Southern Louisiana, where I live, also is quite humid, Costa
Rica probably more so, but when a coin develops full-blown bronze disease
(which, from your description, I can hardly doubt your coins have) in the space of days or weeks, I must wonder whether the coins hadn't been superficially cleaned, but not effectively treated, for pre-existing bronze disease
before they were photographed, sold, and shipped. I have had
to be careful of my bronze coins, of course, putting sachets of silica gel in all the storage boxes and not economizing on the air conditioning in the rooms where they are kept. With these precautions, only coins that arrived already with bad symptoms have been troublesome. I'll find the page with a coin of mine that had
bad BD, with photos of progressive treatment. Pat
Be sure to read all 8 replies. Don't try quick fixes. Also see other threads on bronze disease
, but, as I said, drastic, "sure-shot" fixes are likely to make matters worse.