Decoding COE: The "Must-Know" Core for Glassmakers if You Don't Want Your Work to Fall Apart

by Chanicha T

In the world of glass blowing, the beauty of a piece is not solely dependent on the skill of shaping or color mixing.

But there is also a "scientific secret" hidden behind it, which determines whether your work will survive as a complete piece or shatter into glass shards in an instant. This is called "Coefficient of Thermal Expansion" or COE

COE stands for Coefficient of Thermal Expansion, which, to put it simply, means that glass, like many other materials in the world, expands when heated and contracts when cooled. Each type of glass has a different rate of expansion and contraction, and the COE number indicates how much that type of glass will expand or contract when the temperature changes.

There's a simple rule of thumb: "The higher the COE number, the more that piece of glass will expand when hot and contract when cooled."

Why is COE so crucial for glass artists?

The importance of COE becomes immediately apparent when you want to "combine different pieces of glass," whether it's for blown glass with color gradients, decorating with glass rods, or fusing glass sheets, known as Glass Fusing

The golden rule to remember is that glass pieces intended to be fused together "must have the same COE." If two pieces of glass with different COE values are fused together in a hot kiln, the glass may appear smooth and well-bonded. However, during the cooling process, the glass with a high COE will contract quickly and significantly, while the glass with a low COE will contract slowly and only slightly.

The result is that the piece will crack or explode immediately upon cooling, or, even more frighteningly, it may not break immediately but could sit on a table for several days and then suddenly explode due to accumulated internal stress that it can no longer withstand.

Now, let's look at the most popular COE values in the glass industry, categorized by work type and technique:

COE 3.3 / 33* (Borosilicate Glass / Heat-resistant glass): Very low expansion and contraction, making it highly resistant to sudden temperature changes (Thermal Shock). Hot glass won't easily break when exposed to cold water, making it ideal for flameworking, scientific glass tubing, jewelry, and intricate handmade miniature glass art.

  • COE 96 (Soft Glass): Melts easily at lower temperatures than the first group, has good viscosity, and offers a wide variety of colors. It is the most popular standard value for furnace glassblowing and glass fusing.
  • COE 90 (Popular Soft Glass): This soft glass is highly precise and widely used in Glass Fusing.
  • COE 104 (Moretti / Effetre): Has the highest expansion rate in the group. The glass flows well and softens quickly when heated. It is most commonly used for beadmaking using the flameworking technique.

The matter of COE or the Coefficient of Thermal Expansion is a critical issue that cannot be overlooked. Try to understand it thoroughly and choose the right type of glass for your application. We guarantee that your glass pieces will be beautiful and exactly what you envision.

Note: COE 3.3 and COE 33 refer to the same type of glass; the difference is merely in the way they are referred to and the units of measurement used.

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