strain point

Glass Tips from Verrier

Information on stained glass, kilnforming and other areas of glass working

Saturday, 22 June 2019

Strain Points and Annealing Ranges

I received the following question a while back and thought my response might be useful, although very informal.

“Can you dumb down the concepts of ‘annealing point’ and ‘strain point’? I understand anneal point to be a fixed point (depending on the glass) but the strain point is a range. is this correct? I understand the concept of a hold at the anneal point but I’d like to understand how to bring it down through the strain point.”

I really dislike the idea of dumbing down concepts in kiln forming glass. Glass chemistry is incredibly complicated. Glass physics is still little understood. Glass is a very complicated subject. The marketing of glass for kiln forming has led us all to think it is a simple matter of recipes. Well it’s not.

Having got that rant out of my system. Let’s go ahead.

The annealing point is roughly defined as the temperature at which the glass (if it is the same temperature throughout) will relax most quickly. In the practical kiln forming that we do, it is not possible to ensure that the glass is that temperature throughout. So it is better to think of an annealing soak at the annealing point to allow the glass to become a more even temperature throughout its thickness. As thicker glass means the heat has further to travel from the centre to the surfaces, a longer soak is needed for thicker glass.

The annealing occurs during the slow cool past the lower strain point. The annealing occurs best with a slow, but steady drop in temperature. So annealing is occurring over a range, not at a point. We all rely on a combination of the manufacturers’ recommendations, various writings we read, and experience to determine that rate, although Bullseye have published a chart which is most helpful, whichever glass you use .

Strain points.

There is an upper and lower strain point, although this is disputed by some. There are mathematical definitions for these as well as observational definitions. I do not understand the mathematics of either. In lay terms, the lower strain point is that temperature below which no further annealing can take place. It is safe to assume this is 50C below the annealing point (I think it actually is 43C, but I’m not certain of this number).

So it is safest to control the cooling to at least 5C below the lower strain point. Bullseye find that cooling from the annealing soak to 370C is best – this is much more conservative than is theoretically required – 146C below the annealing soak point. This does take care of any problem of thermal shocking of the glass during the cooling.

The upper strain point might be more properly described as a softening point. This also has scientific definitions. The way I think of it is as being the temperature above which no annealing can occur. Another is to think of it as a point beyond which the molecules of the glass are in relatively free motion – which increases with temperature. This again can be considered (on the rise) as 50C above annealing. However on the way down it is safer to consider it to be not more than 30C above annealing. This is because the glass temperature lags behind the air temperature (which is what our controllers measure).

So there is no point in soaking more than 30C above annealing in an attempt to equalise the temperature throughout the glass. However, if you really need to equalise temperature at some point above the annealing point, it might be better to slow the cooling from the working/top temperature and do the final equalisation of temperature at the annealing point.

To answer directly, the strain point by definition of language cannot be a range. There are two points which form the possible annealing range, although the lower one is the critical one. The upper one I described earlier as the softening point. The softening point forms the upper part and the strain point forms the lower part of a range in which the annealing can occur. So the concepts are the opposite of what you propose. They are points which are the boundaries of the annealing range.

To complicate things further, not all glass from one manufacturer has the same annealing point. The published annealing point is a compromise that their experiments and experience have shown to be most suitable. Bullseye glass for example has three annealing points, 532C for opals, 505C for cathedrals and 472C for gold bearing glasses. NOTE: these figures may not be exact; they come from memory rather than documents. Since this list of annealing points was published, Bullseye have conducted further experimentation that shows the best annealing soak occurs at 482C which is below transparent and opalescent, but above gold bearing annealing points.

Schott recommends a range for annealing, not a point, to accommodate these variables. Bullseye , Uroboros , and Spectrum have published annealing points that are practical for people kiln forming in smaller kilns that are less well controlled than the factory lehrs.
If you look at the Bullseye site – education section, you will find a lot of useful information. Especially informative are their tech notes. Spectrum – to a lesser extent than Bullseye – gives helpful information. The information from both sites should be absorbed and the principles applied to other glasses.

Finally, kiln forming is deceptively simple. I have spent 29 years discovering how much more there is to learn. This is one of the reasons that glass is such an exciting medium – people keep discovering new things.

Reviewing the above, I realise that I have not answered your question “. how to bring down through the [lower] strain point”. My answer is that you should look at the manufacturer’s site for each glass that you use. Look at their rates for annealing for different thicknesses of glass (some also take into account the size). Consider them. Then look at some of the other sites for their published annealing rates for various thicknesses. Comparison of their rates will reveal differences. Think about what they are, how they relate, and whether they reveal that they are using the same principles with slight variations.

Also, if you can, get a copy of Graham Stone’s book “Firing schedules for glass, the kiln companion”. It provides a handy guide to annealing rates. But DO NOT use it as a book of recipes. Read all the commentary about the schedules, as they (combined with the introductory parts) give principles and tips about how to think about the cooling of the glass. Bob Leatherbarrow has recently published an excellent book on kilnforming schedules, available from his website.

By the way, experience is so often lost, or misremembered, that keeping a log is essential. My first log consisted of loose leaf binder, so I could file all the same kind of firings for various glasses together (this was in the days when there was not much fusing compatible glass, and I couldn’t afford Bullseye at UK prices. I was discovering lots about glass firing and using some schedules that I now wonder how I had any success. I did learn a lot from my failures and recorded them too. Now I use a log, usually an out-of-date A4 size diary, sometimes a manuscript book that is big enough to record observations and illustrations. Bullseye have a good record form on their site.

I congratulate you on your desire to understand the processes. Too many only want to put the glass in and turn the kiln on. That is the desire a number of kiln manufacturers pander to when they put pre -programmed schedules on the controllers. So, don’t take any of this as criticism of you or your comments. It is meant in a constructive manner – even though I am told frequently that the manner is blunt, even rude.

Best wishes on continued successful kiln forming.

Revised 22/06/19

An informal description of the concepts of strain points and annealing points.