D.M. Procida wrote:

What will loosen decades-old dried brass polish in fine crevices (on a
decorative Persian tray)?

I've heard suggestions from lemon juice and vinegar to WD-40.

Thanks for the various suggestions made here.

In the end, the only thing that works to extricate the polish is to use
a toothpick, but since this tray must have something several hundred
metres of fine crevices, that's not really an option.

Neither soaking overnight in a solution of washing-up liquid, nor
applying more aggressive solvents (vinegar, white spirit, various
alcohols) makes any difference, and doesn't appear to loosen the dried
material enough to brush it out - I think the crevices are just too fine
and the polish too tenacious.

(The tray was large enough for me to do a somewhat controlled experiment
with different solvents on different parts of it, but since none of them
appeared to make any difference at all, there's nothing interesting to
report about that.)

Daniele