Potash is produced by leaching ashes and sea plants and then evaporating the resulting solution in large pots. Sea plants were the primary ingredient in Triana. The reside left us called “pot ash”. This was boiled witholive oil, soda and lime. It could be scented with almond oil, Rosemary and other herbs and spices. Triana was known for it boiling pots leaching the ashes.
In the photo above shows ashes being steeped from a wood fire to leach out caustic soda. The liquid runs into a cauldron where it is boiled to reduce water content.
[ES: Flicker. Potash. Aug 4, 17; ES: Wikipedia.org/Potash; and Rodison. “Studies.” 2001:145]