Another approach to avoiding bubbles is to plan on two firings. This works well for pieces that have multiple layers, with glass or other inclusion in the middle.
For the first firing, put the middle pieces flat on the kiln shelf with one layer of glass on top. Take this to at least a tack fuse, although full fuse temperature is better as there should be no remaining gaps for air to be trapped within. Now turn this over and clean it well. Place this part in the kiln with the middle layer up. Place the top layer over this piece – now right side up – and take to the full fuse. Remember that now you are firing a thicker piece than in the first firing so take the temperature up more slowly.
This is most often applied to three layer pieces, but in principle can be applied to any number of layers.
Supporting the edges
Arrangement of glass sheets