Thanks Rob. Yes, it's about time I gave it a go.
The answer seems fairly self explanatory, but I'll ask anyways... why such low temps? Is that pretty standard for holographic materials?
As for the exposure, the white paint on my gas tank is that dark, solarized, blue color, while the gold paint gets the interference colors. So I'm both over & under it seems, but generally it was tough light to shoot in, and I knew that going into it.
I'm also looking forward to giving it another go, and sooner this time. The whole procedure was pretty easy, and I spent more time mixing the chems than shooting & processing.
Gallium seems very interesting. It looks as though it was first made in 1875, so it definitely stands to reason that Lippmann might not have known about it, or enough quantity didn't exist on the face of the earth at the time! Surely mercury would've been easily available and cheap.
Any idea what kind of quantity one would need, theoretically? It's density is 6.1 g/cm³. 100 grams will cost you $170 from one supplier, but undoubtedly there are deals out there. Very intriguing...
Thanks for the word on Hermann Krone.