Reducing Annealing Time for Float Glass

Credit: Bullseye Glass Co.

Annealing float glass seems take a long time. The annealing point (Tg) is higher than most fusing glasses, although float glass is part of the family of soda lime glass. This group of glasses should be cooled slowly from annealing temperature to 427ºC/800ºF and below to reduce risks of thermal shock.  This makes a greater temperature range over which to  anneal float than fusing glasses, consequently it extends the cooling time and increases energy expenses.

It does not have to be this way.  Annealing of glass takes place over a range.  This range extends below the published annealing point (Tg).  This is the temperature at which equalisation can most quickly take place, but it is not as energy efficient as starting in the lower range.  Annealing points (Tg) vary between manufacturers, but these are some of them:

Pilkington Optiwhite              559ºC/1039ºF

Pilkington Optifloat               548ºC/1019ºF

USA float (typical)                548ºC/1019ºF

Australian float (average)      550ºC/1022ºF

The annealing range extends to a practical 38ºC/68ºF below the Tg temperature.  Annealing at a lower temperature can be as effective at the lower portion of the range as at the Tg.  Using a lower annealing soak temperature reduces the temperature range of the first cooling stage by as much as 38ºC/68ºF, and reduces the cooling time without increasing risks of breaking.  It also creates a denser glass according to scientific research.  Denser glass is arguably a stronger glass.

This means that the annealing of float glass can take place at the following reduced temperatures:

Pilkington Optiwhite              521ºC/971ºF

Pilkington Optifloat               510ºC/900ºF

USA float (typical)                510ºC/900ºF

Australian float (average)      512ºC/954ºF

 

This reduces the first cooling stage for 12mm/0.5” Pilkington Optiwhite from 2 hours 24 minutes to 1 hour 43 minutes.  Forty-one minutes may not seem much but in electricity costs is significant.  Also using the Bullseye concept of a three stage cooling, further savings can be made.  Their research shows the second cooling stage to 371ºC/700ºF can be increased by 1.8 times the first cooling rate, saving further time and energy.  The chart which shows these rates is Annealing Thick Slabs -  Celsius and - Fahrenheit.

More information on annealing is available in the ebook Annealing: Concepts, Principles and Practice

Annealing float glass at the lower part of the annealing range reduces the time and cost of firings.