90 COE vs 96 COE Glass: What’s the Difference and Which Should You Use?

Choosing compatible glass is one of the most important steps in glass fusing. A common question beginners and experienced artists ask is: What is the difference between 90 COE and 96 COE glass, and can they be fused?

This guide explains COE, why it matters, and how to choose the right glass for your projects.

What does COE mean in Glass Fusing?

COE (Coefficient of Expansion) measures how much glass expands and contracts during heating and cooling.

• Expressed as a number (e.g., 90 or 96)
• Determines whether glass pieces will move at similar rates in the kiln

When glass with different expansion rates is fused, stress forms during cooling. This can lead to cracking, shattering, or delayed breakage.

What is 90 COE glass?

90 COE glass is one of the most widely used fusible glass systems.

Characteristics

• Designed specifically for kiln fusing
• Known for color consistency and compatibility within its system
• Popular among artists using brands like Bullseye

Common uses

• Jewelry and cabochons
• Plates and bowls
• Decorative art panels
• Kiln-formed sculpture

Note: Artists often choose 90 COE for its predictable firing behavior and extensive color palette.

What is 96 COE Glass?

96 COE glass is another major fusible glass category with slightly higher expansion. It’s worth noting that many beginners gravitate toward 96 COE due to accessibility and product variety.

Characteristics

• Formulated for compatibility within its own glass family
• Often associated with Oceanside/System 96 products
• Widely available in sheet, frit, stringer, and accessory forms

Common uses

• Beginner kiln projects
• Functional dishware
• Suncatchers and decorative panels
• Mixed media fused glass art

Key Differences Between 90 COE and 96 COE Glass

Functionally, both behave similarly during firing. The main difference is compatibility.

Can you fuse 90 and 96 COE Glass together?

Short answer: No, mixing 90 and 96 COE glass is not recommended.

Why mixing causes problems

• Different expansion rates create internal stress
• Cracks may appear immediately or days later
• Finished pieces can break unpredictably

Even when glass appears fused successfully, hidden stress can weaken the piece. Advanced compatibility testing exists, but it is unreliable for production work and not recommended for beginners.

How to Test Glass Compatibility 

If you don’t know the COE of your glass:

Simple test method

1. Cut two small strips of glass
2. Fuse them fully
3. Observe after cooling for cracks or curvature
4. Use a polariscope for stress detection if available

However, testing should supplement, not replace, known compatibility.

Choosing Between 90 and 96 COE Glass 

The best choice depends on your current materials and workflow. Remember, you should always commit to one system per project.

Choose 90 COE if:

• You already own Bullseye-compatible glass
• You want an artisan-focused palette
• You follow firing schedules designed for 90 COE

Choose 96 COE if:

• You are starting fresh with beginner kits
• Your molds and supplies are geared toward System 96
• You want widely available accessory glass

Tips for Avoiding Compatibility Mistakes 

• Label scrap glass by COE
• Store 90 and 96 COE separately
• Buy glass from the same compatibility line
• Double-check frit, stringer, and embellishments
• Keep a firing log with material notes

These habits prevent costly kiln failures and broken finished work.

Ready to Choose the Right Fusible Glass? 

Using compatible glass is the foundation of successful kiln work, and having reliable materials makes every project easier. Whether you work with 90 COE or 96 COE, choosing high-quality sheet glass, frit, and accessories ensures consistent firing and stronger finished pieces.

Explore Hollander’s selection of fusible glass, frit, molds, and fusing supplies to find everything you need for your next project. And if you run into problems during firing, review this helpful guide on troubleshooting common issues in glass fusing to diagnose defects and improve results.