Mirror, mirror on the wall

–  Who’s the fairest of them all? –  But what if the mirror had not been as clean and bright as it should have been? Neither the Queen nor Snow White could have asked the magic mirror that famous and memorable question. Mirrors have played an important role in films, not only in ‘Snow White and the seven Dwarfs’, but also in some older such as ‘Taxi Driver‘ or ‘The Shining‘ and some recent such as ‘Harry Potter and the Philosopher’s Stone’ or ‘Black Swan’. Even Justin Timberlake dedicated a song to mirrors! What would we do without mirrors? We could not even know how we are or look like (or how we dress)!

But what happens when a mirror gets old? Why do mirrors grow old? Let’s find it out!

You have probably seen one of those old mirrors with dark spots. Actually, this is what makes a mirror old. Something like this:

Old mirror with black spots.

Why is this happening? Well, to answer this, we need to know how a mirror is made of.

First of all, I want to point out that I will not go through the physics of mirrors and optics (which is however an amazing topic). But in case you want to know more about it, I recommend you to read the post of How Stuff Works.

History of Mirrors

Early mirrors (6000 BC) were just polished stones, such as obsidian. Later on, in the bronze age, mirrors were polished discs of different metals: silver, copper, bronze, etc. However, these were extremely heavy and tiny. In the Middle Ages, the development of mirrors jumped due to glassmaking technology. Nonetheless, the molten metals used to coat the glass used to break the glass. It was common to use a tin-mercury amalgam, in which mercury was boiled away by heating the piece.

Silver coatings

Modern mirrors were invented in 1835 by the German chemist Justus von Liebig. He described the technique as silvered-glass coatings or silvering. This technique is based on the deposition of metallic silver by chemical reduction of the “Tollen’s reagent”, a silver reagent prepared from silver nitrate. You can see the two-step mechanism to prepare the Tollen’s reagent in the next image:

Preparation of Tollen’s reagent.

In the next and final step, silver is deposited on glass by reduction of the Tollen’s reagent and the parallel oxidation of an aldehyde (e.g. glucose):

Tollen’s test. Redox reaction with reduction of silver and oxidation of an aldehyde.

In the making of a mirror, the glass is perfectly cleaned and rinsed with deionised water and polished with cerium oxide, to avoid any bubble or imperfection in the mirror. In addition, it is pre-covered with tin. This video shows you how a mirror is made.

Aluminium coatings

Modern mirrors are made of aluminium rather than silver because of their cheaper cost, quicker fabrication and excellent reflectivity. However, aluminium is prone to oxidizing, so a protective coating is usually applied. The fabrication process of aluminium mirrors is similar to the silver coatings, but in this case, molten aluminium is directly applied under vacuum, which bonds better to the glass. 

Comparative reflectivity of aluminium (Al), silver (Ag) and gold (Au) at different wavelengths. Source: Wikipedia.

The graphic displays the comparative reflectivity of silver, aluminium and gold at different wavelengths. As you can observe, aluminium covers a broader range than silver or gold.

Now that we know a little bit about mirrors, why do they spoil?

Black spots

The appearance of black spots and dark edges in a mirror is a sign that the back or coating is lifting and being worn away. This process is called desilvering. Moisture is the most common cause of desilvering, but there are some others as poor ventilation around the mirror, changes in temperature or exposure to chemicals. Luckily, coatings nowadays have improved a lot and there are much less desilvering in the modern mirrors than in the oldest ones.

Now you know what happens, be careful when placing and cleaning your mirror, unless you want a vintage one!

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