If you are percussing glass you need a very strong platform, below center if you are going to strike it with a hard hammer, Glass likes antler. Elk is my choice. Copper works OK if the bopper has a small impact point.
If you are doing slabs try angling your flakes obliquely so they travel further. Once you reach the center
you will have enough curvature to flake at a 90 degree angle if you choose.
Zuma