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How to Determine Glass Compatibility

How to identify different COE of Glass and Glass Compatibility

There are many different production methods and recipes for making glass. As a result, there are almost as many different types of glass as there are glass artists who use them. Types of glass range from basic window glass (called "float glass") to brightly colored stained glass (also called "art glass"), many of the types of glass come in numerous sub-types and categories.

In addition, there are many types of glass coatings, such as iridescent and dichroic, which have unique properties when applied to glass. New types of glass and glass coatings are constantly being developed.

All of these different glass types are candidates for fusing, slumping, and other kiln-forming processes. Some can be used off the shelf, but others require testing to make sure they will work in the kiln. That’s because it’s likely that you will want to combine more than one different sheet of glass in your projects. If so, then you’ll need to make sure the glass you select is "compatible." Using incompatible glass may cause cracking or even shattering of the piece when it cools.

What are the Different types of Recycled Glass and Compatibility?

Different Glass COE (Coefficient of Expansion) will have different heating and cooling temperatures in a kiln and temperatures are also affected by glass density.

Most of our glass artist customers are used to hearing COE 90 by Bullseye Glass or COE 96 by Spectrum Glass as the type of COE.

Here are a few other common COE types:

  • COE 32 - CORNING GLASS: Also called Pyrex or Borosilicate glass.
  • COE 84 - 87 - WINDOW PANE: Standard window pane glass. It is also referred to as "float" glass.
  • COE 104 - EFFETRE: Also called Moretti glass. It is commonly used for lampworking, torch work.

The difference in expansion and contraction can be significant as the glass contracts during the cooling process. Regardless of how small the difference is between the COE of the glass, if you are trying to fuse two different COE together, it counts! When using different types of COE it is also important they be kept in separate areas of your glass studio. This will help prevent you from accidentally intermingling them when assembling glass to be fired.

Sometimes you can get away with using two different COEs of glass where the COE differences are small. Test fire samples of approximately 3 inch square to provide enough glass to test the compatibility.

Two methods to determine the COE compatibility:

  • Stick with glass that has already been "Tested Compatible" by the manufacturer. Manufacturers typically charge more for "Tested Compatible" glass but it avoids the issue entirely.
  • Conduct a compatibility test: Glass can look fine to the naked eye, but when using a polarized film to view the glass, you can see the differences. Simply fuse a small piece of glass and then examine it by sandwiching it between two strips of polarized film. Compatibility testing can also allow you to identify more glass colors and textures from a stack of unidentified glass. Testing will allow you to save money and time!
9th Jun 2018 Ann Sanborn

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