A beautifully fused piece can crack days after it comes out of the kiln, even when the firing looked perfect. Often, the cause is not the firing schedule at all. It is incompatible glass. If you have been asking what does COE mean in glass, the short answer is that it is one of the key guides to choosing glass that can be fused together safely.
COE matters most in kiln-formed work, including fusing, slumping, tack fusing and cast glass. It is less relevant to traditional leadlighting and copper foil work, where glass is cut and assembled mechanically rather than melted together. Knowing the difference can save a great deal of wasted material, kiln time and frustration.
What does COE mean in glass?
COE stands for Coefficient of Expansion. It describes how much a glass expands when heated and contracts as it cools. The number is usually expressed as a value such as COE 90, COE 96 or COE 104.
Glass does not stay exactly the same size as its temperature changes. It expands in the kiln and contracts again during cooling. When two glasses move at different rates, they can place stress on one another. In a fused piece, that stress may produce an immediate crack, a break during cooling, or a crack that develops later without any obvious cause.
A COE number gives makers a practical starting point for combining glass. For example, Bullseye glass is commonly described as COE 90, while Oceanside Compatible glass is COE 96. These systems are made and tested so that glasses within each manufacturer’s compatible range can generally be fused together when used and fired appropriately.
The important point is this: COE 90 glass should not be treated as interchangeable with COE 96 glass. They are different systems and should be kept separate in kiln work.
COE is a compatibility guide, not a guarantee
It is tempting to think that two glasses with the same COE will always fuse safely. In practice, it is more nuanced than that. COE is a useful number, but it does not fully describe the behaviour of a particular glass.
Compatibility is also influenced by the glass formula, viscosity, heat history and the way a material has been manufactured. Two glass products may have a similar stated expansion figure but still be unsuitable for fusing together. This is why reputable kiln-glass manufacturers test their ranges for compatibility rather than relying on the COE number alone.
For dependable results, use glasses, frits, powders, stringers and rods labelled as compatible within the same tested system. A COE 90 transparent sheet, for instance, is intended to work with other tested COE 90 kiln-forming products from that system. The same principle applies to COE 96 products.
This is particularly relevant when using special-effect materials. Dichroic glass, iridised surfaces, handmade stringers, murrine and inclusions can all behave differently in the kiln. Check that each item is intended for your chosen glass family before incorporating it into a finished piece.
Why incompatible glass cracks
During a firing, glass passes through several important temperature stages. At high temperatures, it softens and flows. As the kiln cools, the glass becomes rigid again. The most critical part of the firing is annealing, where the glass is held and cooled carefully through the range in which internal stress can develop.
If all the glasses in a piece contract at a similar rate, annealing allows the piece to cool into a stable state. If one element contracts more or less than another, the materials pull against each other. The result is residual stress trapped inside the glass.
Sometimes the evidence is obvious: a crack runs from an inclusion, a corner breaks away, or the piece splits in the kiln. Other times, the work appears sound until it is cut, cold-worked, transported or simply left on a shelf. That delayed failure is one reason experienced fusers are careful about compatibility from the start.
A correct annealing schedule cannot make incompatible glass compatible. Annealing relieves stress created by firing and cooling, but it cannot overcome a basic mismatch in expansion between two glasses.
Which glasswork needs COE matching?
COE matching is essential whenever glass will be heated enough to bond, flow or become one piece. That includes full fusing, contour fusing, tack fusing, kiln carving, slumping a pre-fused blank, pate de verre and many cast-glass processes.
It also matters for lampworking. Glass rods used to make beads or sculptural work need to be selected within a tested compatible family. A bead may look fine on the mandrel but crack later if incompatible colours or decorative elements have been combined.
By contrast, COE is usually not the deciding factor for stained glass windows, leadlighting, copper foil panels or mosaics. A leadlight can combine cathedral glass, opalescent glass, textured glass and handmade sheet glass from many different manufacturers because the pieces are held by lead came, zinc, copper foil, grout or adhesive. They are not fused together in a kiln.
