Anomalous Annealing Schedules

Why do schedules vary so widely?

I frequently observe strange annealing schedules promoted on the internet. These include:

• extremely long anneal times with rapid cools risking thermal shock.

• single slow cool rates, using more time and electricity than necessary

  • to 371°C/700°F.  There are better cool arrangements that fit the needs of cooling glass.

  • to room temperature.  Again using more time and electricity than needed.

  • somewhere between.  These still do not adhere to the requirements of the glass.

• Cool rates with staged soaks. There is no use for soaks below the strain point (about 470°C/800°F) as any annealing stress is locked in by that time.

• Cool rates unrelated anneal soak lengths.  Adequate cool rates are directly (but inversely – longer soak = slower cool rates) related to the length of the anneal soak.

• Cool rates ending at 371°C/700°F which are inadequate for thick glass.

They are strange, because there is a well researched document giving annealing times and cooling rates published by Bullseye for Fahrenheit and Celsius, 

It may be understandable that users of fusing glasses other than Bullseye are cautious about the use of this table.  But it is a well researched table supported by independent academic research, which is applicable to all soda lime glass. [McLellan and Shand (1984), Glass Engineering Handbook, 3rd Edition, New York, McGraw Hill]  Only the annealing temperature needs to be changed, as all the rates and times apply equally to all fusing glasses, and even float glass.

Yes, sometimes changes are required for:

• Profile

  • Contour fuse needs times and rates for 1.5 times the thickest part

  • Tack fuse needs times and rates for 2 times the thickest part

  • Sharp tack or sinter needs times and rates for 2.5 times the thickest part

• Contrasting colours, and differing viscosities need times and rates for at least 3mm thicker than the profile indicates

• Extreme forming such as drops and deep slumps also need times and rates for at least 3mm thicker than the profile indicates

These matters are explained in greater detail in my e-book Annealing Concepts, Principle and Practice available from Bullseye and Etsy