Castell 9000 twin sharpening box, green The quality of a sharpener, especially the blade, is crucial for optimum sharpening of wood-clad pencils. Due to their robust and hard leads, graphite pencils can be sharpened to a very thin point. Given their softer leads, colour pencils are sharpened to a shorter and blunter point. For optimum sharpening, we recommend replacing the sharpener now and then because the sharpener blade will become blunt over time. This results in more tip breakages.