TANBARK
Before the days of Dacron, cotton canvas was the common material for sails. Cotton is heavy, and stretches, and, when left furled too long, provides a perfect place for mold and mildew to form. To combat those destructive growths, sailors used to treat the sails by "tanning" them with the bark of the Acacia catechu tree. The sails were boiled in water filled with the tree bark, or else the mixture was swabbed on, giving the sails some protection from mildew, and turning them a handsome reddish-brown colour. That colour became known as "tanbark".

