Well, the apparently most widespread procedure seems to have been utilizing coconut water, i.e. vinegar, for cleaning and simultaneous etching. (Just to emphasize in case you subsumed it under fruits...)
However, considering period accounts and early pics with “black” blades, I’m willing to bet that warangan was known and utilized, even if of limited availability due to financial constraints and/or access to skilled artisans (with expected variability based on period, area, and possibly status).
There are enough pattern-welded Moro kris (including but not limited to twistcore) that IMVHO just scream for any enhanced contrast from advanced etching methods.
These maritime societies knew each other well and traded for centuries if not millennias!
Regards,
Kai
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