Silver-backed mirror must be glued down using a special mirror adhesive; otherwise the reflective silver will oxidize and turn black. Mirror tile is a different situation.
Mirror Vs Mirror Tile
Note that this warning is about pieces of broken mirror being used as tile. Mirror tiles such as the colored glass mirror tile we sell have a special epoxy coating over the silver to protect it from adhesives. In that case, you can use Weldbond and do not have to worry.
Case Study: Lotus Mosaic
Donna said that this was her first mosaic, and so the fact that it has critical problems really hit me hard. First projects are leaps of faith with a lot of emotional investment, and so we can love them in spite of their warts and blemishes because they are honest warts and blemishes, but the possibility that we can make a simple mistake that causes the whole project to fail completely just seems unfair. (Fortunately that doesn’t happen often with mosaic.)
Thinset is caustic, and so it oxidized the silver backing more or less immediately, which left clear glass over mortar and totally destroyed the reflective property of the mirror pieces.
How could this have been avoided?
A Revised Method
One way to make this project would have been to temporarily face-mount the mosaic on mounting tape or mounting paper and then press the mosaic into a bed of mirror adhesive. Once the mirror adhesive hardened, the mounting tape or paper could have been removed, and the mosaic could be grouted.
The problem with this revised method is that mirror adhesive is sticky and petroleum based and difficult to clean up. You would need to put on enough so that it thoroughly covered the silver backing (to protect the edges from grout) but not so much that the adhesive squeezed up into the grout gaps, which would have to be removed before you could grout.
That means you would need to make a small practice sheet and spread out some adhesive to figure out how thickly you needed to spread it to avoid problems. The extra step of a small experiment such as this can save you a lot of time and stress in the long run.
Yet Another Caveat
Mirror adhesives are used for traditional mirror stock that is made by electroplating silver on the back of glass. There is a cheaper variety of mirror made from acrylic, and mirror adhesive will ruin that material. I believe that if your mirror is glass, you can assume it has silver electroplating, but I’m not sure.
Alternatives To Mirror
Part of the ethos of mosaic is the ability to salvage something broken and make something new from the pieces. I think that is why so many people want to use broken mirror in their mosaics, and that is fine, but you have to think about how the finished work will look. The thing about mirror is that it reflects what is in front of it, and so you might find that you don’t actually like what it looks like once it is installed on location. This issue has bit more than a few people, and so you might consider using stained glass instead of mirror. Colored glass mirror tile is also superior for this reason.