When it comes to glass, I often get asked assorted questions such as…
“Is this recycled glass?”
“I make stained glass so I already have a bunch of glass - can I just fuse them together?”
“I have a bunch of wine bottles - can I just use those?”
As nice as it would be to just fuse whatever glass strikes your fancy, unfortunately, that’s just not a good idea when it comes to fusing glass together.
If you’re new to fused glass, you may have come across the term COE or Coefficient of Expansion. It sounds technical, but don’t worry–it’s easier to understand than you might think, and it’s an important concept to know when working with glass! In this blog post, I’ll break down what COE means, why it matters in glass fusing, and how different types of glass are suited for different purposes.
What is COE?
COE, or Coefficient of Expansion, refers to how much the glass expands when it’s heated and contracts when it’s cooled, so if you attempt to fuse glass together that have different COE’s, they're not able to work together because of these different expansion rates. However, different types of glass expand and contract at different rates. For example, glass with a COE of 90 expands less than glass with a COE of 96 when heated. These numbers might seem small, but when you’re heating glass to temperatures over 1,400°F (the temperature needed for fusing), even slight differences in expansion can cause problems.
Why Does COE Matter in Fused Glass?
When you fuse glass, you typically layer different pieces of glass together. If those pieces have different COE’s, they expand and contract at different rates during the heating and cooling process. This creates stress between the layers, which can lead to cracks or even breakage. You could find the piece broken when you open up the kiln to admire your glass creation. Or the design could stay fused together but maybe a year from now the stress in the glass becomes too much and it just randomly breaks one day. By using glass with the same COE, you ensure that all the layers expand and contract at the same rate, keeping your fused piece stable and durable.
Common COE Types in Fused Glass
There are a few common COE values used in fused glass, each corresponding to different glass manufacturers:
• COE 90: Often used with glass from Bullseye Glass Company
• COE 96: Common with glass from Oceanside Glass & Spectrum
• COE 104: Generally made with Moretti, Effetre or Lauscha glass
These numbers don’t indicate the quality of the glass—they simply tell you how much the glass expands and contracts. You can create beautiful projects with any of these COEs, as long as you’re consistent and only use glass with the same COE in each project.
The type of glass I use is Bullseye 90 COE, as that was what I was taught with, has a great line of colors, and is the most accessible in my area. Once you determine which COE you are going to use, it’s a good idea to just stick with that COE in your studio. If you don’t, you run the risk of contamination and accidentally mixing COE’s. If you do end up with multiple COE’s in your studio, be sure to keep them marked/labeled and separated from your other glass, so they don’t get mixed together!
Why Are There Different Types of Glass for Different Purposes?
Different glass artists use different types of glass depending on what they’re creating:
1. Fused Glass (COE 90 or 96):
Engineered and designed specifically kiln work, this glass is made with specific COE values to ensure compatibility when fusing.
2. Lampworking (COE 104):
Used in flameworking, like making beads or small sculptures, where the glass is melted with a torch. COE 104 glass is softer and melts easier in the flame, making it a good choice for creating designs and handles rapid temperature changes well.
3. Stained Glass (COE varies):
When designing a stained glass panel, you can use any kind of glass you want that works best for your project! Since it won’t be heated up in the kiln, COE doesn’t matter.
4. Float Glass (COE varies):
This is your typical window glass. It has a different COE than art glass, so it isn’t compatible with fusing art glass. It can make for good scrap glass when practicing your cutting skills–just label it and don’t mix it with your fusible glass!
5. Borosilicate Glass (COE 33):
Known for its durability and resistance to thermal shock, this glass is used in scientific glassware, some lampworking and is also known as Pyrex (like your cookware). Its low COE allows it to withstand extreme temperature changes without cracking.
The Takeaway
When working with fused glass, always check the COE of the glass you’re using and make sure all the pieces in your project have the same COE. This simple practice will help you avoid cracks, ensure your pieces are stable, and give you the confidence to create beautiful fused glass art.
Happy fusing!
— Lindsay