NANIWA flattening stone (truing stone)
A flattening block is used to dress stones that are no longer flat enough to sharpen properly. A flat stone is indispensable for effective sharpening, especially of flat chisel and plane blades. To flatten a stone quickly, simply move it over the surface of the flattening block with a circular or back and forth movement.
To speed up the process, you can spread a little silicon carbide powder on the block (see drawing). If you do use a powder, you must afterwards rinse the sharpening stone thoroughly with water.
The flattening block comes with removable rubber feet to stop it from sliding around during the flattening process: