Choosing glass by color alone can produce a beautiful layout that fails in the kiln. Compatibility depends on how each glass responds to heat, including its expansion, contraction, viscosity, and softening behavior. For most kiln-formed projects, identifying the COE family before cutting or layering is the first safeguard against internal stress and cracking.

Understanding what the number measures makes the comparison easier and helps you select a firing schedule that supports strong, durable work. The complete glass fusing guide provides broader project guidance, while the next section establishes the science behind COE.

What Does COE Mean in Glass Fusing

COE stands for Coefficient of Thermal Expansion. It describes how much a glass material expands when heated and contracts when cooled. In kiln-formed glass, this measurement is also a practical compatibility guide. The COE number helps you determine whether two pieces can share a fused project without developing damaging internal stress.

Thermal expansion is a predictable physical process. As glass moves through a firing cycle, its dimensions change with temperature. During cooling, the glass contracts again. When compatible pieces respond at similar rates, they can remain bonded as the project returns to room temperature. For a broader overview of firing methods and project planning, see this guide to how to choose between COE 90 and COE 96.

How the COE numbers compare

The number represents the approximate expansion rate, expressed in units of 10^-7 per degree Celsius:

• COE 90: expands at approximately 90 x 10^-7/degrees C.
• COE 96: expands at approximately 96 x 10^-7/degrees C.

That difference may look small, but it matters when separate glass pieces are fused into one structure. COE 90 and COE 96 belong to different glass families, so they should not be treated as interchangeable materials. Always confirm the compatibility rating supplied by the manufacturer rather than assuming that similar colors, thicknesses, or shapes can be combined safely.

Why mismatched COE glass cracks

When glasses with different expansion rates are heated and cooled together, they do not move in exactly the same way. One layer may expand or contract more than the other. Once the pieces are bonded, that unequal movement creates internal stress. If the stress exceeds the glass’s tensile strength, the finished piece can crack.

The damage is not always visible when the kiln opens. A mismatch can produce a crack immediately, or the accumulated stress may cause failure hours or days later. Correct COE selection is therefore part of the design process, not a minor labeling detail. Pair it with a firing schedule designed for the selected glass family and proper annealing to relieve internal stress.

COE 90: Bullseye and Oceanside System Properties

COE 90 is a major compatibility system used in kiln-formed glass, most closely associated with Bullseye Glass. Its nominal expansion rate is approximately 90 x 10^-7/°C, a specification that describes how the material expands during heating and contracts during cooling. That number is not interchangeable with a COE 96 label. Glass from different systems can respond differently throughout the firing and annealing cycle, so the compatibility designation must remain consistent within a project.

Thermal behavior and firing considerations

Compared with COE 96, COE 90 generally requires higher fusing temperatures because it is the harder system of the two. A schedule developed for a softer, lower-temperature glass should not be transferred automatically to Bullseye work. Use the manufacturer’s recommended firing and annealing guidance, then test unfamiliar combinations or thicknesses before committing valuable material to a finished piece. A controlled annealing segment is essential for managing internal stress as the glass cools.

The expansion rate is only one part of compatibility. Viscosity, softening behavior, and the way a particular formulation responds to heat also influence the finished result. Two pieces marked with a similar COE still need to be confirmed as compatible through the manufacturer’s system and product guidance. For background on how these properties affect a kiln schedule, review this guide to understanding glass compatibility.

Color, surface, and product selection

Bullseye’s COE 90 system is known among fusing artists for rich, deep color and dependable stability across a broad range of kiln-formed applications. Available sheet formats include transparent, opalescent, and streaky styles, giving artists control over translucency, layering, and visual movement. These choices are useful for everything from small jewelry components to dimensional panels and sculptural work.

The system also includes dichroic glass for reflective accents and high-contrast details. Art Glass Supplies maintains Bullseye COE 90 materials alongside CBS dichroic inventory, allowing artists to source compatible project components in one place. Explore the current COE 90 glass collection or browse the dedicated COE 90 dichroic glass selection before planning your layup.

Source: Swart Glass COE reference; relative fusing behavior documented by COE 96 manufacturer compatibility guidance.

COE 96: Wissmach and Uroboros System Properties

COE 96 is a glass compatibility system associated with Oceanside and also used across compatible Wissmach and Uroboros product lines. Its expansion rate is approximately 96 x 10^-7/°C, meaning the glass expands and contracts at a different rate from COE 90 during the kiln cycle. Use it as a complete system rather than combining it with unrelated COE families.

A softer glass with a lower-temperature working range

Compared with COE 90, COE 96 is generally softer and matures at lower kiln temperatures. That difference affects more than the final firing temperature. Softer glass can influence how edges move during a full fuse, how layers settle, and how a piece responds during slumping. Follow a firing and annealing schedule designed for the specific glass formulation and project thickness; a lower-temperature system is not a substitute for correct temperature control.

The COE 96 palette is especially useful when a project depends on strong, brilliant color. Artists can build designs with Oceanside sheet glass, add specialized accents from the Fusers Reserve line, or use compatible dichroic glass for reflective detail. Product availability and color selection make the system practical for both small decorative pieces and larger compositions that require a coordinated palette.

Why COE 96 can suit larger projects

COE 96 is often an economical choice when a project uses substantial amounts of sheet glass or repeated color combinations. Its cost advantage can make larger panels, production runs, classroom work, and broad mosaic-style compositions more manageable without reducing the need for careful compatibility control. Buying within one system also simplifies material organization and reduces the risk of accidentally combining incompatible pieces.

Explore the COE 96 glass collection to compare Oceanside sheet glass, Fusers Reserve, and dichroic options. Confirm each product’s compatibility designation before cutting, layering, or firing, particularly when adding specialty glass to a larger design.

