A query about reducing a bubble appeared on the internet recently. The bubble was from between the shelf and the single layer glass. It was a relatively shallow dome that did not seem to have thinned the glass much.
There is quite a bit of information on reducing the incidence of bubbles. Among them are my blog posts on the subject.
My view is that large thin bubbles cannot be repaired successfully. As the bubble forms and grows, it pushes a proportion of glass to the side. This thickens the glass at the edge of the bubble. Bursting the bubble and filling it with something (e.g., a piece of glass, or frit) leaves marks at the thickened edge of the bubble, so it remains a mark in the finished piece.
However, glass with a low uprising between the shelf and the glass can be successfully repaired, if the uprising is low and the glass has not thinned. In the case mentioned, the risk in simply re-firing right side up is that the bubble will increase in size. The weight of the glass may not be sufficient to pull it down except at higher temperatures – which is where the risk of increasing the size of the bubble occurs.
Instead, flip the piece over. Allow the weight of the glass to flatten the uprising. You can use a much lower temperature to flatten the glass by taking advantage of the weight of the whole piece. This lower temperature means that you will not mark the surface so much as at higher temperatures. Don’t worry if the uprising is not central, you do not have to balance the glass on the point of the bubble for this process to work.
Take the piece to 620°C maximum for as long as it takes to flatten. The rate of advance should be slow – not more than 100°C per hour. This steady, slow input of temperature will allow the glass to relax at lower temperatures than rapid increases.
You should use the smoothest separator surface that you can – Thinfire or Papyros, or a smoothed kiln wash. This together with a low temperature will give minimum markings.
You must observe the process from about 560°C to be able to stop the slump when the piece is flat and advance to the annealing segment of the firing.
This post gives a further alternative. Use two shelves to compress the uprising flat. Although the post is talking about thinning a pot melt, the principle is the same. Place fibre paper around the edge equal to the thickness of the glass piece and place another prepared kiln shelf on top. You do not need to invert the surface of the piece to do this. It may be that you will need a fire polish to remove any marks on top.
Do not drill holes. Especially not in the case of a shallow bubble. The glass has not significantly thinned and so can be rescued. Drilling a hole will only leave an unwanted mark.