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Glass Fusing Calculator — Firing Schedule, Anneal Time & Energy Cost

Get precise firing schedules, anneal times, and energy costs for glass fusing projects

Total Firing Time

10.8 hrs

Peak Temp

1,490°F

Energy Cost

$2.26

Units

Firing Summary

10.8 hrs
total firing time
Peak Temp
1,490°F
810°C
Energy Cost
$2.26
2500W kiln

Complete Firing Schedule

SegmentRateTemp (°F)Time
Initial Ramp+300°F/hr70→1,0003h 6m
Bubble SqueezeHold1,00030m
Ramp to Peak+600°F/hr1,000→1,49049m
Peak HoldHold1,49010m
Flash CoolAFAP1,490→9605m
Anneal Cool-100°F/hr960→8001h 36m
Anneal SoakHold8001h 0m
Final Cool-200°F/hr800→1003h 30m
Total10h 46m

Material & Cost Estimate

Glass Area80 sq in (0.56 sq ft)
Layers2 layers
Glass Cost (est.)$16.67–$27.78
Energy Cost$2.26
Total Est. Cost$18.93–$30.04

Energy Cost by Kiln Size

Small (1,500W)$1.36
Medium (2,500W)$2.26
Large (4,000W)$3.62

Key Temperatures (COE 90 (Bullseye))

Process°F°C
Slump1,225663
Tack Fuse1,350732
Full Fuse1,490810
Casting1,500816
Anneal Point960516
Strain Point800427

Safety & Best Practices

COE Compatibility: Never mix COE 90 and COE 96 glass. Mismatched expansion rates cause cracking during cooling.
Kiln Wash: Always coat shelves with kiln wash before fusing. Glass fused to bare shelves is nearly impossible to remove without damage.
Ventilation: Vent the kiln during initial ramp (below 1000°F). Some coatings and binders release fumes at low temperatures.
Annealing: Never skip or shorten the anneal soak. Thicker pieces need longer anneal times—add 15 min per mm over 6mm.
Thermal Shock: Never open the kiln above 1000°F except for flash cooling. Rapid temperature changes cause stress fractures.

Example Calculations

1COE 90 Full Fuse — Double Layer, 8×10", Medium Kiln

Inputs

Glass TypeCOE 90 (Bullseye)
ProjectFull Fuse
ThicknessDouble Layer (6mm)
Dimensions8" × 10"
KilnMedium (2,500W)

Result

Total Firing Time10.8 hours
Peak Temperature1,490°F (810°C)
Energy Cost$2.27
Glass Cost (est.)$16.67–$27.78
Anneal Soak60 minutes

A standard full fuse of two COE 90 layers on a medium kiln takes about 10.8 hours. The 30-minute bubble squeeze at 1000°F removes trapped air between layers. Energy cost is approximately $2.27 at $0.12/kWh.

2COE 96 Tack Fuse — Single Layer, 6×6", Small Kiln

Inputs

Glass TypeCOE 96 (Spectrum/Uroboros)
ProjectTack Fuse
ThicknessSingle Layer (3mm)
Dimensions6" × 6"
KilnSmall (1,500W)

Result

Total Firing Time9.7 hours
Peak Temperature1,325°F (718°C)
Energy Cost$1.22
Glass Cost (est.)$2.50–$5.00
Anneal Soak60 minutes

A tack fuse of single-layer COE 96 glass skips the bubble squeeze (not needed for one layer) and peaks at a lower 1325°F. The small kiln keeps energy cost to about $1.22.

3COE 90 Slump — Triple Layer, 10×12", Large Kiln

Inputs

Glass TypeCOE 90 (Bullseye)
ProjectSlump
ThicknessTriple Layer (9mm)
Dimensions10" × 12"
KilnLarge (4,000W)

Result

Total Firing Time11.1 hours
Peak Temperature1,225°F (663°C)
Energy Cost$4.66
Glass Cost (est.)$37.50–$62.50
Anneal Soak105 minutes

Slumping a thick 9mm piece requires an extended anneal soak of 105 minutes (60 base + 45 extra for 3mm over 6mm). The peak temperature is lower at 1225°F since the goal is shaping, not merging.

Frequently Asked Questions

Q

What temperature do you fuse glass at?

Fusing temperatures depend on the project type and glass COE. COE 90 (Bullseye) full fuse peaks at 1490°F (810°C), tack fuse at 1350°F (732°C), and slump at 1225°F (663°C). COE 96 runs slightly lower: full fuse at 1475°F, tack at 1325°F, slump at 1200°F.

  • COE 90 full fuse: 1490°F (810°C) — glass fully merges into smooth, flat surface
  • COE 90 tack fuse: 1350°F (732°C) — pieces bond but retain texture and shape
  • COE 90 slump: 1225°F (663°C) — glass softens enough to drape over or into a mold
  • COE 96 temperatures run 15–25°F lower than COE 90 for all project types
  • Never mix COE 90 and COE 96 — mismatched expansion causes cracking
Project TypeCOE 90 (°F)COE 96 (°F)COE 90 (°C)
Slump12251200663
Tack Fuse13501325732
Full Fuse14901475810
Casting15001485816
Q

How long does a glass fusing firing take?