There are still practical considerations in these projects. Glass thickness, texture, cutting behaviour, colour transmission and the suitability of a glass for the location all matter. But you do not need every piece in a traditional leadlight panel to share the same COE.
Common COE systems you will see
COE 90 and COE 96 are the systems most often encountered by kiln-forming artists in Australia. Bullseye is a well-known COE 90 compatible system, with sheet glass, frit, powders, stringers, rods and accessory products designed for kiln work. Oceanside Compatible is a COE 96 system with a wide selection of colours and styles for fusing.
COE 104 is commonly associated with soft glass used in lampworking. It should not be assumed compatible with COE 90 or COE 96 kiln glass. Other expansion systems exist as well, particularly in specialty rods, scientific glass and production glass, so always check the manufacturer’s information rather than guessing from appearance.
The number does not indicate quality. A COE 90 glass is not better than COE 96, nor is one inherently easier to fuse. Your choice may come down to the colours, effects, product formats and firing characteristics you prefer. The most useful approach is to choose one compatible system for a project and build your materials around it.
How to choose glass without mixing systems
Start by deciding whether the project is kiln-formed or assembled cold. For a leadlight panel, select glass for its colour, texture, opacity, thickness and cutting qualities. For a fused bowl, jewellery piece or kiln-formed tile, select a tested compatible glass family first.
Read product labels closely. Terms such as “fusing compatible”, “COE 90” or “COE 96” are useful, but they should be read alongside the manufacturer and product range. A label saying “COE 96” does not automatically mean it is suitable with every other item that happens to carry a 96 designation.
Keep offcuts organised. Mixing scraps from different systems is an easy mistake, especially when clear glass, black glass or small pieces of frit look similar. Label storage containers and retain identifying stickers on larger sheets until they have been used. Many makers find it helpful to maintain separate scrap trays for each kiln-glass family.
Be cautious with unknown glass. Window glass, recycled bottles, stained glass offcuts and unlabelled sheets can be useful for experimentation, but they should not be combined with known compatible kiln glass in work you intend to sell, gift or rely on structurally. Their expansion and firing behaviour can vary considerably.
Can you test glass compatibility yourself?
A simple strip test can provide an indication when you are uncertain about two glasses. Fuse a narrow strip of one glass across a base piece of the other, then inspect the cooled sample for stress. If the top strip contracts differently, the base may curve or show stress-related issues.
However, a strip test is not a substitute for using a tested system. It can reveal a clear incompatibility, but it may not expose every problem, particularly where colours, thicknesses and firing schedules change. For production work or pieces with sentimental value, working within a manufacturer-tested range remains the sounder choice.
If a fused piece has cracked and you are unsure why, check compatibility before changing your kiln schedule. Then consider whether the piece was annealed for its thickness, whether it cooled too quickly, and whether it contains an untested inclusion. Glass problems often have more than one contributing factor.
COE questions makers often ask
Can I fuse COE 90 and COE 96 glass together?
No. They are different expansion systems and should be treated as incompatible for kiln work. They may look fine initially, but stress can cause cracking during firing or later.
Does clear glass work with every COE?
No. Clear glass also has a defined expansion and must match the compatible system used for the project. Keep COE 90 clear separate from COE 96 clear and from unknown clear offcuts.
Does COE matter when slumping?
Yes, if the blank has been made by fusing multiple pieces of glass. The components must be compatible before the slump firing begins. A single compatible sheet can be slumped without combining different glasses, but its firing schedule should still suit its thickness and mould.
Is all stained glass suitable for fusing?
No. Most stained glass is made for architectural or decorative assembly, not kiln forming. It may change colour, devitrify, distort unpredictably or be incompatible with other glasses when fired.
COE is not a complicated rule once it becomes part of your material habits: choose a tested system, label your offcuts, and treat unknown glass as experimental. If you need help matching sheet glass, frit, stringers or dichroic products for a kiln project, the team at Art Glass Imports can help you start with materials that are made to work together.