To begin with, I use ZYP or MR-97 as a kiln release on these types of texture molds so that I can get a lot of texture in the completed firing. I wondered whether it could be my kiln release that was causing this condition. It's not often that I create unintentional, large bubbles in my fused glass pieces, so that's why I decided that it was not from my kiln release, since I use it often without any issues. A lot of time I fuse two sheets of glass together at the same time that the glass is slumping on a texture mold to save a step.
1. First fire together the two layers of glass that you want to use.
2. To prep the mold, drill about four holes with a small drill bit to allow trapped air to escape from under the glass when fusing. When drilling, drill from the front of the mold just in case some of the mold chips off when the bit exits the mold. If the back of the mold is slightly chipped, there is no harm done to the texture on the front side of the mold.
3. Place your mold on kiln shelf posts to elevate it to allow for more even heating from underneath.
4. Finally place your pre-fired and fused glass blank on top of the texture mold (prepped and set up) in your kiln. It will greatly reduce your bubbles. Be sure to add a bubble squeeze in your kiln schedule both when you fuse the two layers together and for the second step when you fuse the double layer thick glass blank on the textured tile mold.