The Saga of Recluce

…the inner lining of the bark of the river willow should be scraped, then dried until it is firm and stiff. Then it must be ground into the finest of powders with a polished hardwood mortar and pestle…

The White Order - chapter:XXXVII

…if the leaves be brown, and dried, and powdered, then they may be used as to purge the bowels…save that never more than a thimble be used for a full-grown man…and never be offered to a child or anyone of less than four stones…