"CoE Equals Compatibility" - Kiln Forming Myths 10

CoE equals compatibility.

This is as persistent myth.  CoE is an abbreviation for Coefficient of Linear Expansion.  It is not an abbreviation for Compatibility.  

Apparently, CoE is used by manufacturers of glass that is being marketed to capitalise on the popularity of fused glass without the necessity of carrying out the testing and quality control required to ensure compatibility.  It is also used as a marketing device by wholesalers and retailers possibly to make greater sales.  It is used by individuals who have been lead into sloppy thinking about the materials they are using.

There are several facts to reinforce the assertion that CoE does not equal, nor is a shorthand for, compatibility.

·         Glass marketed as CoE90 or CoE96 has to be tested by the user.  Many users have often found that the compatibility with their other glass is suspect and inconsistent. This comes from breakages that occur with one sheet of glass but not another.

·         The System 96 range was made by two glass manufacturers who had testing and quality control to ensure their whole range is compatible.

·         Uroboros makes fusing compatible glass that many claim to be compatible with Bullseye.  In general, that is the case.  But many have found that it is important to test the compatibility of the glasses from Uroboros and Bullseye against each other before committing to a project, as the compatibility is not (and cannot) be guaranteed.

·         Not all float (window) glass is compatible between manufacturers.  Even the coloured glass is marketed with a range of 6 CoE points.  And some float glass is not compatible with the accessory glass. There is even a float glass that has a CoE of 96, but it is nowhere near compatible with System 96 glass.

·         There are physical reasons too.  Coefficient of Linear Expansion is tested as the average expansion between 20°C and 300°C.  This is the brittle range for glass.  We are much more interested in what happens at the glass transition point – the small range of temperature where the glass changes from a viscous liquid to a solid – generally between 480°C and 530°C. 

·         At the glass transition there is a surprising (to me) reduction in the CoE before a rapid rise.  This variation is influenced by the viscosity of the glass.  Also, at this temperature the CoE is much higher than at the measured region and cannot be taken as a guide to what is happening at the transition point.

·         In the early attempts to make compatible glass for fusing, it was discovered that the closer to the same CoE the glass was made, the less compatible it became.

·         Viscosity is the important element in the making of compatible glass.  The change in viscosity at the glass transition point must be balanced with the expansion characteristics of the glass.  A more viscous glass requires to be balanced by a different CoE glass than a less viscous one. Thus the CoE is being adjusted – not the viscosity – to balance the forces within the glass.

·         Finally, I believe the CoE of Bullseye’s clear glass is actually 90.6 rather than 90, so if we are rounding, Bullseye might be called CoE91. 

Whether the clear CoE90 or CoE96 of other manufacturers is the same as the Bullseye, System96, or Uroboros is not the relevant point.  The relevant point is whether it is compatible.  Whether these other companies have the quality control to ensure all their glass is compatible with the claimed fusing glass without further user testing is the essential point.  At this time, it appears that they do not have that capacity.  So, those using glass marketed as CoE90 or CoE96 will need to continue to test for compatibility with each sheet they use.

Other posts on Compatibility are here:
Is Coe Important?
What is Viscosity?
CoE varies with temperature
Defining the glass transition stage

All myths have an element of truth in them otherwise they would not persist.

They also persist because people listen to the “rules” rather than thinking about the principles and applying them.  It is when you understand the principles that you can successfully break the “rules”.