As a function of time, the surface of silver becomes covered by a granular layer of black coloured Ag2S grains. The thickness of this layers increases with time. During this process, the colour of the surface changes gradually. In the early stage of corrosion, silver becomes yellowish and only in a later stage it becomes black. It is said the colour changes with the thickness of the film formed. Once the thickness of the film is larger than the wavelenght of visible light, the colour of the pigments becomes dominant.
The yellow colour is considered as an interference colour. But how can the colour be the the same from all angles at which you look at it. Iridescence is supposed to give different colours at different angles and result in "rainbow colours".
Iridescence also depends on the film thickness. It is optimal for films thinner than 500 nm (for interference effects in visible light). Thin tarnish layers on silver can vary from yellow to red to blue before the layer becomes black, its final colour. The attached photograph shows red and blue interference colours on the outer edges of a silver object (Photograph courtesy of Margo Brunn, Provincial Museum of Alberta).
For thicker tarnish layers, the rainbow effect (caused by iridescence) gradually vanishes.
Thank you for ypur answer Sergei. In many cases there is not really a rainbow efffect visible, rather 1 dominant colour with very subtle changes in hue when looking from other directions. The question is: how are thin tarnish layers colouring the surface so uniformly? HAs it siomething to do by the fact that tarnisg layers are granular and that also scattering should be taken into account?
Olivier, I am not sure the tarnish layers are always granular. Like in any crystal growth, the final shape depends on many different parameters. However, since tarnishing normally takes place under the influence of surrounding factors (gas atmosphere, moisture, temperature, etc), we can expect a rather inhomogeneous layer with nonuniform structure. If you want to say that a uniform coloring is a result of random light reflection by nonuniform surface, I would probably agree with you. Regarding the scattering contribution, it definitely should be taken into account for the same reason.
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