lye -
A strong causticalkaline solution of potassium or sodium salts, obtained by leaching wood ashes. It is much used in making soap as well as its use in biodiesel.
Wikipedia
Lye is a corrosive alkaline substance, commonly sodium hydroxide or historically potassium hydroxide. Previously, lye was among the many different alkalis leached from hardwood ashes. Now, lye is commercially manufactured using a membrane cell method.
OmegaWiki Dictionary
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lye The alkaline solution that is obtained from the leaching of wood ashes.\n(Source: MGH)