A beautiful firing can still fail on the way down through the kiln. A crack that appears after the glass has cooled is often a compatibility problem, which is why the question of COE 90 versus COE 96 matters before you cut the first piece. For fused glass work, choosing a compatible glass system is not a minor technical detail. It is the basis of a sound, durable result.
COE stands for coefficient of expansion. It describes how much glass expands when heated and contracts as it cools. Glasses made to the same tested system are formulated to move together through the firing and cooling cycle. When they do not, internal stress can remain in the finished piece, even when it looks perfect at first.
COE 90 versus COE 96: the practical difference
COE 90 and COE 96 are two distinct kiln-forming glass systems. In simple terms, COE 90 glass is designed to fuse with other compatible COE 90 products, while COE 96 glass is designed to fuse with compatible COE 96 products. They should be treated as separate families.
Bullseye is the best-known COE 90 system. Its sheet glass, frit, powder, stringers, rods, confetti and many compatible speciality products are formulated and tested to work together when fired to the manufacturer’s recommended schedules. This makes it a popular choice for fusers who want a broad palette, reliable repeatability and access to established technical information.
COE 96 is widely associated with the System 96 range, including compatible Oceanside products and accessories. It is also a well-supported system with a generous range of sheet glass, frits, noodles, stringers and dichroic options. It offers its own colours, textures and visual character, so the choice is not simply about one system being better than the other.
The key point is straightforward: do not fuse COE 90 and COE 96 together in the same project. A full fuse does not make incompatible glasses compatible. In fact, higher temperatures can make the stress issue harder to identify because the pieces may appear to blend beautifully before cracking later.
Why the COE number is not the whole story
The COE label is useful, but it is not a universal guarantee. Glass compatibility is more complicated than one number printed on a label. COE is an average measurement over a particular temperature range, and two glasses with a similar expansion figure may still behave differently when heated and cooled.
That is why serious kiln-forming manufacturers test their products as a system. Their compatible glasses are formulated not only around expansion but also around other thermal characteristics, including viscosity and strain behaviour. A manufacturer’s COE 90 range should be used with that manufacturer’s compatible products unless the product is specifically stated to be tested for cross-compatibility.
This is especially relevant when buying speciality materials. Dichroic glass, iridised glass, glass paint, frit, murrine, enamel, inclusions and recycled offcuts can all introduce uncertainty. Check the product description rather than assuming that a familiar-looking material will fuse safely with your base glass.
A label that says “90 COE” does not automatically mean it can be combined with every other 90 COE product from every source. The same applies to COE 96. When a project matters, use a tested system and keep the packaging or product details with your offcuts.
Choose one system at the start of a project
The easiest way to avoid expensive mistakes is to decide whether a project is COE 90 or COE 96 before selecting glass. Begin with the base glass, then choose every fusible component to suit that system.
For example, if you begin a platter with Bullseye COE 90 sheet glass, select COE 90 frit, stringers, rods and compatible dichroic. If you begin with an Oceanside or other System 96 base glass, stay within the tested COE 96 range for your decorative elements. Clearly label scrap containers, particularly for transparent glass that can be difficult to identify once cut down.
This approach becomes even more valuable in a shared studio. A mixed scrap tray may be fine for mosaic work or decorative cold applications, but it is not suitable for random kiln fusing. Separate storage is less exciting than choosing colours, but it saves time, glass and kiln shelf clean-up.
When colour or texture drives the choice
Sometimes the glass itself makes the decision for you. You may need a particular transparent colour, opal tone, streamer pattern or reactive effect available in one system but not the other. That is a sound reason to choose a system, provided the rest of the design follows it.
It is also worth looking at sheet size, product availability and the repeatability you need. A one-off small pendant allows more room for testing than a set of commissioned cabinet inserts or a production run of slumped dishes. If you expect to remake an item later, choose materials you can source consistently and record what you used.
Firing schedules matter after compatibility is sorted
Compatible glass can still crack if it is fired or cooled poorly. COE is not an annealing schedule. Each manufacturer provides firing guidance developed for its own glass, and that guidance should be your starting point.
The thicker the work, the more carefully it needs to be annealed. A simple two-layer fused tile may tolerate a standard schedule well, while a thick casting, multiple layers of glass, dense frit work or a substantial sculptural piece needs a slower cooling cycle and often a longer anneal soak. Complex forms can also heat unevenly, particularly where glass is piled high in one area and thin at the edges.
Do not assume that a schedule used successfully for one project will suit the next. Kiln size, element placement, shelf load, glass thickness and the shape of the work all affect results. Keep notes on the glass system, layers, firing temperatures, holds and annealing cycle. Those notes quickly become more useful than a generic firing chart.
A break test can answer a compatibility question
When you are uncertain about two glasses, make a small test piece before committing to a larger design. Fuse the glasses together using an appropriate schedule, then inspect the cooled sample carefully. A common workshop method is to deliberately break the test and examine the break line under polarised light for stress.
This can reveal incompatibility, but it is not a shortcut around manufacturer testing. A small sample may not reproduce the stresses in a large bowl, thick panel or layered casting. Treat a successful test as useful information, not a blanket approval for every firing situation.
If the glass source, COE system or product history is unknown, the safest choice is to keep it out of kiln-formed work. Unknown glass can still be valuable for leadlighting, copper foil work, mosaics and other cold construction methods where the pieces are not being fused together.
Does COE matter for stained glass and leadlighting?
For traditional stained glass, copper foil panels and leadlight construction, COE compatibility is generally not the deciding factor. Glass is cut, fitted and held mechanically with lead came, copper foil and solder rather than melted into a single sheet. You can combine a wide range of art glasses for colour, texture and light transmission without needing them to share a kiln-forming system.
That said, glass selected for a panel should still suit the job. Consider its thickness, texture, cutting behaviour, strength and how it will sit beside neighbouring pieces. A heavily textured glass may look striking but be difficult to foil neatly, while a very thin sheet can need careful handling in a large leadlight.
The line changes when you add kiln work. A fused cabochon, fired glass inclusion or kiln-formed border must be made from compatible materials within its own component. Once it is fully annealed and installed mechanically into a leadlight or jewellery setting, it does not need to match the COE of the surrounding unfused sheet glass.
A reliable buying and studio habit
Write the COE system directly on every bag of frit, packet of stringers and container of offcuts as soon as it enters the studio. Keep COE 90 and COE 96 tools, scraps and project trays separate where practical. It sounds basic, but most accidental mixing happens after labels have been removed and small pieces have accumulated.
When selecting materials, start with the finished result you want, choose one tested glass family, then build the project within that system. If you are unsure whether a speciality glass belongs with your base sheet, pause before firing and check the stated compatibility. A few minutes of care at the bench gives your work the best chance of leaving the kiln exactly as you intended.