I my last post I spent a fair amount of time licking my wounds do to a massive air bubble that formed under a piece I was firing. I was fairly confident I could fix things by doing another tack fuse.
Well it worked…sort of.
The bubble fused flat and there’s a hole where a check of glass broke off. Not a big deal since that will be filled in with frit.
The point where I got hammered was the cool down after the anneal soak. After the piece had been out of the kiln for about a day a crack formed along the boundary of the pot melt.
I spent a couple of days scratching my head of over this one before I realized what the problem was. I used a cool down schedule which I though was appropriate for a piece that had an overall thickness of ~9 millimeters. the problem was my piece was not uniformly 9 millimeters. Here’s the cool down portion of my schedule.
| Ramp | Target | Hold (in minutes) |
| FULL | 960 | 80 |
| 75 | 800 | 0 |
| 150 | 700 | 0 |
| 200 | 100 | 0 |
I think this would have been OK if the piece was uniformly 9 mm thick but this was too aggressive relative to what I was trying to do. 3mm glass is going to cool much faster than 9 mm and that’s what introduced stress along the boundary of the pot melt. If anything I need treat as though I were cooling down something that was 12 – 15 mm thick due to the varying glass thicknesses. Here is the schedule I plan on using next time.
| Ramp | Target | Hold (in minutes) |
| FULL | 960 | 100 |
| 50 | 800 | 30 |
| 75 | 700 | 30 |
| 100 | 100 | 0 |
I’m thinking (knock on wood) this should do the trick.
I’ll keep ya posted.
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