Can You Mix COE 90 and COE 96 Glass

No. Never mix COE 90 and COE 96 glass in the same fused piece. These are different compatibility families, even though the numerical difference may appear small. COE 90 and COE 96 expand and contract at different rates during the kiln cycle. When the layers are fused together, they cannot move uniformly as the temperature rises and falls.

That mismatch creates internal stress at the boundary between the glasses. If the stress exceeds the tensile strength of the glass, the piece can develop cracks or separate between layers. Mixing different COE ratings is one of the primary causes of stress cracks in fused glass, and a successful firing does not prove that the combination is safe. The damage may be hidden until the piece cools completely or is handled later.

Why the cracks may not appear right away

Incompatible glass can crack immediately after firing, but failures may also appear hours or days later. Residual stress can continue accumulating as the piece reaches room temperature and experiences ordinary handling or temperature changes. A careful annealing schedule can relieve stress created by the firing process, but it cannot make two incompatible COE families compatible.

For reliable kiln work, select one system before you design the project. Check the compatibility information for every sheet, stringer, frit, powder, enamel, and dichroic element you plan to include. Keep COE 90 and COE 96 materials in clearly separated storage areas, and use a firing schedule intended for the selected family. These precautions protect both the finished piece and the time and materials invested in it.

For a deeper explanation of the risks and compatibility rules, read our guide to mixing COE 90 and 96.

Choose your glass system and shop COE 90 glass or shop COE 96 glass before starting your next firing.

Which COE Should You Choose for Your Projects

COE is the first compatibility decision in kiln-formed glass. Once you choose a system, keep its sheet glass, frit, stringer, powders, and accessory materials within that family unless the manufacturer specifically identifies them as compatible. The right choice depends less on a universal winner than on your preferred color behavior, working temperature, budget, and finished object.

COE 90 and COE 96 comparison for glass fusing projects

Consideration	COE 90	COE 96
Color palette	Deep, subtle, and nuanced color options, including the broad Bullseye range.	Vibrant, bold color effects associated with the Oceanside system.
Fusing temperature	Generally requires a higher fusing temperature.	Generally softer and reaches a fusing maturity at a lower temperature.
Cost	Often the higher-cost option, depending on the material and color.	Often the more affordable option for building a larger material selection.
Stability	Excellent stability for detailed work, with less tendency toward devitrification in suitable firing conditions.	Slightly softer behavior, which can be useful but requires careful control of heatwork.
Best use cases	Detailed compositions, sculptural work, and projects where controlled structure matters.	Jewelry, suncatchers, and larger-scale projects where vivid color and material economy are priorities.
Manufacturer support	Bullseye is the primary manufacturer associated with this system. Explore COE 90 glass.	Oceanside is the primary manufacturer associated with this system. Explore COE 96 glass.

What should beginners choose?

Beginners should usually choose the system with the clearest material availability and the strongest support from their local supplier, instructor, or studio. COE 96 can be approachable because it is softer and generally fuses at a lower temperature. COE 90 is equally practical when your class, project instructions, or existing inventory is built around Bullseye. Starting with one family makes compatibility easier to manage while you learn tack fusing, full fusing, slumping, and annealing.

When does a professional need a specific COE?

Professionals should choose based on the visual language and physical demands of the collection. COE 90 suits artists who prioritize stable, detailed, or sculptural forms. COE 96 can make sense for bold color, jewelry production, suncatchers, and larger work where cost and lower-temperature firing influence the production plan. In either system, store the glass separately to prevent an accidental mix-up, and always use the firing schedule designed for that COE family. A kiln schedule cannot make incompatible glass compatible.

Frequently Asked Questions

What is the difference between COE 90 and COE 96 glass?

COE 90 and COE 96 identify glass families with different thermal expansion rates. COE 90 expands at approximately 90 x 10^-7/C, while COE 96 expands at approximately 96 x 10^-7/C. COE 96 is generally softer and fuses at a lower kiln temperature than COE 90, so each family needs its own compatible materials and firing approach. Source: Swart Glass

Can you fuse COE 90 and COE 96 glass together?

No. Treat them as incompatible families unless a manufacturer specifically identifies a tested combination as compatible. They expand and contract at different rates during heating and cooling, which can create internal stress, cracks, or breakage. Damage may appear immediately after firing or develop hours or days later as stress accumulates. Read the compatibility guidance.

Which glass is easier to cut, COE 90 or COE 96?

COE 96 is often easier to cut because it is generally softer than COE 90. Cutting comfort still depends on the specific sheet, thickness, cutter, and technique, so use controlled pressure and inspect every score before breaking the glass. Easier cutting does not change the compatibility requirement: keep the two COE families separate.

Are firing schedules the same for COE 90 and COE 96?

No. Their different expansion rates, softening behavior, and maturation temperatures mean you should use a schedule designed for the specific glass family. Follow the manufacturer guidance, include the appropriate annealing segment, and adjust only after controlled testing. A schedule cannot make incompatible glass compatible. See firing and compatibility guidance.

How should I prevent COE 90 and COE 96 mix-ups?

Store each family in a separate, clearly identified area, and keep scraps organized by COE rather than by color alone. Confirm the product label before assembling a project, especially when using small offcuts or decorative pieces. Separate storage is one of the simplest ways to prevent accidental mixing and protect a kiln firing. Review studio organization guidance.

Ready to Choose Your Fusing Glass?

Understanding the differences between COE 90 and COE 96 helps you select a compatible glass system and plan your next kiln-firing project with confidence. Shop COE 90 and COE 96 glass for sheet, color, and project materials that match your preferred system. Keep each COE family separate, follow its firing schedule, and get started with the glass that fits your technique.