Fusing Calculator

Professional Glass Firing Schedule & Annealing Estimator

Segment	Rate (°F/hr)	Temp (°F)	Hold (min)	Description

What is a Fusing Calculator?

A fusing calculator is a specialized technical tool used by glass artists and industrial manufacturers to determine the optimal heating and cooling cycle (known as a firing schedule) for kiln-formed glass. Glass fusing involves heating two or more pieces of glass until they bond together. Because glass is sensitive to thermal shock and internal stress, the fusing calculator must account for thickness, the Coefficient of Expansion (COE), and the specific desired outcome (e.g., a full fuse versus a tack fuse).

Who should use it? Anyone from hobbyists working with microwave kilns to professional studio artists using large electric kilns. Using a fusing calculator helps prevent “thermal shock” (breaking during heating) and “annealing cracks” (breaking during cooling), ensuring the longevity and structural integrity of the glass piece.

Fusing Calculator Formula and Mathematical Explanation

The fusing calculator doesn’t use a single linear equation. Instead, it employs a multi-segment logic based on the physics of glass transition. The key variables include the Glass Transformation Range and the Annealing Point.

The Variables of Glass Firing

Variable	Meaning	Unit	Typical Range
Thickness (T)	Total height of glass stack	mm	3mm – 20mm
Rate of Heat (R)	Degrees per hour increase	°F/hr	100 – AFAP
Target Temp (Tp)	The peak processing heat	°F	1200 – 1500
Soak Time (S)	Duration held at temperature	Minutes	10 – 60

The Logic Derivation

1. Initial Heat: 400 / T. As thickness increases, the heating rate must decrease to prevent the center of the glass from being significantly cooler than the surface.
2. Annealing: This is the most critical phase. The fusing calculator calculates the duration of the soak at the annealing point (where stress is relieved) based on the square of the thickness (T²).

Practical Examples (Real-World Use Cases)

Example 1: 6mm Standard Full Fuse (COE 90)

An artist is creating a 6mm thick coaster. Using the fusing calculator, the inputs are set to 6mm and “Full Fuse”. The result provides a peak temperature of 1480°F with a 10-minute soak. The annealing phase requires a 30-minute soak at 900°F followed by a controlled cool-down. Total time: approx 11-12 hours.

Example 2: 12mm Thick Slump (COE 96)

A heavy bowl requires a 12mm thick blank to be shaped into a mold. The fusing calculator calculates a much slower initial ramp (approx 150°F/hr) to protect the thick glass. The slump temperature is lower (1225°F), but the annealing soak is extended to 2 hours because of the increased thickness, preventing later breakage.

How to Use This Fusing Calculator

1. Input Thickness: Measure your glass stack with a caliper. Even 1mm difference matters for the fusing calculator logic.
2. Select Firing Type: Choose ‘Full Fuse’ for a smooth finish or ‘Tack Fuse’ if you want to keep the texture of the individual glass pieces.
3. Define COE: Ensure your glass types are compatible. Mixing COE 90 and COE 96 will cause the glass to shatter.
4. Review the Schedule: The fusing calculator will generate a segment-by-segment table. Program these into your kiln controller.
5. Monitor Results: Always record your results to refine future firings based on your specific kiln’s behavior.

Key Factors That Affect Fusing Calculator Results

• COE (Coefficient of Expansion): Different glass chemistries (90 vs 96) expand at different rates. The fusing calculator adjusts target temperatures accordingly.
• Glass Color: Darker colors absorb heat faster than clear or opal glass. This may require slightly slower heating than the tool suggests.
• Kiln Size: Large kilns have more thermal mass and cool slower. The fusing calculator assumes a standard medium-sized kiln environment.
• Mold Material: If slumping, ceramic molds take longer to heat than stainless steel molds.
• Altitude: High-altitude firing may require slightly higher temperatures to achieve the same “look” as sea-level firings.
• Devit Risk: Holding glass too long in the “devit range” (1300°F – 1350°F) can cause a cloudy surface. The fusing calculator minimizes time in this zone.

Frequently Asked Questions (FAQ)

Why does the fusing calculator suggest such a slow cooling rate?

Glass contracts as it cools. If the outside cools faster than the inside, the glass will crack. This is called “Annealing.”

Can I use this for tempering glass?

No. A fusing calculator is for kiln-forming. Tempering is a specific industrial process involving rapid air-cooling.

What happens if I mix COE 90 and 96?

The piece will appear fine initially but will likely crack within hours or days due to incompatible expansion rates.

Does thickness affect the peak temperature?

Not significantly, but it affects the soak time. Thicker glass needs more time at peak temp for the heat to penetrate the center.

Is “AFAP” safe for all kilns?

AFAP (As Fast As Possible) is usually used for the “Flash Cool” phase. Use it only if your kiln manual supports it.

What is a “bubble squeeze”?

It is an extra soak at 1225°F added to the fusing calculator logic to allow air to escape between layers.

Why is my glass rounded on the edges?

Surface tension causes glass to want to be 6mm thick. If your stack is thinner, it shrinks; if thicker, it spreads.

How do I calculate for 3D cast glass?

Casting requires much longer annealing cycles than flat fusing. Increase the thickness input in the fusing calculator significantly for safety.

Related Tools and Internal Resources

• Glass Weight Calculator – Estimate the weight of your glass projects before firing.
• Annealing Time Tool – Detailed deep-dive into cooling cycles for thick castings.
• COE Compatibility Checker – Ensure your glass scraps are safe to fuse together.
• Kiln Cost Estimator – Calculate the electricity cost for each firing cycle.
• Slumping Mold Guide – Best practices for choosing and preparing glass molds.
• Thermal Shock Prevention – Safety guide on handling hot glass and kiln safety.