Resources
Why Mirrors Turn Black at the Edges — and How to Stop It
Black edges are not dirt and they do not clean off. They are the coating failing, and the cause is almost always moisture reaching the edge.
Short answer: the black is desilvering — the reflective coating on the back of the glass breaking down. It almost always starts at the edge, because that is where the coating is exposed and where moisture and cleaning chemicals reach it. It cannot be cleaned off or repaired in place; the panel has to be replaced.
What is actually happening
A mirror is glass with a metallic coating on the back, protected by paint layers. The face you look at is sealed by the glass itself. The vulnerable part is the perimeter, where the coating stops and the layers are exposed in cross-section.
When moisture reaches that cross-section, the coating corrodes. It shows first as a dark line creeping in from the edge, then as spots and cloudy patches. People often try to clean it, which does nothing, because the damage is behind the glass rather than on it.
The common causes, in the order we see them
| Cause | Where it shows up |
|---|---|
| Humidity reaching the edge | Bathrooms, changing rooms, spa and pool areas |
| Cleaning fluid running to the edge | Any mirror sprayed directly rather than onto a cloth |
| Water sitting on a bottom edge | Mirrors set directly onto a countertop or splashback |
| Poor ventilation | Rooms where steam has nowhere to go after a shower |
| Damage to the backing paint | Rough handling, or fixings driven through the back |
Two of those are installation and maintenance rather than product. Mirrors mounted with a small gap behind them, and cleaned by spraying the cloth rather than the glass, last noticeably longer than the same panel installed hard against a wet wall.
Why copper-free glass exists
Traditional mirror backing uses a silver layer with a copper layer over it. The copper is the part that corrodes readily in humid conditions, and it is the usual driver of edge blackening.
Copper-free mirror removes that layer and uses a different protective build. It does not make the mirror immune — nothing does — but it removes the most reactive component, which is why it is the normal call for bathrooms, wet rooms and any commercial washroom.
Our copper-free stock is supplied by a mirror mill that holds an SGS test report on the coating; we can quote the report number on request. The round copper-free high-humidity mirror and the rounded rectangle copper-free low-iron project mirror are the two formats we carry.
What to specify for a wet room
Four lines, and they cover most of it:
- Copper-free mirror glass
- Sealed or protected edge treatment
- Mounting that leaves an air gap behind the panel
- Cleaning instruction: spray the cloth, not the mirror
The last one belongs in the handover pack rather than the purchase order, but it changes the outcome more than most people expect — and it costs nothing.
When it is already happening
There is no field repair. A desilvered panel is replaced, and the useful question is why it failed: if the edge is black all along the bottom, water is sitting there and the replacement will go the same way unless the detail changes. Fixing the room usually matters more than upgrading the mirror.
Send the room conditions with the panel sizes and we will suggest the build. The specialty mirror range covers the copper-free formats, and we will come back within 24 hours.