Wednesday, 8 April 2020

Long Annealing Times

I’ve noticed a lot of people are using long annealing times for full or contour fused glass of 6mm thickness.  Whether this comes from the advice for tack fused pieces to be annealed as though there were twice the total thickness of the piece, or not, is uncertain. 

Temperature Equalisation

This term employs the concept of what happens at the annealing point chosen for your glass.  The soak at the annealing point - or even at some degrees below - is the process of ensuring the glass is all at the same temperature before proceeding to the annealing cool.

A “flattish” fused piece of 6mm needs only a half hour when the temperature equalisation soak is at the annealing temperature.  When using a temperature equalisation soak about 30°C-35°C below the annealing temperature – as Bullseye, and now Wissmach, does – you may need an hour.

Tack fusing is much more difficult to anneal properly than a full or contour fuse.  The general rule of thumb has become that you must schedule the firing as though it is twice the actual maximum height. 

E.g., if you have a two-layer piece with other pieces distributed around this flat base, you have a 9mm thick piece in total height. Scheduling for this piece as though it is 18mm thick requires longer soaks and slower cools.  In this case, you should schedule for a three-hour soak at temperature equalisation, whether using the annealing point or a lower temperature. 

Other kilnformers have found that firing tack fused pieces as though they are one and a half times the actual maximum thickness provides a perfectly adequate result.  It will be up to each person to decide which approach they take.  Once the current lock down is over, I am going to do some work on the actual requirements.

Annealing Cool

People seem to cool a tack fused piece at a rate suitable for thinner pieces. It appears that the general practice is to use rates suitable for a for 12mm thick piece regardless of the calculated thickness. 

But the annealing cool is still too rapid for a tack fuse.  If you need an anneal soak for 19mm, you need to cool for that thickness too. An annealing cool for a 19mm piece is one half of the rate of that for a 12mm piece.  It is during this too rapid cool that stress can be induced, especially on a tack fused piece.

The temperature equalisation soak and anneal cool can take as much as twice - or more – as long as a flat piece of the same total height.  This needs to be allowed for in the scheduling of the firing.

The length of anneal soak and the rate of anneal cool both need to be related to the appropriate thickness.  There is the Bullseye chart for annealing thick slabs the rates of which, can be applied to any fusing glass.  You only need adjustment to the temperatures for your glass, if not Bullseye.

No comments:

Post a comment