Lye

From DwarfFortressWiki

Lye is used to make soap or potash, though potash can be made directly from ash. It is made with a Lye maker dwarf at an ashery and requires 1 ash.

Why make potash out of lye instead of ash?

• to train your dwarfs by making them do extra tasks
• if you have left over lye from making soap
• lye can be bought at the "embark" screen but ashes cannot