A firing can look perfect through the kiln peephole, then crack with a sharp ping as it cools. That is why kiln firing schedules are more than a set of temperatures to copy into a controller. They are a sequence of controlled heating, soaking and cooling stages that must suit the glass, the build thickness, the kiln and the result you want.
For fused glass makers, a reliable schedule is one of the best ways to protect both your work and your materials. It gives glass time to heat evenly, reach the required degree of fusion, release internal stress during annealing, and cool safely through the most vulnerable temperature ranges. The right schedule is rarely the fastest one.
What a kiln firing schedule controls
A kiln controller divides a firing into segments. Each segment tells the kiln how quickly to rise or fall in temperature, what temperature to reach, and whether to hold there. Those instructions affect the surface, shape and structural stability of the finished piece.
The heating rate matters because glass does not warm evenly by itself. The outside and edges generally respond first, while the centre of a thick piece lags behind. If the kiln climbs too quickly, those temperature differences create stress. A piece may survive the firing and still fail later, sometimes days after it comes out of the kiln.
A hold, also called a soak, keeps the kiln at a chosen temperature for a set time. At the top of the firing, this allows layers to fuse or glass to settle into a mould. At annealing temperature, the hold allows the whole piece to equalise and release stress. Cooling rates then become just as significant. Rushing through the annealing range is one of the most common causes of unexplained breakage.
Start with the glass manufacturer's schedule
Use the schedule supplied for your compatible glass system as your starting point. This is particularly important with kiln-forming glass, where tested firing information is designed around a particular COE and production range. COE compatibility is essential, but it is not a guarantee that unrelated glasses will behave identically in the kiln.
A manufacturer's schedule is a tested baseline, not a promise that every project can use the same settings. Your kiln may fire hotter or cooler than the display indicates. A small test kiln responds differently to a large front-loader, and an element placement can affect heat distribution. A shallow, single-layer pendant also needs a very different approach from a 12 mm casting or a large platter on a mould.
Keep a firing log from the beginning. Record the glass range, colours and special products used, finished thickness, shelf preparation, mould, programme, kiln load and result. Notes such as “good fuse, slight devitrification” or “centre cracked after opening” will be far more useful than trying to remember what happened several months later.
Heat slowly through the early stages
The first part of a firing is often slower than beginners expect, especially with thicker work. Glass needs time to warm through evenly. This is also the stage where moisture can cause trouble. Ceramic moulds, shelf primer and kiln wash should be thoroughly dry before firing. Entrapped moisture can create steam and damage a piece or disturb its surface.
For a basic two-layer fuse, moderate heating may be appropriate. For a thicker stack, a piece with many overlapping elements, or a larger format, slow the rate and consider an equalisation hold before the top temperature. The aim is to reduce the temperature difference between the outside and centre of the glass.
This is not merely cautious firing. It is sound workshop practice. If a schedule succeeds with small samples but produces cracks in larger panels, the early ramp and the anneal are the first places to look.
Choose the top temperature for the finish you want
The peak temperature and top hold determine how much the glass moves. A tack fuse retains texture and individual contours. A contour fuse softens sharp edges while preserving some relief. A full fuse produces a smoother, more level surface as layers melt together.
There is no single “full fuse temperature” that applies to every kiln and glass range. Many compatible fusing glasses will begin to tack fuse in the low-to-mid 700°C range and fully fuse closer to 800°C, but the true result depends on hold time, kiln calibration, glass thickness and the way heat reaches the work. A few extra minutes at peak can noticeably flatten a piece, soften a patterned surface or allow reactive colours to change more than expected.
When testing a new colour combination, dichroic product, frit blend or decorative treatment, use small samples first. Reactive effects and some surface treatments can be beautiful, but they need their own tested process. Test tiles cost far less than a failed commission.
Annealing is where strength is earned
Annealing is not the optional cooling stage after the interesting part of the firing. It is the process that makes a kiln-formed piece durable enough to use, transport and sell.
At the annealing temperature recommended for your glass system, the glass is held long enough for heat to equalise and internal stress to relax. From there, it must cool slowly through the strain range. Thicker glass requires longer holds and slower cooling because the centre remains hotter than the surface for longer.
A common rule of thumb is that 6 mm work can use a relatively straightforward anneal, while thicker work needs a substantially more conservative schedule. But thickness is only part of the picture. A piece with uneven thickness, a heavy central inclusion, a thick rim or a deep casting should be treated according to its thickest section, not its average thickness.
If a piece breaks cleanly into two or more sections after firing, inspect the break. A crack that runs through the body with little relation to a design line often points to thermal stress. A break around an incompatible inclusion or a particular decorative glass may indicate a compatibility issue instead. Both can occur in the same piece, which is why careful testing and firing notes matter.
Slumping requires its own schedule
A successful fuse schedule does not automatically suit slumping. In a slump firing, glass is heated until gravity allows it to bend into or over a mould. The goal is controlled movement, not a high-temperature melt.
The shape and material of the mould affect the firing. A shallow drape mould may slump quickly, while a deep bowl mould can need more time for the glass to reach the base without thinning excessively at the rim. Opening the kiln too early to check progress can introduce a sudden temperature change, so resist the temptation unless your kiln is already at a safe viewing temperature.
Slumping can also reveal issues that were not obvious after fusing. A slightly uneven blank may distort, and a piece with residual stress may crack as it takes up the mould shape. Anneal the slump firing properly, even when the glass has already been annealed in its fuse firing.
Adjust one variable at a time
When a firing does not give the result you expected, avoid changing the entire programme. Alter one variable, then test again. If the piece is under-fused, increase the top hold slightly before making a large temperature jump. If texture is disappearing, reduce the hold or lower the peak a little. If a large piece cracks, slow the early heating and extend the anneal rather than assuming the glass is faulty.
Several practical signs can guide your next test:
- Sharp, unwanted cracks after cooling usually call for a more conservative anneal or cooling rate.
- Glass that has not fully joined may need a longer top hold, a slightly higher peak, or a review of how the layers are stacked.
- Excessive flattening suggests too much heatwork, meaning the combined effect of temperature and time was too great.
- A cloudy or dull surface may be devitrification, contamination, or an extended high-temperature exposure rather than a simple firing-temperature problem.
Build safety into your firing routine
Good kiln practice includes more than the programme. Place work so heat can circulate around it, avoid overloading shelves, and use a clean, suitable shelf separator for the temperatures you plan to reach. Check that moulds are stable and supported. Allow the kiln to cool naturally unless the glass manufacturer specifically advises otherwise.
Do not force-cool by opening the lid or door. Glass can look cold enough to handle while its interior is still carrying damaging stress. Follow your glass system's guidance for opening temperatures, and be more patient with large, thick or complex work.
At Art Glass Imports, we regularly see that the makers who get the most dependable results are not necessarily using complicated programmes. They understand what each segment is doing, test their own kiln, and give thick or ambitious work the time it needs. Start with a proven schedule, make careful adjustments, and let your firing notes become part of your studio knowledge.