If you are going to knap glass, be careful! Having knapped several different materials, I can tell you that glass and obsidian will slice you in a heartbeat. You should always wear safety glasses when knapping, but this is especially true with glass and obsidian! One more thing is you need a leg pad with these materials, trust me, I learned the hard way after destroying several pairs of jeans from the tiny obsidian shards embedded in the fabric. As for the lens, I would start by pressure flaking in a back and fourth zig zag pattern around the edge (make shure you grind your platforms) and then I would try to visualize the point ect. that you want to get out of the lens. The convex shape should help you drive long flakes. Remember to take your time and set up good platforms (slightly below center, well ground for strength). Also you may not want to set your flakes side by side but instead run guide flakes on either side of where you want a big flake, these are guide flakes or relief flakes. The guide flakes work to set up a center ridge. If you use this technique, remember to take a few flakes off the opposite side of the ridge to get it below center. Remember to grind, grind, grind.
