There are a variety of ways to minimise bubbles between layers.
You will have noted that there are smoother and rougher sides to the glass. Putting the two rough sides together will increase the number of small bubbles in the finished piece. But the opposite –smooth to smooth – does not produce the smallest bubbles. The fewest small bubbles are produced when one smooth side is touching the rough side of the other layer of glass. This “roughness” allows any air to find a way out. Smooth to smooth tends to produce fewer but much larger bubbles.
Supports during the forming stage