Appreciate the insight. You're pretty smart and right on. Floor & Decor blames the adhesive as well. My installer is a nice guy and says he will make it right but I'm cautious about using the same tile again...however, wife likes it so I'll try and get some assurances from the retailer on what is a fool proof installation if we go that route again. There were NO cautions on the box of tile as to special handling or needing a unique application. It appears to be a freak happening.
Thanks

The adhesives used to install tile are reactive by their nature-a chemical reaction takes place to cure the adhesive and grout, and metals frequently can be affected by that. glass and ceramic are pretty neutral as far as reactions go, but as mosaicists we like to push the envelope and learn these things by trial and error. You can either appreciate the effect or try to combat it. Using mirror in mosaics frequently causes the above effect due to the silver. We use different adhesives or use a mirror sealant on the pieces to prevent the oxidation. Such a sealant might help with these tiles.

WW
Thank you again. My wife is committed to using the same tile again (with the tiles of silver foil interspersed in the mosaic). What kind of "mirror" sealant would you recommend in case I have to seal every single tile of silver foil to be sure we don't get the chemical reaction again?
Appreciate the info
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http://www.delphiglass.com/soldering-supplies/flux-finishing-chemicals/mirror-edge-sealant

Here is a product, but there are automotive products that do the same thing. Its just a spray, and you could just hit the silver ones. Do you know what type of adhesive your tile person used? In mosaic we use thinset mortar. DRY. not premixed. Premixed thinsets are mastic based, and they WILL fail. especially outdoors. Your environment also matters. If you live in a dryer frost free area, it could be find for a number of years. The reason I ask is because mirror uses a special mirror mastic. I would never use mastic for anything I want to endure However, tile people often have different opinions about that. And I know when mastic is used in my region out of doors because I often pick up the broken tiles that have fallen off to use in my work :) I have obtained some very expensive ceramic that way. At any rate, here is an article for you. Good luck!

http://tiletools.com/mastic-vs-thinset-which-should-i-use/

I just purchased the same tiles and Floor and Decor and we are having the same issue. They were installed last week and they are getting that black almost mold looking appearance. I assumed it was the material they used reacting with the tile paint. Have you had any luck fixing it or getting a refund? Thank you!