Wednesday, December 5, 2007

Glass COE - Why Can't We All Just Get Along

Not all glass is created equal, which means that it is not all compatible either. While this wouldn't be a big deal if I only ever made beads using one color per bead, but that would be very boring for me and for you!

One of the first things new glass artists learn is that the COE, Coefficent Of Expansion, of the glass colors being used must all be the same. What's that, you say?

I know that sounded like a bunch of boring science-speak, so let me boil it down for you. When glass gets hot is expands. As it cools, it contracts. In order for a glass bead or any other object to survive the annealing and cooling process, the amount that each glass expands and contracts must be the same as all of the other glass in the piece.

If one color expands twice as much as the others, it will expand and contract too much, and the beautiful piece of art will crack and sometimes break apart due to the stress this incompatibility causes.

One minor exception is frit if used sparingly. Many of the frits on the market do not match the COE of the soft glass, but they have a secret that will enable them to be compatible when used on the surface of a bead as decoration. What's their secret? The lead content in the glass enables it to be compatible with glass that has a COE that is close to the frit. It is not so compatible that it can be covered with clear glass, a process known as encasing.

However, a thin layer on the surface of the bead can be used quite successfully as long the frit does not comprise more than five percent of the bead's glass. This is called the 5% Rule in glass circles.

If you have any questions or would like to suggest topics, I can be reached through this blog or through my web site or shop at

Have great afternoon!

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