Was this a particularly tricky offering from Picaroon, or did I have too much good wine with dinner? I suspect a combination of both.
Southwest corner took a long time to yield, mainly because of two completely unknown (to me) words.
FOI 21 (yes, I had to go that far down)
LOI embarrassingly 24. I’m no chemist.
Classic Picaroon with some very granular clues, nice surfaces and plenty of misdirection. If I’m pushed, I’d go with 18a, simply because I laughed when the penny dropped.