... I wonder if ash is corrosive

...
Back in the day they burned trees to make potash.
Potash (
/ˈpɒtæʃ/) includes various mined and manufactured
salts that contain
potassium in water-
soluble form.
[1] The name derives from
pot ash, which refers to plant ashes or
wood ash soaked in water in a pot, which was the primary means of manufacturing the product before the
Industrial Era. The word "potassium" is derived from "potash".
[2]
Potash (especially potassium carbonate) has been used in bleaching textiles, making
glass, ceramic, and making
soap, since about AD 500. Potash was principally obtained by
leaching the ashes of land and sea plants.
en.wikipedia.org
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