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”. But to understand what desilvering is, it’s helpful to understand how mirrors are made. And to understand how mirrors are made, we need to know a little about the history of mirrors and how glass itself is made.
The History of De-Silvering
The world’s first mirrors, dating back to at least 2,000 B.C.E., didn’t suffer de-silvering issues because they weren’t even made from glass. Instead, they were made from highly polished pieces of metal. The first versions of the mirror as we know today came about in the 15th century when a tin-mercury amalgam was used to silver mirrors. This was a two-step process where both liquid mercury and tin foil were used in conjunction to produce a silvering effect on the glass. This process was standard for creating mirrors all the way up until the early 20th century. Obviously, for health reasons it was not ideal for mirror manufacturers or consumers to be exposed to mercury, which is highly toxic and can cause a variety of health and mental health disorders.
Quick Tip: If you have an antique mirror dating back to before 1900, there is a chance that it is made with mercury. There are a few techniques used to identify mercury mirrors. The most reliable is a chemical test to determine if mercury is present. If such a test is not available, you can examine the features of the mirror. Mercury mirrors have different reflective qualities than modern mirrors. For example, mercury mirrors sparkle under direct light.
The modern methods of silvering are a bit more high tech. Mirrors start their life like any other piece of float glass. Float glass is molten glass that floats on top of a bed of molten metal. This creates a consistent thickness and smooth, flat finish.
Various methods are used to silver the glass to create a mirror. One method doesn’t actually use silver at all. Sheets of glass are put into a heated vacuum sealed chamber where aluminum is evaporated onto the glass. The molecules of evaporated aluminum rise and when they hit the glass, they cool and harden, creating the reflective backing.
The other popular method does actually use silver. In this method, the glass is thoroughly washed, then rinsed with distilled water. Distilled water is used so that minerals do not contaminate the glass. Next, a layer of liquid tin is applied to the glass, then a liquid layer of silver. The tin acts as a primer which allows the silver to stick to the glass. The silver is applied in an extremely thin layer. Any excess is rinsed from the glass and reused. The glass is heated to dry the backing, which is usually sealed on by a layer of paint. The mirror can then be cut to the desired shape and size.
What Causes De-Silvering?
Whether new or old, many mirrors desilver at some point during their lifetime. Dark edges around the outside of a mirror is a telltale sign that the silver backing is lifting and being worn away. The dark spots will eventually work their way in further.
Moisture is usually the primary culprit and cause of desilvering. This is mostly seen in bathrooms where steam from hot showers is abundant. Inadvertently splashing water from the sink can also penetrate the corners and edges of a mirror. Even cleaning a mirror improperly can lead to desilvering. This is why we always recommend spraying glass cleaner onto a towel rather than directly on the glass. And we suggest no ammonia cleaners be used on mirrors. Moisture on a mirror once or twice might not have any effect, but repeated exposure over time will undoubtedly take its toll.
What Can You Do if Your Mirror Desilvers?
There are a few options to repair mirrors with black edges. You will want to consider the value of the mirror when choosing the best solution. Sometimes the best solution may be to simply purchase a new mirror. Another option is to buy a frame for the mirror to hide the dark edges. A wide frame is best because the dark spots will likely grow over time.
The last solution is to take the mirror to a professional so it can be resilvered. Unfortunately, there are only a handful of known places across the US that re-silver mirrors.
The resilvering process is much like the silvering process itself. It is a time and labor-intensive process, so this will probably only be a good option is the mirror is of some sentimental or monetary value (contact a local antique specialist for assistance).
First, the painted backing must be removed. Then the existing silvering must be stripped from the glass. From here, the mirroring process can begin. Just as in a factory, the mirror should be washed and prepped, then the silvering product will be applied and sealed. It is recommended that this process is done by a professional since it involves the use of chemicals and can take practice. Glass must also be handled carefully throughout the process so that it does not scratch or break, which could cause injury.