuse firing cycle for a 6 mm COE 90 piece might look like this: heat 300°F per hour (149°C/hour) to 1100°F (593°C) and hold 20 minutes, then 500°F per hour (260°C/hour) to about 1490°F (810°C) and hold 10 to 15 minutes. Around 1100°F (593°C), the glass softens, and in the 1400s, it flows and fuses into one smooth layer.

After that, cool at about 999°F per hour (537°C/hour) to 900°F (482°C), hold for 60 minutes to anneal, then cool around 100°F per hour (38°C/hour) to 700°F (371°C) before shutting off so the stress stays low. Understanding the firing stages of glass helps you recognize what's happening at each temperature point. Of course, the hold times can vary by the size of the project and your particular kiln.

Glass Casting and Slumping Schedules

Casting and slumping ask for gentler ramps and longer soaks, especially when the glass is thick or stacked.

An open face casting that is about 1 inch thick might climb only 150 to 200°F per hour (66 to 93°C/hour), soak at a mid-range to clear bubbles, then hold at casting temperature for several hours so the mold fills all the way. The anneal hold can last many hours, followed by very slow cooling through the 900°F (482°C) zone.

Slumping a single sheet into a mold is usually quicker. You might heat at a moderate rate to around 1200 to 1300°F (649 to 704°C) and hold only until the glass just sags into the shape you want.

Then you return to that same careful anneal around 900°F (482°C) and a controlled cool down, so the new curve in the glass does not come with surprise cracks.

Ceramic and Pottery Firing Schedules

With ceramic and pottery it is important to pay attention to the temperatures as explosions can happen.

Bisque Firing Schedules

Bisque firing starts slowly, so the clay does not explode from the force of moisture leaving the material. The first climb is usually around 80-150°F per hour (27-66°C/hour), so leftover water can leave safely. Around 212°F (100°C), free water boils off.

As the temperature rises through the 400-600°F range (204-316°C), organics and binders burn out. Near 1060°F (571°C), quartz inversion happens, where the crystal structure in the clay shifts and the body expands. All of this is normal. It just needs time.

A typical cone 04 slow bisque firing cycle can take about 13 hours. You warm slowly through the moisture and burnout zones, then keep a steady climb up to about 1940°F (1060°C). The goal is not shiny beauty.

The goal is sturdy, porous ware that can take glaze without cracking. When you open the kiln, pieces should feel hard but still be able to absorb water from a damp sponge. That strange half-cooked state is exactly what you want. Proper preparation before firing ensures your pieces survive this critical stage.

Glaze Firing Schedules

Glaze firing is faster at the start because the clay has already survived bisque. You can ramp more quickly through the low range, then slow down as you approach glaze maturity so bubbles can rise and gases can escape.

Early on, the glaze dries and begins to soften. As you reach higher temperatures, gases from the body and glaze move out, bubbles form, then the melt levels and smooths. A cone 04 glaze firing schedule might run at 250-300°F per hour (121-149°C/hour) up to around 1800°F (982°C), then slow to about 100°F per hour (38°C/hour) to hit cone 04 with a short soak for smoothing. Cone 6 schedules often go 300°F per hour (149°C/hour) to about 2000°F (1093°C), then around 120°F per hour (49°C/hour) to cone 6.

Cooling is not just "turn it off and walk away." A controlled cool helps the glaze surface stay smooth and can affect color and crystal growth. Some special glazes use a down-fire schedule, where you cool to a certain point, then hold or even reheat slightly to grow visible crystals.

It feels wrong to heat again after peak, but the glassy glaze network listens to these tiny temperature moves and grows patterns you cannot fake with a brush.

Reduction Firing in Gas Kilns

Reduction firing in a gas kiln lives and dies by the damper. With the damper more open, plenty of oxygen enters, and the kiln fires in oxidation. This is where you want to be while organics burn off, often up to about cone 012.

To create a reduction, you close the damper partway and adjust the gas so the burners have more fuel than oxygen. You watch for a soft, lazy flame licking out of the peep holes and a slight smell of unburned fuel. That is the kiln telling you the atmosphere inside is stealing oxygen from the clay and glazes instead of the air.

