"Can you fuse decals between 2 layers of glass. If so do you need to cover with powder?"
Yes, you can fuse decals between 2 layers of glass. We didn't do exhaustive testing on this issue, and most of the tests that we did for doing so were only 4"x4", so larger projects may yield different results. Here's a basic synopsis of what we found:
1. There was no evidence of the carrier film causing a contamination problem when firing the decals between layers in a single firing.
2. Bubbles between layers seemed slightly larger than normal when firing the decals between layers in a single firing, causing some image distortion.
3. Pre-firing the decal on the base sheet of glass to 1400°F prior to capping it with another layer yields a higher resolution image with a more consistent hue than not pre-firing and than pre-firing to 1225°F.
4. We did not test putting powder between the layers of sheet glass (on top of the decal), but I suspect that you would see fewer large bubbles that cause gross distortion of the image, but many more small bubbles that cause a slightly soften of the image. The main reasons that we covered with powder in the video were to cover evidence of the carrier sheet, and to provide a protective layer for the decal for greater durability and in case we wanted to sandblast the surface and add more imagery. Placing a sheet of glass over the decal achieves these objectives as well.
I hope that this is helpful.