Wednesday, 14 November 2018

CoE Useage

Does anyone know what CoE means?

·         First the proper abbreviation is CoLE.
·         This means Coefficient of Linear Expansion.
·         A coefficient is an average.  This number may be exact at a given temperature, or an average over a range.
·         Linear is the length.  
·         Expansion is measured in fractions of a metre e.g., 0.0000096 metre.
·         The coefficient is given as the average amount of expansion per each degree Celsius.
T     The temperature range used is 0C to 300C.  Expansion characteristics vary greatly at higher temperatures.

So CoLE is the average amount (in metres) that glass expands for each degree (Celsius) increase in temperature from 0C to 300C. 

Whether you call it CoE or CoLE is immaterial, as it still does not equal compatibility.

It does not measure viscosity. Viscosity is a (possibly the major) element in making a range of compatible fusing glasses.

It does measure expansion rates, but up to 300C only.  It does not tell you how glass expands above that temperature.  Note: it does not behave in a linear pattern as crystalline materials do.

The CoE must be adjusted to match the viscosity to achieve compatible glass.  Spectrum has stated that their glass has a range of CoE of at least 94 to 98 to make compatible fusing glass.  Bullseye have not stated the range, but have indicated their base glass is nearer to 91 than 90.  It is to be presumed that their CoE range is approximately 89 to 93.

The only constant in fusing glass is compatibility

CoE varies within each manufacturer’s range of fusing compatible glass to match the viscosity. And remember the CoE of glass at the critical annealing point is  higher than the low temperature expansion rate. See this post for details.

Viscosity varies according to the materials used in the colouration of the glass, requiring the glass manufacturer to make adjustments in CoE to get compatible fusing glass.  More information here.

CoE does not mean what you think.  It does not mean compatibility.  It does not measure volume expansion at the glass transition point.  It does not measure the most important element – viscosity.  It is not even the correct term for the measure – CoLE is.

Since CoE does not equal a fusing compatible glass, its continued use can lead people (especially novices) to believe the simple number means any glass (mistakenly) labelled with that number will be compatible with others so labelled.  This leads to unexpected incompatibilities for newcomers to the field.


What can you use instead? It is easy – use the manufacturer’s name.  Where the manufacturer is making more than one range of fusing compatible glass use the manufacturer’s nomenclature.


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