Glass color compatibility is one of the most important — and most misunderstood — variables in kiln-formed glass work. You can have perfect technique, the right kiln, and a carefully programmed firing schedule, and still end up with cracked or delaminating work if your glass pieces aren't compatible with each other. Here's what you need to know.
COE stands for coefficient of thermal expansion. It's a measure of how much a material expands when heated and contracts when cooled. Different glass formulations have different COEs. When you fuse two glasses with significantly different COEs together, they expand and contract at different rates during the firing and cooling cycle. If the difference is large enough, the stress exceeds the glass's tensile strength and the piece cracks — sometimes immediately, sometimes hours or days later as the stress slowly accumulates.
The most commonly referenced COE values in studio glass are COE 90 (Bullseye, most Spectrum colors) and COE 96 (Oceanside, formerly Spectrum 96). These two are the most popular systems and are NOT compatible with each other. Fusing a COE 90 piece with a COE 96 piece will eventually crack.
Flamework and lampwork artists often use soft glass with a nominal COE of 104 (Moretti/Effetre, Lauscha). This is the most common glass for bead making at the torch. COE 104 glass is NOT compatible with COE 90 or COE 96 kiln-forming glass. They're different formulations, different chemistries, different applications.
This confusion comes up frequently when flamework artists start adding kiln work to their practice. The glass you've been using at the torch generally won't work in a fusing kiln alongside your fusing glass.
Within a glass line — say, Bullseye COE 90 — all colors are formulated to be compatible with each other. But even within a compatible system, not all glasses are perfectly matched. Some specialty colors (especially blacks, whites, and some reactive colors) behave slightly differently from base colors and may require testing before using in large pieces.
This is why glass test strips and small test firings are standard practice. Fire a small fused sample of your color combination before committing to a large piece. Look for cracks, delamination, or stress lines in the cooled test piece.
Dichroic glass — glass with a metallic coating that creates iridescent color effects — is available in both COE 90 and COE 96 formulations. Make sure you're buying the right COE for your glass system. Dichroic on the wrong base glass will crack just like any other incompatibility.
The internet is full of people who claim they've been fusing mixed COE glass "for years without problems." This can be true for small pieces or thin layering where the stress doesn't build to failure point quickly. But for serious studio work, mixing COE systems is a recipe for delayed failure — pieces that look fine in the kiln and crack on the shelf weeks later.
Choose a system — COE 90 or COE 96 — and work within it. Label your glass storage clearly. Don't mix batches from different systems in the same bin.
A good kiln like the Hot Shot 12G or Hot Shot 16G fires whatever glass you put in it. COE compatibility is entirely about your material choices — the kiln's job is to hit the right temperature on the right schedule. Your job is to make sure everything inside is compatible before it goes in.
Browse our glass kiln lineup — the 12G and 16G are both excellent for COE 90 and COE 96 work.
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