One of the most effective ways of reducing bubbles is to adjust the schedule to allow the top glass to slump down onto the bottom sheet before the glass is soft enough to stick at the edges and trap air. This is commonly referred to as a “bubble squeeze”.
A common method is to insert a soak at the slumping temperature of the glass. You will have found that the glass will take up the form of a simple slump at a lower temperature than more angular forms. Use this lower temperature for 30mins to an hour. You may want to extend that soak time depending on the thickness and complexity of the layup.
Another method is to start the squeeze about 55C above the annealing soak temperature and increase the temperature slowly (27-55C per hour) until you are at the slump temperature.
You can also combine the two above methods by soaking at the slump temperature for 30 minutes to an hour – or longer for thick and complex pieces – after the slow rise.
If your kiln is a side fired one, you need to be especially careful, as the edges of the glass become hotter than the centre. Two options are available - fire more slowly, or place baffles around the outside of the piece to prevent direct radiation of the heat onto the edge of the glass.