Because the reflective backing is corroding. A mirror is glass carrying a thin silver layer and protective paint on the back, and moisture reaching that layer from the exposed edge eats it away, so the black creeps inward from the perimeter. Accurate Glass & Mirror, a family run glass shop in Bergen County NJ, cuts replacement mirror to size on site in Lodi.
The edge is the weak point because it is the one place the silver stands exposed. Once water or vapor wicks under the backing paint, that silver oxidizes and darkens, and damage travels along the perimeter rather than sitting in a spot. Which is why this failure shows up as a creeping border and almost never in the middle of a mirror.
Three habits cause most of it. Spraying cleaner straight at the glass, where it runs down and pools along the bottom edge. Mounting a mirror flat to a wall that gets wet, with no air behind it and no clearance underneath. And running a bathroom without an exhaust fan, so every shower loads the room with vapor that has to condense somewhere. Add a mirror clipped tight over a backsplash and its bottom edge sits wet after every shower. Panels bedded flat in a full spread of mastic trap whatever finds its way behind them, because nothing back there ever gets a chance to dry out.
There is no putting silver back. Products promising to repair a desilvered mirror either paint over the back or hide the border behind a frame, and corrosion keeps advancing underneath either way. Once the black reaches into the viewing area, the honest answer is new glass. Replacement is also easier than people expect, since we cut mirror to your dimensions, polish or bevel the edges, and set it with clips, a channel or mastic depending on the wall.
The next one lasts longer if conditions change with it. Spray cleaner onto the cloth instead of the mirror. Dry the bottom edge whenever you dry the face. Run the fan during a shower and after it. Where a mirror must live in a wet spot, a polished or beveled edge with a small standoff, or a frame keeping water off the perimeter, buys years. And if you actually like the look of aged glass, antique mirror does that deliberately and evenly, rather than failing along one edge.
Not structurally. Desilvering is a coating failure rather than a crack, and the glass remains as strong as the day it went up. What deserves checking is whatever sits behind it, because a mirror whose edge stays wet often hangs on a wall or backsplash that is also staying wet. Inspect the drywall and the caulk line while the old mirror is down.
Better mirror stock uses a more corrosion-resistant backing than builder grade sheet, and it earns that in a bathroom. The larger factors remain installation and air: clearance behind the glass, a sealed or polished edge, a mounting method that does not pond water at the bottom, and an exhaust fan that runs. Good glass in a bad spot still fails at the edge.
Once black reaches the reflection, new glass is the only honest fix. Bring the size to our Lodi showroom on Industrial Road and we will cut mirror to it, polished or beveled, for a vanity, a wall, a full-length spot or a home gym.
