The case of the spotty crystals

By Gersil N. Kay, IESNA

Here is a mystery in the process of being solved. In a white marble powder room, the interior designer supplied mirrors and crystals to be lit by glass fiber optics lighting.

Crystals were attached to the front of the mirrors and the silver backing was removed from the reverse side to allow the light emitted from 5mm light guides to shine through.
A year after installation, black dots appeared on all the crystals. Only light rays, not electricity, flows through the light guides.
What caused the black dots? Could it be the silver reacting with light? Can the spots be removed? We are waiting for answers both the manufacturer of the glass fiber optics, and the mirrors.