Many people are under the impression that CoE can tell you a wide number of things about fusing glass.
What does CoE mean?
The first thing to note is the meaning of CoE. Its proper name is the coefficient of linear expansion. It tells you nothing certain about the expansion in volume, which is as or more important than the horizontal expansion.
It is an average determined between 0°C and 300°C. This is fine for materials that have a crystalline structure. Glass does not. Glass behaves quite differently at higher temperatures.
It may have an average expansion of 96 from 0°C-300°C – although there is no information on the variation within that range – but may have an expansion of 500 just above the annealing point.
The critical temperatures for glass are between the annealing and strain points. One curious aspect to the expansion of glass is that the rate of expansion decreases around the annealing point. The amount of this change is variable from one glass composition to another.
The CoE of a manufacturer’s glass is an average of the range which is produced. Spectrum has stated that their CoE range is from 94 to 98. This kind of range can be expected in every manufacturer’s compatible glass.
CoE does not tell us anything about viscosity, which has a bigger influence on compatibility than CoE alone.
Comparison of CoE and Temperature
Among the things people assume that CoE determines is the critical temperatures of the strain, annealing and softening points of various glasses.
Unfortunately, CoE does not necessarily tell you fusing or annealing temperatures.
Most float glass is assumed to be around CoE 83. The characteristics depend on which company is making the glass and where it is being made.
Pilkington float made in the UK has an annealing point of 540°C and a softening point (normally the slump point) of 720°C.
Typical USA float anneals at 548°C and has a softening point of 615°C.
Typical Australian float has a CoE of 84 and anneals in the range 505°C -525°C.
Uroboros FX90 has an annealing point of 525°C compared to Bullseye at 482°C, and Wissmach 90 anneal of 510°C.
Wissmach 90 has a full fuse temperature of 777°C compared to Bullseye's 804 - 816°C.
There is a float glass with a CoE of 90 that anneals at 540°C and fuses at 835°C.
Bullseye has a slump temperature of 630°C-677°C and Wissmach’s 90 slumps between 649°C and 677°C, slightly higher.
Kokomo with an average CoE of 93 has an annealing range of 507°C to 477°C. Kokomo slumps around 565°C
Artista with a CoE of 94 has an annealing point of 535°C and a full
fuse of 835°C, almost the same as float with a Coe of 83.
Wissmach 96 anneals at 510°C with a full fuse of 777°C and a slump temperature of 688°C.
Spectrum96 anneals at 510°C and full fuses at 796°C.
Whether the Spectrum glass to be produced by Oceanside Glasstile will have the same characteristics of Spectrum 96 remains to be seen.