The following is from results of a laboratory test of Bullseye clear (1101F)
Temperature range.......................COE
20C-300C (68F - 572F).................90.6
300C-400C (572F - 752F).............102.9
400C-450C (752F - 842F).............107.5
570C-580C (1058F-1076F)............502.0
Bullseye glass is probably typical of soda lime glasses designed for fusing.
The change of CoE by temperature is further illustrated by Kugler (a blowing glass) who state their CoE by temperature range. Remember CoE is an average expansion over a stated range of temperatures)
CoE 93 for the range 0C-300C
CoE 96 for the range 20C - 300C
CoE 100 for the range 20C - 400C
The extension of the range by 100C beyond the brittle phase of glass has a distinct effect on the average expansion over the (larger) range.
This shows why it is not helpful to refer to CoE without also mentioning the range of temperature.
In addition, here is an illustration of the effect.
Image credit: Kerwin and Fenton, Pate de Verre and Kiln Casting of Glass,2000, p.32
It is understandable and common sense that as the temperature increases, so the rate of expansion increases and this applies to most solids. Glass behaves differently as the graph above shows. The change in expansion of Bullseye glass shows a relatively consistent average expansion until the strain point is reached. Once out of the brittle phase, glass expansion rates change very much more rapidly. It is not be coincidence that viscosity of glass changes at almost the same rates. It is the viscosity that is controlling the CoE, not the other way around.
Revised 5.1.25
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