A typical full fuse firing takes 10–12 hours from start to finish. The heating ramps take about 4 hours, holds add 40–70 minutes, and the controlled cooling (annealing) phase takes 5–6 hours. Thicker pieces need longer anneal soaks, adding 15 minutes per mm over 6mm.

  • Initial ramp to bubble squeeze (1000°F): about 3 hours at 300°F/hr
  • Bubble squeeze hold: 30 minutes (for double+ layers)
  • Ramp to peak temperature: about 50 minutes at 600°F/hr
  • Anneal cool (960°F to 800°F): about 1.5 hours at -100°F/hr
  • Final cool (800°F to 100°F): about 3.5 hours at -200°F/hr
ProjectPeak °FTotal TimeAnneal Soak
Tack Fuse (6mm)1350~10.3 hrs60 min
Full Fuse (6mm)1490~10.8 hrs60 min
Slump (6mm)1225~10.5 hrs60 min
Full Fuse (9mm)1490~11.5 hrs105 min
Q

What is annealing and why is it important?

Annealing is the controlled cooling process that relieves internal stress in fused glass. Without proper annealing, glass develops invisible stress that causes cracking hours, days, or even months later. The anneal soak holds glass at 960°F (COE 90) or 940°F (COE 96) for 60+ minutes, then cools at -100°F per hour.

  • Anneal point for COE 90: 960°F (516°C) — temperature where stress begins to relieve
  • Anneal point for COE 96: 940°F (504°C)
  • Base anneal soak: 60 minutes for glass up to 6mm thick
  • Add 15 minutes per additional mm over 6mm (9mm = 105 min total)
  • Strain point (800°F): below this temperature, glass is rigid and safe to cool faster
ParameterCOE 90COE 96
Anneal Point960°F (516°C)940°F (504°C)
Strain Point800°F (427°C)800°F (427°C)
Base Soak (6mm)60 min60 min
Extra per mm >6mm15 min/mm15 min/mm
Q

How much does it cost to fire a glass kiln?

A typical glass fusing firing costs $1.50–$5.00 in electricity depending on kiln size and local rates. A medium 2,500W kiln running for 10.8 hours at $0.12/kWh costs about $2.27. Small 1,500W kilns cost around $1.36, while large 4,000W kilns run about $3.63 per firing.

  • Small kiln (1,500W): $1.00–$2.00 per firing at $0.10–$0.15/kWh
  • Medium kiln (2,500W): $1.50–$3.50 per firing at $0.10–$0.15/kWh
  • Large kiln (4,000W): $2.50–$5.00 per firing at $0.10–$0.15/kWh
  • Kilns average about 70% duty cycle — elements cycle on/off during holds
  • Monthly cost for weekly firings: $6–$20 depending on kiln size
Kiln SizeWattsCost at $0.12/kWhCost at $0.15/kWh
Small (8×8")1,500W$1.36$1.70
Medium (12×12")2,500W$2.27$2.83
Large (18×18")4,000W$3.63$4.53
Q

What is the bubble squeeze and when do I need it?

The bubble squeeze is a 30-minute hold at 1000°F (COE 90) or 990°F (COE 96) during the initial heating ramp. It allows trapped air between glass layers to escape before the glass seals together. You need it whenever stacking two or more layers of glass (6mm+ total thickness).

  • Temperature: 1000°F for COE 90, 990°F for COE 96
  • Duration: 30 minutes — enough for air to migrate to edges and escape
  • Required for: double layer (6mm), triple layer (9mm), and thicker builds
  • Not needed for: single-layer slumps or single-layer draping projects
  • Skipping causes trapped bubbles between layers that cannot be removed after fusing
LayersThicknessBubble Squeeze?Hold Temp (COE 90)
Single (3mm)3mmNot neededN/A
Double (6mm)6mmRequired1000°F, 30 min
Triple (9mm)9mmRequired1000°F, 30 min
Q

Can I mix COE 90 and COE 96 glass together?

No. Mixing glass with different COE (coefficient of expansion) values causes stress cracking because the glasses expand and contract at different rates during cooling. Even a small COE mismatch creates enough internal stress to crack the piece during annealing or weeks later. Always use glass from the same COE family.

  • COE = coefficient of expansion — how much glass expands per degree of temperature change
  • COE 90 and COE 96 differ by ~6.7% in expansion rate — enough to crack any piece
  • Some cracks appear during annealing, others develop days or weeks after firing
  • Test compatibility: stack two pieces and fire a small test tile first
  • Even within the same COE, some colors have slight mismatches — use tested-compatible glass
ScenarioSame COEMixed COE
Expansion RateMatched~6.7% mismatch
Stress During CoolMinimalSevere
Crack RiskVery lowNear certain
ResultSafe, durableCracks within hours–weeks

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Last Updated: Mar 9, 2026

This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

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