Surface studies of corrosion in second surface silver helios

[erik winkler]

What tarnishes mirrors?....
http://www.sciencedirect.com/science?_o ... ff4244d858

And

http://www.sciencedirect.com/science?_o ... 90fcfc6ef2

Again it seems that we need lead in our paint.
But in this case real lead and not leadoxide.

[Patrick Mackle]

The first account seems to apply to sputtered or vacuum deposition mirrors. They are deposited with a thin layer of metal only and no paint is involved.
This is because they are usually encased or in a fairly neutral environment. If in time the reflective metal layer becomes blemished, the thin metal layer is easily removed and
a fresh new layer is applied. This way the mirror is always kept in a fresh condition.

The second account is a test of painted mirror in about the worst situation a silver coated glass could be subjected to.
Basically the salt is breaking down or leaching out soluble solids in the paint coating, which allows the corrosive salt to penetrate the resulting voids and reach the mirror metal, tarnishing and dissolving it.
Last year I was sent some 24" search light reflectors off of an old British ship that needed resilvering. Besides being painted with a very dense mirror paint, they had lead foil backing
that had been spun on to conform the their dished shape and the lead foil edge was formed around the edge of the dished mirror to hold it in place. Then the whole back was rolled
and formed with chicken wire.
The salty sea water eventually figured a way to defeat even this carefully planned barrier after more than 50 years of service.
Pat