90 COE

90 COE glass is made by to main manufacturers; Bullseye, and Uroboros.

It comes in a variety of colors, textures and thickness.

This product is available to purchase in sheet glass, noodles, stringers, confetti, powders and frit.

Be sure when purchased that your glass has been factory tested for compatibility.

If you're using Bullseye glass the annealing temperature should be around 960 degrees Fahrenheit.

Glass testing for compatibility has only been around since the late 1980’s.

So, it has only been in the last few years that glass workers have been able to buy tested glass.

What does the COE mean?

The COE or coefficient of expansion is the fractional change in the area, length or volume per unit change in temperature of the glass at a given constant pressure.

This just means that the different pieces of glass fused together have to be compatible in the rate of heat expansion and cooling contraction.


Bullseye glass is manufactured in Portland, Oregon.

They make flat glass as well as rod with the same 90 COE.

Bullseye is the glass of choice for many who do fused pieces.

Their glass is a little more expensive, because it is guaranteed to be compatible.

Bullseye glass is tested and guaranteed to be compatible even if fired up to three times to 1500 degrees Fahrenheit.

If the glass is fired hotter than 1500, there is no guarantee. When fired hot, some colors can change COE.

This is particularly true of oranges, reds and yellows.

Heat can change the chemical composition of the glass.

The exact chemical composition of the glass determines the compatibility characteristics.

This change can make the glass incompatible when taken to extremely high temperatures or fused more than a few times.


Uroboros started in 1973.

They produce 163 color combinations in 14 unique styles, and are available in many different styles or textures.

Each sheet is ladled from their furnaces and formed by hand using old-world techniques.

Uroboros started testing compatible fusible glass in 1989.

They also produce 96 COE glass, called System 96.

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