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What Causes a Mirror to Tarnish?

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The Case of the Malicious Mirror Mastic


Dear Glass Detective,

I have had 3 sets of 2 brilliant-cut glass mirrors made over the last 20 years. The mirrors had birds carved in them and they looked fantastic when new. However, none have lasted more than a few years without tarnishing badly—the silvering turns misty and foggy. These were made by 3 different companies. The one thing that they had in common was all three companies stuck silver foil on the rear to protect from moisture because they were being fitted in a bathroom. I have since read that a primary cause of silvering failure was if the silver backing came into contact with glue or adhesive as this could cause a chemical reaction. I would like your thoughts and comments on this.

Kind regards,
Gordon Usher



Dear Gordon,

Thank you for making contact with the Glass Detective regarding your apparently ongoing problem with tarnishing and deteriorating mirrors. You stated that you have now had fabricated decorative mirrors installed by three (3) different companies and in each case, the silvering on the mirrors tarnished over time. I certainly must applaud your persistence. You truly must have liked the look of those custom mirrors and I, too, am a big fan of glass art installations.

I have a back carved and painted backsplash in my kitchen that I absolutely love. It has been in place for about twelve years now and I have had no problem with it whatsoever. It is not a mirror, but a back-painted low-iron piece of glass that was sandblasted (carved) and then painted with various colors including reflective colors that look very mirror-like. Then the painted glass surface was treated with a protective overcoat of sealer to prolong the painted glass’s life. It is a large piece of glass that gets a fair amount of humidity and heat. It is generally cleaned on a daily basis. It sets on the countertop (on top of clear silicone blocks) and is held in place with a mastic material designed for use with glass and mirrors for this type of installation. I have also observed many other glass and mirror installations using custom fabricated products so I know these types of installations can be done in such a way that they can hold up very well for an extended period of time—many years in fact.

You also inform me that the mirrors had “carved” bird images on them. You don’t say whether the birds had been carved on the face of the mirror or on the backside of the mirror (the side that would have been silvered). However, for our discussion purposes, I am going to assume the carving was done on the silvered (back) side of the mirror. Lastly, you state that each of the companies that have performed these installations has done so while using a silver foil that they, in your words; “stuck” on the rear (backside) of the mirrors as they were being fitted in a bathroom to “protect from moisture”. You’ve read that the primary reason for mirror failure occurs when a mirror comes in contact with “glue” or adhesives that cause a chemical reaction. So, based on the information you are providing and my interpretation of it, here are my thoughts:

I believe that the most common causes of mirror failure (de-silvering, misting or tarnishing) are caused by improper fabrication or improper installation techniques. In my experience (and I have been around for a very long time) the use of the wrong adhesives, that is, adhesives not formulated to be used with mirrors, is the number one cause of mirror failures. Tarnishing, fogging, black spot formation, and de-silvering can occur. However, there are other causes as well. These include:

  1. Using the wrong type of cleaner (ammoniated cleaners are really hard on mirrors).
  2. Lack of ventilation in rooms where mirrors are installed that have prolonged, high levels of humidity such as shower rooms, steam rooms, near pools or hot tubs.
  3. Lack of proper protection on the silvered side of the mirror after it has been fabricated.

Please also remember too, that over some period of time, almost all mirrors will show some signs of deterioration.

I am baffled by the silver foil process that you refer to but maybe if I saw a picture of what they do with this foil I could better understand it. My initial thought is that the foil might be doing more harm than good. Then again, you state that all three of these companies used this silver foil technique/approach so maybe there is something new that I am not aware of that is the current approach to mirror installations. Without seeing it first hand, I am not quite sure what to assume. It is a curiosity to me for sure.

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A few years ago I became quite involved in a legal matter when a mirror fell off of a column in a retail clothing store and injured a young child. The mirror that fell had been installed less than six months before the accident. By the time I had been called and arrived at the site, the broken mirror had been cleaned up and thrown out. However, we were able to remove other mirrors (quite easily by the way) that had been installed at the same time. These mirrors would also have started falling off in the near future. The problem was that an adhesive typically used on wood products had been used to install the mirrors and it simply didn’t hold once this adhesive had fully dried and cured.

It was quickly determined that the improperly installed mirrors had been put in place by a carpentry company that had performed remodeling work in this store. They had been asked to do the mirror work by the store manager and simply bought mirrors (cut and beveled to size) from a local glass shop and then put them up with the adhesive/glue they were using at the time for various wood items. They just did not know any better. This lack of experience in working with mirror products resulted in an injury to a customer’s child and a hefty lawsuit against the store, the general contractor and the carpentry subcontractor. The proper mirror adhesive would have cost less than $10 per tube.

I have seen this situation on a number of occasions over the past 30 years or so. There are mirror adhesive products (mastics) that are appropriate for mirrors. They will not only do a good job of holding a mirror in place but they also are chemically formulated NOT to attack the protective back on a mirror. I used the term “protective back” because the silver that is deposited on the glass surface to make it a mirror is covered/coated with other materials (coatings) in an effort to keep chemicals and moisture from reaching the silver and causing damage. That’s one of the reasons it’s important to use a glass professional for mirror installations.

When you perform fabrication on the silvered side of a mirror, you are exposing the silver at the edges of the fabricated areas. A protective coating or film needs to be applied to “seal” up these exposed areas. Those who work in this medium should know how and what is needed to protect these critical areas.

Just as your real silver pots, bowls and tableware will tarnish easily when exposed to the atmosphere over time, so too will the silver in the back of a mirror if it is not protected. And unfortunately, when the silver on the back of the mirror starts tarnishing (oxidizing) there is not much you can do to clean it up.

So, go back to your fabricator to ask them what procedure they’re following to protect the silvered side of your mirrors after they have carved into them. And, back to your installer(s) to ask them if they are using an approved mastic material for their mirror installation needs. And if you need help in identifying qualified mirror fabricators and/or installers in your part of the country, please feel free to reconnect with us and we will try to be of assistance. I truly hope that this information is of some value to you and thank you again for contacting the Glass Detective.

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Lyle Hill

Lyle Hill has been in the glass and metal industry for more than 40 years. In this time he has managed glass retail, contract glazing, mirror, architectural window, window film, and automotive glass businesses throughout America. He obtained an MBA from IIT with a focus on Technology and Engineering Management. Hill is also a columnist for glass industry trade magazines and often called the “face” of the glass industry. He has also authored books including “The Broken Tomato and Other Business Parables,” which is available through Amazon. Find out more about Lyle on Linkedin.

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2 Responses

  1. Hi, I just had some cabinets built in my closet. They have mirrored doors on them but the inside of the cabinet doors are just the mirror backs which I’m not in love with. Any ideas on how I can finish them off without harming the mirror? Thanks so much


    1. Hi Ronda,
      Thanks for the question. You’re off to a great start knowing that you need to treat the mirror backing delicately so that the mirror is not compromised. It’s difficult to answer without seeing the cabinets, but it sounds like you could probably have the mirror backs encased in wood to match the cabinets. By encasing the mirror, the wood casing would be fastened to the wood cabinet, leaving the mirror backing untouched.

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