Having problems with markings on wet plate tintypes. It looks like oil on water. It is still seen after varnishing. What is the cause of these defects?
I only get this on tintypes not on ambrotypes. And my process is exactly the same for both types.
I think it has something to do with the paint layer on the black plates you're using. You could try applying a coating on top of that (perhaps just black paint, or a clear varnish), then pour collodion over that. See if the same iridescence still emerges.
It is the phenomenon of Thin Film Interference. This is often seen when a thin layer of gasoline lies atop water, such as when a drop of gasoline drips onto calm water while fueling an outboard motor on a small boat alongside a dock.
It’s also seen just after a light rain when the thinner components of motor oil float on wet asphalt.
It doesn’t require oil. Many thin transparent materials will produce this effect.
The optics behind Newton rings are similar, the cause and solution tend to be different. In a very general sense, you could group them all under 'interference caused in the interface of two layers' - but that doesn't get you very far in solving the issue, I suppose.