The Annealing Range Concept

There is a lot written about the annealing temperature of a glass being at a single exact temperature. This is another fundamental misunderstanding of the concept - much like CoE meaning compatibility.

The annealing point is mathematically defined as the temperature at which a glass reaches a particular viscosity. This is the temperature at which stress can most quickly be relieved. It is denoted as Tg. Each glass has its own Tg according to colour and composition. The manufacturer recommends a good average Tg for their glass. The first section of this blog post gives a description of the glass transition point. 

A description of the physical changes that occur during annealing. 

An informal discussion of the limiting factors on the annealing range is given in this blog. 

A description of the effects of attempting to anneal at the upper part of the annealing range. 

A description of why annealing at higher temperatures is counter productive. 

Bullseye used to publish three different annealing temperatures for transparent, opalescent, and gold bearing colours and gave an average of these to be the annealing temperature. This was before they began conducting research on annealing of thick slabs. As a result, they were able to determine annealing in the lower portion of the range produces good anneals with reductions in time spent in cooling.

A description of the annealing range and the advantages of low temperature annealing is given in this blog post. 

Although written to counter the mistaken view that CoE can determine the annealing temperature, this blog indicates that the annealing temperature is a choice within a range of temperatures. It also connects annealing soaks with cooling rates. 

The general point is that the annealing soak can occur at any point between the softening point at the top and the strain point at the low part of the temperature range. There are good reasons to avoid annealing above the annealing point (Tg). There are also good reasons to anneal near the strain point of the glass – saving time, electricity, and producing a denser glass. Annealing is critical, but the temperature at which you do it is less so.

 

All this has provoked me. There is so much more to say. So, I have begun a writing an eBook on Annealing – Concepts, Principles and Practice.  In the meantime, more information is given in the eBook Low Temperature Kilnforming