These Faithfull Tile & Glass Drill Bits have extremely hard tungsten carbide spear tips and should be used at low speeds whilst applying light pressure only. They are ideal for drilling glass windows, mirrors, bottles and ceramics. Lubricate with water for best results. For use in variable speed drills only.The Faithfull FAIGDSET4 is for drilling glass windows, mirrors, bottles and ceramics - for use in variable speed drills only. These drillbits have extremely hard tungsten carbide spear tips and should be used at low speeds whilst applying light pressure only. Note: Lubricate with water for best results The Faithfull FAIGDSET4 is a set of 4 glass drills in a plastic case and comes in the following sizes: 3, 4, 5 and 6mm.