View Single Post
  #12  
Old 2007-09-03, 10:57am
prairieson's Avatar
prairieson prairieson is offline
Senior Member
 
Join Date: Sep 17, 2005
Posts: 903
Default

Quote:
Originally Posted by Dennis Brady View Post
Are you suggesting that the viscosity of COE 104 is the same as 96?
Perhaps in some cases it is, but that's not the point. The point is that viscosity is at least as important in determining compatibility as COE. They work hand in hand.

The two (viscosity and COE) occur at two different temperature ranges. COE affects the glass at well below the strain point, it is an average of COE's from room temp to 300° C. Viscosity affects the glass at higher temperatures, between the strain point and softening point.

In order for glass to be compatible these two properties must offset each other. It's referred to as compensating errors. If the COE of two glasses sets up a strain in one "direction", then the viscosity must cause strain equal and opposite to cancel the effect.

Now, to address further your original question. If two glasses with COE's of 104 and 96 respectively, have the same viscosities, then no they won't be compatible. But, those two glasses are compatible if the viscosities are such that they cancel the effect of differing COE's... but important nonetheless.

The Reader's Digest Condensed version of the whole thing is...

COE's and viscosities are equal -> glasses are compatible
COE's are equal and viscosities aren't equal -> glasses are not compatible
Viscosities are equal and COE's aren't equal -> glasses are not compatible
COE's and viscosities aren't equal -> glasses might be compatible

The first of the three links I provided explains this much more eloquently than I, and in much more depth. The other links are discussions regarding other factors affecting compatibility, such as batching, working time/temp and its effect on crystal growth and COE, and other such esoteric brouhaha.
Reply With Quote