The thing we were all warned about not to drink as children, major component in modern drain openers and indispensable material to medieval tanners, linen finishers, and innumerable other trades. 

Lye was locally made from various and sundry caustic ingredients, including fowl droppings, human urine, wood ash, and powdered limestone, mixed down with rainwater to desired strength. 

Lye was used to bleach, to soften, and to scour, and to dissolve unwanted bits of adhering materials (flesh or fat from hides during tanning, for instance).