Rory, What do you think of chalk sharpeners? I know that many tailors sharpen their chalk with their shears or a razor blade, but I've never been very good at it. I once bought a plastic chalk sharpener, but it didn't work that well, probably because it wasn't that sharp. On Instagram, there are a couple of people who make handsome wood chalk sharpeners that feature razor blades. I also saw a post by one of my former tailoring teachers in which he made one for himself, but that would be beyond my capabilities.