A broken mirror brings you luck or something, right? Well, it does bring you a lot of trouble because you have to clean up all the shards of glass. But does it have to be made of glass? Isn’t it just some metal coating on the back of the sheet of glass? Couldn’t that coating be put onto something more durable?
It actually is true – a mirror is nothing more than just a thin coat of metal on a piece of glass. Glass itself doesn’t provide you with reflectivity – it is all that metal. Mirrors are manufactured by applying a reflective coating to a suitable substrate. That substrate is almost exclusively just glass, and reflective coating is typically silver or aluminium. Because metals oxidize, they can be additionally coated with some tin chloride or paint. A good mirror will always be protected with a frame to avoid damaging the edge of the glass. However, if it wasn’t glass, it wouldn’t damage so easily, right?
Well, there are plastic mirrors. Usually they are made with children in mind, but they have different applications as well. You can actually make a mirror without any substrate at all. You just have to polish a sheet of metal to a high degree and it will reflect very well. These kind of mirrors so exists and are used, but usually serve just a decorative purpose rather than being a sort of a tool to actually look at yourself. Some parts of building facades are polished to that degree.
You could make a lighter mirror using plastic or less likely to shatter using metal. However, glass is used not for no reason.
The truth is that glass is the most suitable material to make mirrors. Plastics scratch too easily and metal is too expensive and too heavy. Furthermore, by cleaning a metal mirror you would reduce its polish – it is almost impossible to not introduce micro scratches that will reduce reflectivity. Here are some more reasons why mirrors have to be made with glass substrate:
- It is cheap (much cheaper than metal sheets anyway);
- We have lots of it;
- We can make very flat and smooth pieces of glass;
- It is hard (doesn’t scratch easily);
- It is easy to maintain (no need for special chemicals);
- It is very rigid (it makes it easy to paste mirrors onto);
- It does not tarnish (old mirrors are as transparents as the new ones);
- It provides protection (prevents oxidation of the metal);
- Even cheaper glass is optically very transparent.
Finally, it is a bit of a tradition. Glass is a high quality material (that is why you see it on phones) and we expect it when we are buying mirrors. It doesn’t age at all, it doesn’t require a lot of maintenance and it is not very expensive. And breaking, although annoying, is not that big of an issue, because mirrors are rarely moved around.