Markings on tintypes?

[-chrille-]

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.

 

[koraks]

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.

 

[retina_restoration]

I think it has something to do with the paint layer on the black plates you're using.

I agree. I used to see this on “trophy plate” aluminum, and mostly on certain brands. I don’t use trophy plate anymore because of its many flaws.

 

[-chrille-]

Thanks! Will try coating on some of the plates and see if that helps.

 

[Ian C]

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.

https://www.google.com/search?q=thi...MuNLgH5QXCBwkwLjIuMC4zLjLIB1c&sclient=gws-wiz

It is the basis of antireflection coatings applied to filters and lenses.

https://en.wikipedia.org/wiki/Thin-film_interference

 

[-chrille-]

Good explanation.

I also think about Newton rings but that might be the same physical properties?

 

[koraks]

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.