Black spots cannot be cleaned or polished away, because they are the silver backing corroding behind the glass rather than a film on its face. A mirror in that condition is usually answered with fresh mirror glass, which Accurate Glass & Mirror cuts to the original opening at its Lodi, NJ shop. Resilvering exists, but it is a specialty process kept for antique and heirloom pieces.
A mirror is a sheet of glass with a thin layer of silver applied to the back and a protective paint sealing that silver in. Everything in the reflection is happening on that back surface. When humid air, water or cleaner works past the paint, the silver oxidizes, and the result reads as black or gray blooming, almost always creeping in from an edge or a corner first.
That is why nothing you apply to the front does anything. Polish, compound and every spray on the shelf act on the face of the glass, and the face of the glass is not what failed. It is also why the marks look slightly different depending on where you stand, since you are looking through the thickness at a layer that is no longer there.
Origin decides this more than size does. An antique piece, a beveled mirror in its original frame, anything with hand worked detail, or a mirror that came with the house and cannot be matched can go to a resilvering specialist, who strips the failed backing and lays a new reflective coat. For a plain builder grade bathroom or closet mirror, that work rarely makes sense against having a new piece cut to the opening.
There is a middle path worth knowing. If the blooming is confined to a narrow band along one edge and the field of the mirror is still clean, a frame or a metal edge trim covers it and looks deliberate. It buys time without stopping the silver from lifting underneath. Some homeowners go the opposite direction and order antique mirror on purpose, where the mottled surface is the point.
Most desilvering starts at an edge, and most of it is preventable. Spray cleaner onto the cloth instead of at the glass, so nothing runs down and stands along the bottom edge. Dry that bottom edge after a steamy shower. Run the bath fan long enough to clear the room rather than long enough to feel finished, keep ammonia heavy cleaners away from the perimeter, and set the mirror so the back can breathe against the wall instead of trapping moisture behind it.
Generally yes, and humidity is the reason. A hall or bedroom mirror lives in dry air, while a mirror over a vanity takes steam several times a day and takes cleaner that pools along its lower edge. Ventilation and drying that edge do more for how long it lasts than the grade of the mirror itself.
Usually, though slowly and unevenly. The corrosion follows wherever moisture can reach, so a mark that started in a corner tends to travel along the edge before it moves into the open field. Cutting off the moisture supply slows the march considerably. Nothing applied to the front of the glass reverses what has already gone.
A photo and two dimensions are usually enough to say whether you are looking at edge blooming or a mirror that is finished. Replacement glass is cut and edged in our own Lodi shop, so odd sizes and notched pieces are routine. Call (201) 460-1313.
