Frank, I'm stepping right outside my comfort zone now. :shock: Thinking aloud...... cold lustering techniques can also be used. I think this can produce very light (I have heard people incorrectly describe it as "flashed") iridescence.
I haven't scratched any fake carnival (and perhaps a definition is needed for "fake" but this is perhaps not the place for it), but I would not think it is possible to scratch the iridescence off - for example - the Far Eastern Good Luck, Peacocks etc. The iridescence on pretty much all Carnival (as far as I am aware) is, in old fashioned parlance, burnt on. Iridised when HOT (directly after moulding/shaping).
Cold iridising of a sort can be done by "water ambering" in certain natural springs. It has also been done as "one-offs" on some old EAPG pieces (mainly Heisey) through a post production "decorating" process that I believe was applied cold luster. The iridescence on them is pale and very watery - and of course, they are "rare" (in other words, only a handful of them were iridised).
It's a big topic. I've only scratched the surface :roll: