Bubble Mystery

A question was asked about a collapsed bubble. There were two pieces in the kiln and one (strips) was fine and the other (flat plate) had the collapsed bubble.  Both on the same dried shelf.  The question also asked if the collapsed bubble piece could be flattened by fusing again.

Collapsed Bubble

The bubble collapsed because it had not burst by the time the cool toward annealing had begun.  As the air pressure under the bubble dropped, and the weight of the thinned glass bubble sank down as there was not enough air pressure to hold it up.

The glass is now thinner at the centre of the bubble than the main part of the piece, and thicker at the edge of the bubble. I don't think it is possible to successfully flatten it to become an even thickness across the whole piece. To get the same thickness across the whole piece would require high temperatures and long soaks there. 

Another possibility is to use a pressing solution. 

My suggestion is to add elements or repurpose it. I don’t think any repairs would present a good-looking piece.

Diagnosis

The on-line diagnosis of the possibilities for the cause of the bubble was extensive and sometimes inventive.  It was finally determined the bubble was from under the glass, that is, between the glass and the shelf. A slight depression in the shelf is the usual explanation.  The user tested the shelf for smoothness and found no depressions.

It was clear the bubble came from under the glass.  All the suggestions about how bubbles can form under glass were given, but none seemed to apply.

How can you get a bubble on a dry shelf that is perfectly flat and that has not been subjected to too rapid or too high a temperature?

Solution

The answer is that a little spot of grit or tiny ball of fibre paper can keep the glass raised up enough for air to be trapped.

Prevention

It is not enough to test the shelf is flat.  You need to use clean kiln wash with a clean brush to avoid any grit being brought to the shelf. It is also a good reason to vacuum the shelf before each use in case any dust or grit has fallen onto the shelf. Covering the shelf or putting it into a cupboard will also reduce the possibility of small bits of grit falling onto the shelf.

Of course, if you smooth the kiln wash with a nylon or similar fine cloth, you will remove any specks of grit.  A vacuum of the shelf after smoothing is still a good idea.

Conclusion

It is as important to keep tools and materials clean as it is to clean the glass you are going to kilnform.