Oh wow that is absolutely beautiful!! Exactly what I was looking for! Experimented for ages with layer weight and colour ramps but this is so elegant and looks so realistic! Really appreciate your help, thank you for making this available for everyone to use!
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Youâre welcome!
where do i download the node groups
The top post has all the links 
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Thank you.
Iâm late to this party, but I found a free-to view paper on copper patinas which gives a pretty good description of their optical properties and causes of color. Also a graph of % reflectance against wavelength for bronze, brass and copper metal. The general conclusion Iâm reading is that the brown and green colors are mainly from the chemistry and light-absorbance of the lower Cu2O and upper Cu(II)-compound layers. The samples examined werenât flat or uniform in thickness enough for film interference to be a factor.
https://www.sciencedirect.com/science/article/pii/S0010938X19305712
Interesting read! I indeed donât think the green patina is a result of interference, but âsimplyâ a diffuse color.
However, I donât see any evidence that interference does not play a role in the darkening of copper. The authors state that Mie scattering (small particles inside the cuprite layer) may play a role, but admit that they donât see evidence for such small particles in the SEM images. Furthermore, if you check the Supplementary Figure S3, the cuprite layer is pretty much conformal to the copper substrate for thicknesses from about 0.7 micron and up. The substrate + cuprite layer only has to be flat on the scale of about a wavelength to get an interference effect, where the existing roughness leads to scattering in multiple directions and a visual averaging of the interference effect over angle.
Having said that, I can totally imagine a multitude of phenomena playing a role for thinner layers, especially if the growth of the layer is not uniform (for example, island growth). I donât know how the initital layer of cuprite grows.
The interference model gives quite some wacky colors for very thin layers of cuprite (0.01 - 0.05 micron), which we typically donât see in architecture. That it is real can be shown when copper is undergoing a thermal treatment, see for example here: https://www.pinterest.com/pin/461196818068902201/
Either way, in my opinion the interference model gives some quite convincing aged copper looks, regardless whether it is 100% accurate or not
I updated the three materials that can use the Principled shaderâs thin film interference feature (Blender 4.3+) in the Blend file that is in the top post. The visual results may change slightly, but speed is much improved for these three materials according to my limited testing. The other materials are unaffected as that kind of interference is not yet supported in Blender. If conducting materials become available in Blender, Iâll further update this file.
Additionally, I added a lot of info on how to hook up these nodes to a shader.
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