Why is the annealing temp of glass 950 F-1000 F?

Question

If glass has variable coefficients of expansion, why isn't the annealing

temperature also variable?

Answer 1

It is. As is the glass transition temperature and the liquidus temperature.

According to this page, the annealing temperature (also called annealing point) [is that at which the glass is] at a viscosity, η, of 1013 Poise = 1012 Pa·s,

http://en.wikipedia.org/wiki/Annealing_(...

From the table here, you can see that the viscosity vs. temperature curves for various kinds of glasses are quite varied:

http://en.wikipedia.org/wiki/Glass

For example, the log varies by a factor of 2 between soda-line glass and pyrex so the actual viscosity at a given temperature varies much more. And when you go to optical and other specialty glasses, even the log varies by more than a factor of 10.