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

In the world of glassblowing art, the beauty of a piece isn't solely dependent on the skill of shaping or color mixing. There's a hidden "scientific secret" that determines whether your artwork will survive as a complete piece or shatter into fragments in an instant. This is called the "Coefficient of Thermal Expansion" or COE.

COE stands for Coefficient of Thermal Expansion. To put it simply, glass, like many materials, expands when heated and contracts when cooled. Each type of glass has a different rate of expansion and contraction. The COE value indicates how much a particular type of glass will expand or contract when the temperature changes.

There's an easy rule to remember: "The higher the COE value, the more the glass will expand when hot and contract when cooled."

Why is COE critically important for glass artists?

The importance of COE becomes immediate when you want to "combine different pieces of glass," whether it's for color-layered glassblowing, decorating with glass rods, or glass fusing.

The iron rule to remember is that glass pieces to be fused together "must have the same COE." Because if two pieces of glass with different COE values are fused together while hot in the kiln, the glass might appear smooth and well-bonded. However, when the glass cools down, the result is that the glass with a higher COE will contract rapidly and significantly, while the glass with a lower COE will contract slowly and only slightly.

The result is that the piece will crack or explode immediately upon cooling. Even more frightening is that it might not break immediately, but could sit on a table for days and then suddenly self-destruct 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. We will categorize them clearly by type of work and technique, as follows:

·      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 will not 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 for furnace glassblowing and glass fusing.
• COE 90 (Popular Soft Glass): This soft glass has high precision control and is most 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 flameworking techniques.

The matter of COE, or Coefficient of Thermal Expansion, is crucial and cannot be overlooked. Understand it thoroughly and choose the right type of glass for your application, and your glass pieces are guaranteed to be beautiful and exactly as you envision.

Note* COE 3.3 and COE 33 refer to the same type of glass; the difference lies only in the way it is expressed and the units of measurement used.