A common approach is to switch from clean oxidation to reduction around cone 012 and stay in reduction at least through cone 04. During this "climbing reduction," you keep the temperature rising at roughly 60-80°F per hour (16-27°C/hour).

That slow, smoky climb gives iron and other colorants time to react and shift color. Go too fast, and the reduction can be patchy. Go too slow, and you waste fuel without gaining much. The cones, the color of the flame, and sometimes the dull roar of the burner become your strange little language for holding that sweet, reducing climb. Observing your kiln during firing helps you recognize when the atmosphere is just right.

Factors That Affect Firing Schedule: Success vs Redo

Firing schedules work only if they match the reality inside the kiln.

Each material has its own thermal behavior: clay expands and contracts at key inversion points, glass hates sudden temperature jumps and needs generous time at annealing temperature, and every glaze or glass formulation matures in its own narrow window.

If your schedule ignores those behaviors, you invite cracks, warping, or pieces that look fine but are secretly weak.

The kiln itself also has opinions. A small, low-power kiln with thin insulation cannot follow the same aggressive ramp rates as a big, heavily insulated one. A tightly packed load heats more slowly and unevenly than a light load with plenty of space between pieces.

Shelves, posts, and stacking patterns all change how heat moves. Open vents, drafts in the room, and how often you peek in can nudge the schedule off course. Environmental factors like cold weather and even low voltage conditions can affect your firing cycle timing.

The written program is just the starting point; the real schedule is what actually happens in that specific kiln, on that specific day, with that specific load.

Troubleshooting: What Are Some Common Firing Issues?

Firing issues usually come from heat moving too fast or in the wrong pattern.

If you rush the early stages with damp clay, you get steam explosions, cracking, or bloating.

If burnout is incomplete, organics and carbon can leave smoky patches in glazes or cause pinholes and craters as gases try to escape late in the firing.

Overfiring can slump forms, blister glazes, or drive glass into devitrification.

The fixes are not fancy. Slow the first part of the schedule so moisture and gases have time to leave. Add or extend holds in the ranges where burnout, glass leveling, or glaze smoothing happens.

Use controlled cooling through sensitive ranges like quartz inversion for clay or the thermal shock zone and the annealing range for glass. Check your hardware: elements, thermocouples, burners, and seals, so the kiln can follow the program. Testing for accurate temperatures ensures your controller is reading correctly.

Many "mystery" problems vanish once the schedule and the kiln's real behavior are actually in sync with the material.

Best Practices for Programming and Documenting Your Schedules

Keep track of what really happens in your kiln, not just what you meant to do. Write firing logs that record the program, the kiln used, the load, and any observations: hotspots, slow spots, how long it took to reach key temperatures, and how cones bent. Keeping a detailed log helps you build a reliable reference library over time.

When results are great, you know exactly what to repeat. When something goes wrong, you have clues to what changed.

Build your own library of "known good" schedules and match them to specific clays, glazes, glasses, and kiln setups. Note when you used certain shelf configurations, load densities, and ventilation settings.

Save your controller programs under a clear name so you can repeat them later without guessing. Over time, these tested schedules become as valuable as any recipe book on your shelf. Jump-start programming tips and controller tips and tricks can help you build efficient, repeatable programs.

Dialing In Your Firing Schedules

Firing felt like guesswork, too fast, too hot, cracks one day and perfect pieces the next, with no clear idea why. Every new project meant more trial and error just to avoid wrecking another load.

Now you know how steps, ramp rates, setpoints, and holds work together, and how to shape schedules for bisque, glaze, glass fusing, slumping, casting, and basic heat treatment. Firing feels understandable instead of mysterious.

Next step: pick one schedule from this blog, run it in your kiln, write down what happens, and tweak one thing at a time. Soon you'll have your own reliable "house" schedules you can trust and reuse. If you need guidance on selecting the right kiln for your work or want personalized advice on firing schedules, reach out to our team for expert support.

References

"Kiln Firing." Ceramic Arts Network, Ceramic Publications Company, ceramicartsnetwork.org/ceramic-materials/kiln-firing/.

"Glass Fusing and Slumping." The Corning Museum of Glass, Corning Museum of Glass, www.cmog.org/article/glass-fusing-slumping.