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| Picture credits: Heidi Woj |
I”m assuming this was an anneal problem. All four corners broke/cracked.
Steps: full fuse 2 layers of Tekta with 2mm leaves. Then contour-fused on the flower centers and petals (1-2 layers). [A total of 4 layers at the thickest]
Finally, slumped onto GM97 tray mold. All was well until a couple days later. Thought I used very conservative schedules, but maybe the last anneal wasn’t long enough? Also, I erroneously annealed at 950 instead of 900 on the contour fuse. Any suggestions?
Reactions
My view of the photos show the sunflower centre to be 4 layers top to bottom. A contour fuse needs to be fired as for 1.5 times the thickest part, so 19mm/0.75” at least. However, the picture shows a tack fuse was used which requires firing as for twice the thickest part so 25mm/1”. Anyway, the piece held together during the first firing and any stress in the flat piece will have been relieved during the rise in temperature to above the strain point during the slump firing. Annealing Bullseye at 510°C/950°F can still provide a good anneal, although it takes longer than an anneal at 482°C/900°F. So, although not ideal, the contour/tack firing anneal is not part of the present problem.
Annealing
However, the anneal on the slump is critical.
I have suggested that the anneal for the tack fuse should be as for 25mm/1”, so I recommend the minimum allowable soak and cool is:
- Anneal soak for 4 hours,
- 15°C/27°F to 427°C/800°F, 0”
- 35°C/85°F to 371°C/700°F, 0”
- 120°C/216°F to room temperature
In addition, I have found that annealing a slump for one layer thicker than for the fuse leads to a less stressed piece. So I would anneal the slump as for 28mm/1.125”. This makes for an annealing of:
- Anneal soak for for 4.5 hours
- 11°C/20°F to 427°C/800°F, 0 time
- 24°C/43°F to 371°C/700°F, 0 time
- 65°C/117°F to room temperature.
The cool rates are intimately related to the anneal soak. The first rate is to maintain the small temperature differences at the anneal soak to below the strain point. The next two rates are to avoid introducing large contraction differences within the glass.
Analysis
It is the fast cool that has caused stress sufficient to break the piece days later. The corners broke away/cracked, because the stress in a piece is concentrated at corners and points.
Suggested reading
I have written an e-book Annealing, Concepts, Principles, Practice, which explains the whole annealing process in greater detail. It may be of use. It is available from Bullseye and Warm Glass or me, of course.
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