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Old 11-11-2008, 06:39 PM
Ian Robinson
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Ian Robinson is offline
 
Join Date: Jan 2007
Location: Gateshead
Posts: 2,205
Quote:
Originally Posted by Insomniac View Post
Post script; whilst the U.S coaters I've contacted won't touch overcoated silver because (they say) ,it is still permeable on the molecular level, the boss of Optical Coatings Associates (David) has told me that his overcoating has the problem licked because it's non porous. It would be interesting to know how long they last in practice. But David has been helpful, even offering to send me a coated sample to play with. I'm inclined to stay conservative, especially since over here in NZ I regularly get dew on the primary! I thought that was a near impossibility!
If the overcoating is impervious and will presumeably there is no diffusion of O2 to the metal surface , then it is even less likely moisture will get through too. Lookiing at the reflectivity of standard Al overcoated with SiO2 , Ag looks pretty good for the visual band.

It's the O2 in the air that makes silver tarnish. And Silver metal is insoluble in water.

(Ksp = 1.52 x 10-8 for AgOH)
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