Mastering Kiln Schedules for Glass Fusing

A Complete Beginner’s Guide to Kiln Schedules

Fusing glass in a kiln is a fascinating and rewarding process that allows you to transform individual pieces of glass into a single, unified creation. Whether you are making a simple jewelry pendant or a large decorative plate, success in glass fusing depends on understanding how your fusible glass behaves under heat—and how to control that heat through a kiln firing schedule.

This guide explains the importance of glass compatibility (COE), explores the properties of hot glass, and provides a recommended firing schedule with an explanation of each segment and why it matters.

Understanding COE: The Foundation of Compatibility

COE, or Coefficient of Expansion, measures how much glass expands and contracts as it heats and cools. Each type of glass expands at a slightly different rate. When two glasses with different COEs are fused together, they cool unevenly, creating stress that can cause cracks or complete failure.

• Common COEs for fusing glass are 90 COE and 96 COE.
• Always use glass with the same COE or glass that has been tested for compatibility.
• Even if a mixed-COE piece appears fine after firing, hidden stress can cause it to crack days or weeks later.

At Hollander Fusing, we recommend labeling and storing glass separately by COE to ensure consistency and prevent costly mistakes. Learn more about how to organize and maintain your glass art studio here. 

The Properties of Hot Glass

When glass is heated, it transitions gradually through several physical states. Understanding these transitions helps predict how your glass will behave in the kiln.

• At room temperature, glass is rigid and brittle.
• Around 1000°F (538°C), it softens and moves out of the thermal shock zone.
• By 1450°F (788°C), it flows like thick honey and begins to level and fuse completely.
• Regardless of how many layers you start with, glass naturally seeks to become about ¼ inch (6 mm) thick at a full fuse.

In practice:

• A single 3 mm layer will contract inward at the edges.
• Two 3 mm layers (6 mm total) will maintain their shape and dimensions.
• Three or more layers will spread outward.

This is known as the ¼-inch rule, and it is essential for maintaining the intended form of your project.

Recommended Full Fuse Firing Schedule

For a 6 mm thick project made from 96 COE glass

The following schedule provides both Fahrenheit and Celsius values. It is designed for two 3 mm layers of compatible glass fused into one 6 mm piece. These settings are suitable for most mid-sized kilns, but adjustments may be required for different kiln models or glass types.

Step-by-Step Breakdown of the Firing Segments

Segment 1: Initial Heat

This stage slowly brings the glass from room temperature to a safe pre-fusing level. A steady ramp of 400°F per hour minimizes the risk of thermal shock, especially for thicker projects. For pieces thicker than 6 mm, reduce the ramp rate to 300°F per hour.

Segment 2: Bubble Soak

At 1225°F (662°C), the glass becomes soft enough for trapped air to escape between layers. Holding for 30 minutes allows the layers to settle together and helps reduce air bubbles or voids in the finished piece. This stage is often called a bubble soak or pre-rapid heat soak.

Segment 3: Rapid Heat

Once the glass is safely past the thermal shock zone, it can be heated more aggressively—typically 600°F per hour—to reach the fusing temperature. A faster ramp through the devitrification zone (1300–1400°F / 704–760°C) helps prevent surface crystallization.

Segment 4: Process Soak (Full Fuse)

At 1460°F (793°C), the glass layers fuse completely into one sheet. Hold for 10 to 15 minutes to achieve a smooth, even surface. Longer hold times may cause devitrification or excessive spreading, while shorter holds may result in incomplete fusion.

Segment 5: Rapid Cool

After the fuse is complete, cool the kiln as fast as possible through the devitrification range. This step prevents crystal formation on the surface and ensures clarity.

Segment 6: Anneal Soak

Hold at 960°F (515°C) for one hour to allow the glass to equalize in temperature. Annealing relieves internal stress and strengthens the finished piece. Thicker or uneven projects require longer holds to ensure complete temperature uniformity.

Segment 7: Anneal Cool

Slowly cool through the strain zone at 100°F per hour (38°C/hr) until the kiln reaches 700°F (371°C). This prevents the formation of stress fractures as the glass transitions back to a solid state.

Segment 8: Cool to Room Temperature

After reaching 700°F, the kiln can cool naturally to room temperature. Avoid opening the kiln prematurely—exposing hot glass to cool air can cause cracks from thermal shock.

Key Principles for Successful Fusing

• Always use glass with the same COE to prevent stress and cracking.
• Remember the ¼-inch rule: glass will naturally settle to about 6 mm thickness when fully fused.
• Heat and cool slowly to protect the integrity of your piece.
• Proper annealing is essential; never skip or shorten this stage.
• Keep a firing log to record ramp rates, hold times, and results for consistency in future projects.

Mastering Kiln Schedules Through Practice

There is no single perfect firing schedule for every kiln or project. Each kiln has its own heating characteristics, and each glass composition responds slightly differently. However, understanding the purpose behind every step of the firing schedule allows you to adjust intelligently and achieve consistent, professional results.

At Hollander Fusing, we believe that mastering kiln schedules is the key to unlocking creativity in glass art. By learning how glass behaves under heat—and by respecting the process—you can transform simple materials into lasting works of beauty.