Harder charcoal from magnolia heartwood that is used for intermediate sanding of the Shita Nuri layer.
Traditionally, 'Sumi' charcoal from special types or parts of trees is used with water for sanding the intermediate layers of lacquer.
Only old, well-seasoned wood is used for the charcoal. The complex process in the furnace, during which the wood must not burn up, usually takes two full days.