Heat Up vs Annealing

I am amazed by the effort put into ramp up rates, bubble squeezes, and top temperatures in comparison to annealing.  The emphasis on social media groups seems to be to get the right ramp rates for tack fuses and slumps, bubble squeezes, etc.  Most of the attention is on the way up to processing temperature.

The treatment of annealing and cooling is almost cavalier by comparison.  The attention seems to be on what temperature, and how long a soak is needed.  Then some arbitrary rate is used to cool to 370ºC/700ºF.

Annealing, in comparison to firing to top temperature, is both more complex and more vital to getting sound, lasting projects completed.  Skimping on annealing is an unsound practice leading to a lot of post-firing difficulties.

Annealing is more than a temperature and a time.  It is also the cooling to avoid inducing temporary stress. That stress during cooling can be large enough to break the glass.  This temporary stress is due to expansion differentials within the glass.

People often cite the saving of electricity as the reason for turning off at 370ºC/700ºF.  My response is that if the kiln is cooling off slower than the rate set, there will be no electricity used.  No electricity demands.  No controller intervention.  No relay operation.

Annealing at the lower end of the range with a three-stage cooling provides good results.  The results of Bullseye research on annealing are shown in their chart for annealing thick items.  It applies to glass 6mm and much larger.  It results from a recommendation to anneal at the lower end of the annealing range to get good anneals.  Other industrial research shows annealing in the lower end gives denser glass, and by implication, more robust glass.  Wissmach have accepted the results of Bullseye research and now recommend 482ºC/900ºF as the annealing temperature for their W96.  The annealing point of course remains at 516ºC/960ºF.

Bullseye research goes on to show that a progressive cooling gives the best results.  They recommend a three-stage cooling process.  The first is for the initial 55ºC/º100F below the annealing temperature, a second 55ºC/100ºF cooling and a final cooling to room temperature.

It is a good practice to schedule all three cooling rates.  It may be considered unnecessary because your kiln cools slower than the chart indicates.  Well, that is fine until you get into tack and contour fusing.  Then you will need the three-stage cooling process as you will be annealing for thicknesses up to 2.5 times actual height.

 

Of course, you can find out all the reasons for careful annealing in my book "Annealing; concepts, principles, practice" Available from Bullseye at

https://classes.bullseyeglass.com/ebooks/ebook-annealing-concepts-principles-practice.html

Or on Etsy in the VerrierStudio shop

https://www.etsy.com/uk/listing/1290856355/annealing-concepts-principles-practice?click_key=d86e32604406a8450fd73c6aabb4af58385cd9bc%3A1290856355&click_sum=9a81876e&ref=shop_home_active_4