Mirror Rot is caused by moisture penetrating between the layer of glass and the layer of metal. Once water vapour has wormed its way in, the metal that makes your mirror shiny begins to react and oxidize (rust), turning an ugly black.
Tear off small bits of aluminium foil and use the clear adhesive tape to tape those pieces over the black spots, being sure the shiny side of the foil is against the back of the mirror, and that the foil is as smooth as possible. Once all spots are covered, replace the frame of backing and rehang.
If the black spots are in the middle of the mirror, it is typically caused by moisture or cleaner reaching the back of the mirror and causing damage. Unfortunately in either case, there is no way to remove the black spots. The only solution is to repair the mirror by painting a new finish or disguising the spots.
"Mirror rot" happens when those protective coatings are compromised and the silver nitrate is exposed. Even if your mirror is mounted on the wall, the silver nitrate will begin to react with particles in the air, moisture, or even the products you use for mirror cleaning.
Mirror Desilvering
If you've ever seen an antique mirror, chances are, that mirror had dark streaks running through it or black spots along the edges. You may have even seen this on a newer mirror. What causes these black edges to form on mirrors? The short answer is that it is caused by an effect called “desilvering”.
Can You Repair a De-Silvered Mirror? You can restore your favorite mirror to its original luster with a re-silvering process. Even though a glass professional can remove the backing, strip the remaining silver, and apply new silver, this process is complicated and can get expensive.
Mirroring (re-silvering) is a very wet process. In order to re-silver a mirror, you will have to remove it from its frame. This is a good time to see if the frame itself needs to be strengthened or repaired. The best time to do this is after you have carefully removed the mirror.
When mirrors are exposed to excess moisture, water can get between the layer of silver and the backing. This causes the bond to break and the silvering to chip off behind the glass. This leaves your mirror with unsightly dark spots or black edges where the silver has fallen away.
The key to preventing mirror desilvering or oxidation is to stop any moisture / water vapour being able to penetrate the mirror which can be done by applying a layer of sealant to the mirror edge.
Avoid Harmful Chemicals
Your grandmother may have taught you to use bleach to clean everything, but on mirrors, it can leave streaks and make them look dull over time.
Mix one-part vinegar with four parts water and pour it into the spray bottle. Spray the solution onto the cotton cloth or directly onto the mirror. Before the spray dries, wipe the mirror from side to side or up and down with the cotton cloth. Never wipe the mirror in circles.
On small discolored or tarnished spots on the back of a mirror, use the abrasive side of a kitchen sponge to scrub off the discoloration. Try to avoid enlarging the affected area. Patch these spots with a piece of Mylar film (available at art supply stores) or an ordinary piece of heavy-duty aluminum foil.
A good quality mirror will last many years without issues. The anti-fog properties in a mirror are reliant on the surface, so if it's damaged, it won't have good fog-fighting properties. Instead, try to keep your mirrors clean and damage-free for best results.
Several easy tips can help your mirror regain its former glory: Newspaper and white vinegar. Using a spray bottle, sprinkle some of the vinegar over the filmed spot. Use the newspaper piece to rub off the vinegar until you get a shiny outlook.
Mold can't grow on the clean surface of a mirror. Mold spores can adhere to tiny particles of dust and dirt on a mirror. A clean mirror isn't conducive to mold growth because the surface isn't porous.
The 'black dots' defect in glass consists of fine inclusions, approximately 1 mm in size, that do not dissolve in the silicate melt, thereby creating a reject. The nature and causes of such a defect in glasses and its likely appearance can vary.
Price will vary with age, condition, rarity, special features, materials used, maker or manufacturer, and the current interest which informs the market value. As a rough guide, you can expect antique mirrors in good condition to fetch somewhere between $600 and $6000 USD.
You can test this by holding the edge of a white card against the glass. If the card and the reflection are the same white color, the mirror was made after 1850. If the reflection is more yellow or gray, the glass was made before 1850. Mirrors in any traditional style can be found, some original, some reproductions.
WD-40 Multi-Use-Product is a great alternative for cleaning glass surfaces and mirrors. It removes sticky residues and smears leaving you with clear glass. Always remove the product thoroughly with a dry cloth.
White vinegar, rubbing alcohol and pure water are best for cleaning antique mirrors if you prefer using homemade cleaning assistants. You should avoid using any store-bought or homemade abrasive and harsh cleaning agent as it can cause more harm than good to your old friend.
Rubbing Alcohol and Vinegar/Glass Cleaner
Place some rubbing alcohol on a cotton pad and rub it on the stains. The alcohol dries off quickly so make sure you clean the stains quickly. Spray glass cleaner or vinegar and water on the cloth, if you spray it directly on the mirror, it can create tarnish marks.
Mirror resilvering is not a do-it-yourself project. DIY-ers will actually touch up their mirror resilvering spray, but the results are usually disappointing. To properly resilver a mirror, the original mirror backing must be removed entirely and a fresh chemical coating of silver added.
Silvering is the chemical process of coating a non-conductive substrate such as glass with a reflective substance, to produce a mirror. While the metal is often silver, the term is used for the application of any reflective metal.