Have you tried the Multi-Pastel chalk pencils. They are available at some quilt shops and usually at an art supply store. They are erasable and washable. But remember to wipe or erase any marks before washing (if needed) as chalk will cake when washed. And for any chalk type pencil, keep a sharp point. I usually mark about 10" and then hit lightly with the sharpener. Less to sharpen each time and makes a better mark. The Multi-Pastel is chalk and clay and washable.