The Collaborative International Dictionary of English v.0.48:

Pearlash \Pearl"ash`\, n. (Chem.)
   A white amorphous or granular substance which consists
   principally of potassium carbonate, and has a strong alkaline
   reaction. It is obtained by lixiviating wood ashes, and
   evaporating the lye, and has been an important source of
   potassium compounds. It is used in making soap, glass, etc.
   [1913 Webster]