What is coe?

C.o.e. (coe) stands for Coefficient of Expansion, and is a measure of how much a glass expands or contracts due to changes in temperature. For anyone whose craft requires melting and cooling glass, ensuring compatibility between different colours and different brands is critical. Comparing the coe of the glass used is the most important part of ensuring compatibility.

 

The coe of the various makes of glass used for beadmaking and lampwork varies widely. The table below summarises the various brands found at off-mandrel.com, and groups them by Coefficient of Expansion.

Manufacturer

Coefficient of Expansion

Effetre

Vetrofond

Reichenbach (Rods)

TAG 104

Double Helix

Lauscha

Creation is Messy (CiM)

104

TAG 33

Clear Borosilicate

33

Reichenbach (Frit)

94

Bullseye

90

Schott

91




In general, glasses used together (one in contact with another) should be matched in coefficient of expansion. At its simplest, this means choosing glasses from within one category of the table show above, and for most applications, this is sufficient.


This is not the whole story, however. A small amount of variation in coe can be tolerated, and in fact, most glass ranges vary a little in coe across the range.


It is generally accepted that two glasses which are separated by 4-5 coe points can be considered compatible, subject to appropriate testing. Most manufacturers will warn that users should make appropriate compatibility tests before committing to using a particular glass or combination of glasses in a project.


Two examples of exceptions to the compatibility generalisation:


· Lauscha Clear is incompatible with many coe 104 reactive ‘silver’ glasses; encasing these glasses with Lauscha often causes beads to crack, even though they have been through a proper annealing cycle.

 

· Reichenbach (and other) frits are often used to decorate coe 104 glass beads, even though the coe of Reichenbach frit is around 94 (+/- 2). It is often said that this is ‘safe’ if the quantity of frit used remains less than 15% of the total bead mass.


It may seem that the compatibility issue is a bit of a minefield; it’s often not as complex as it seems. In general, stick to glasses with common published coe values (ie. 104 with 104, 90 with 90, etc), and you’ll not go far wrong. Feel free to experiment with other combinations, but be prepared for compatibility issues. There are many combinations which seem to break the rules, but which work out just fine.

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