The Collaborative International Dictionary of English v.0.48:

Adarce \A*dar"ce\, n. [L. adarce, adarca, Gr. ?.]
   A saltish concretion on reeds and grass in marshy grounds in
   Galatia. It is soft and porous, and was formerly used for
   cleansing the skin from freckles and tetters, and also in
   leprosy. --Dana.
   [1913 Webster]