Glass Fusing Schedule Calculator
Kiln firing schedule from glass type and project.
About this calculator
This calculator builds a kiln firing schedule for glass fusing from three inputs: total layered thickness, the fusing goal (tack fuse around 730°C, full fuse around 800°C, or slump around 650°C), and COE (coefficient of expansion) glass family — COE 90 (Bullseye/Uroboros) anneals near 510°C with a 475°C strain point, COE 96 (Spectrum/System 96) near 515°C/480°C. The schedule ramps at a moderate rate up to 500°C, then deliberately slows through 500-600°C to creep past quartz's 573°C inversion point, where the material briefly changes its crystal structure and is prone to cracking if heated too fast, before ramping quickly to the target top temperature and holding a short soak there. The critical phase is what follows: annealing.
The calculator holds the piece at the annealing point for a soak time that doubles for every 3mm of thickness above a 6mm baseline (thicker glass takes proportionally longer for internal stress to relax throughout its cross-section), then cools through the annealing-to-strain-point range at a rate that's halved for that same thickness step, since thicker glass needs to shed heat much more slowly to avoid locking in residual stress that can cause the piece to crack days or weeks later. Cooling continues at progressively faster rates below the strain point, where the glass is already rigid and no longer stress-sensitive. Energy use is a rough estimate based on kiln interior volume and total schedule time — actual consumption varies significantly by kiln insulation, element wattage, and ambient conditions, so treat it as a ballpark for budgeting, not a utility bill prediction.
Inputs
Results
Top temperature (°C)
800
How to Use This Calculator
- Enter the Total Thickness of the layup in mm and select the Fusing Type: tack fuse, full fuse, or slump.
- Select the COE type (COE 90 for Bullseye, COE 96 for Spectrum) and enter your Kiln Width.
- Indicate whether you have a Digital Controller for precise ramp and hold programming.
- Review Top Temperature, Anneal Soak time, Slow Cool Rate, and Annealing Zone Duration.
- Check Total Schedule Hours and Estimated Energy to plan kiln scheduling and electricity costs.
What each input means
- Total thickness (mm)
- Total layup thickness after fusing. 2 layers of 3mm = 6mm.
- Fusing type
- 0 = Tack fuse (~730°C), 1 = Full fuse (~800°C), 2 = Slump (~650°C).
- COE type
- 0 = COE 90 (Bullseye/Uroboros), 1 = COE 96 (Spectrum/System 96).
- Kiln width (inches)
- Interior width of your kiln for energy estimation.
- Digital controller
- 1 = Digital kiln controller, 0 = Manual (adds safety margin).
What each result means
- Top temperature (°C)
- Peak firing temperature for the selected fusing type.
- Anneal soak (min)
- Hold time at annealing point. Longer for thicker glass.
- Slow cool rate (°C/min)
- Maximum cooling rate through the critical annealing zone.
- Anneal cool duration (min)
- Time to cool through the annealing zone (annealing → strain point).
- Annealing point (°C)
- Temperature at which internal stresses begin to relax.
- Strain point (°C)
- Below this temp, glass is rigid and stresses are locked in.
- Total schedule (min)
- Estimated total firing schedule time.
- Total schedule (hours)
- Total schedule in hours.
- Est. energy (kWh)
- Rough energy consumption estimate.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersTotal thickness (mm) = 6, Fusing type = 1, COE type = 0, Kiln width (inches) = 18 = 5 input(s) provided
- Calculate Top temperature800 = 800
- Calculate Anneal soakAnneal soak = round(30 * thicknessFactor)30 = 30
- Calculate Slow cool rateSlow cool rate = baseCoolRate / thicknessFactor1.5 = 1.5
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does the schedule slow down specifically between 500°C and 600°C?
Quartz within the glass undergoes a structural change (inversion) at 573°C that causes a small, sudden volume shift. Ramping through that narrow window too fast creates thermal stress the glass can't absorb, so the calculator drops the ramp rate from 165°C/hr to 85°C/hr for the 500-600°C phase specifically to give the glass time to pass through that transition safely.
Why does doubling the thickness above 6mm double the anneal soak time?
Annealing works by holding the glass at a temperature where internal stress can relax throughout its full cross-section, and heat needs time to diffuse evenly through thicker material. The calculator models this with a soak time that doubles for every 3mm added above the 6mm baseline, since a thicker piece takes proportionally longer for that stress relaxation to reach its center.
What happens if I use a schedule meant for thin glass on a thick piece?
The anneal soak would be too short and the cool-down through the annealing-to-strain-point range would happen too fast for the glass's actual thickness, potentially locking residual stress into the piece. That stress isn't always visible right away — it can cause a piece to crack spontaneously days or weeks after firing, which is why the calculator scales both anneal soak and cooling rate directly to thickness.
Why does the calculator distinguish COE 90 from COE 96 rather than treating all fusing glass the same?
COE 90 and COE 96 glass families expand and contract at slightly different rates and anneal/strain at slightly different temperatures — 510°C/475°C for COE 90 versus 515°C/480°C for COE 96. Using the wrong annealing point for your actual glass family would mean holding and cooling through the wrong temperature range, which is why the calculator asks which COE family you're firing.
Does the Digital Controller setting change the schedule?
No — it's collected for your own reference when planning how you'll run the kiln, but it doesn't currently change any of the calculated temperatures, soak times, or cooling rates.
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