Bubbles are often blown through frit castings and other thin pieces. This most results from insufficient volume of glass in the mould or on the shelf. Also the design can induce bubbles where there are thinner parts surrounded by thicker parts. As the glass softens, the surface tension of glass - from around 730 - causes it to pull up to equalise at about 6-7mm thick. This causes thinning in certain areas to allow thickening in other areas. This then leads to the risk of blowing bubbles through the glass where the glass has become thinner.
If thinner work is required, you can fire an over-sized piece to about 750C for a short time and then cut it back to the final size. If you want a flat thin sheet, you can also place the glass between two kiln shelves. You need to separate the shelves with a 3mm spacer to keep the upper shelf from coming completely down on the shelf, giving an extremely thin fragile piece of glass.