Even the most expensive and exquisite Venetian glass art costing thousands of Euros might have air bubbles. Air bubbles are a natural part of the process of hand made glass and the random kiln process.

No sheets of glass are perfectly flat. Air can become trapped between sheets if there are imperfections in the glass.  So during fusing, if air is  trapped between sheets of glass as they are fused together, it is sqeezed and contracts to a bubble as the top piece of glass slumps onto the lower piece.  The air cannot escape. 

Most of the time, air bubbles are an interesting part of the uniqueness of each piece.

Some air bubbles are too big, resulting in a very big lump. Such a piece might not be sellable and will be destroyed.

If air bubbles don't detract from the design too much, it might be sold as a 'second'.

Air bubbles are sometimes an intended part of the design. Some techniques involve purposely having air bubbles in patterns in the glass.

So you can see that air bubbles shouldn't be considered as flaws. They are a unique characterisic of each piece, as unique as your skin and your finger print.