Speaking of the mirror, in fact, everyone should know. It is a very important exterior part of clocks and watches. Its function is to isolate water vapor and dust, in order to protect the needle, dial and machine core. But in the 16th century, when clocks and watches were invented, there were no mirrors. Instead, they were covered with a cover, much better than flip-flops or bare-faced phones. Later, the watchmaker used transparent silicate glass as a watch mirror. Of course, in that period, natural crystal was also used to grind the watch mirror. That was the behavior of more local tyrants.
Glass is one of the longest used surface mirror materials, which has been used for hundreds of years. But it is well known that silicate glass is not only fragile, but also easy to produce.
Scratch. In the first half of the twentieth century, silicate glass was upgraded to quartz glass, which improved the scratch resistance and wear resistance.
In the 1940s, a revolutionary mirror material, Acrylic, appeared on the stage. Acrylic acid, also known as PMMA or organic glass, is derived from acrylic (acrylic plastic) in English. Its chemical name is polymethyl methacrylate. Although this material is easy to scratch, it is not easy to break compared with quartz glass and silicate glass. Because of this advantage, acrylic mirrors quickly replaced glass mirrors. Antique watches are mostly arched, because acrylic mirrors are made by pressing technology, which can be stamped arbitrarily after heating the raw materials. It is easy to produce radians, which are later called "bubble mirrors" by cousins. Another advantage of acrylic mirror is that it can be easily processed later, and can be modified with sandpaper or file.
Later, with the development of glass industry, a kind of mineral glass appeared again. The difference of mineral glass is that alumina was added to it during smelting, and chemical and heat treatment were also carried out later. These processes reduce fragility.
After the 1970s, sapphire mirrors began to be used. In modern times, except for a few vintage watches using plastic acrylic mirrors, most of the high-end watches are sapphire mirrors. If the sales tells you that the watch is not a sapphire mirror, many people will turn their heads and leave. After all, the word "sapphire" sounds very big and feels very valuable. But in fact, the "sapphire" used in watch mirrors is not a natural sapphire, but a genuine synthetic sapphire. The price of finished sapphire watch mirrors is not high.
The raw material of sapphire mirror is corundum, with Mohs hardness of 9, second only to diamond. Many of my cousins think it's indestructible, but in fact, sapphire mirrors are also very fragile.
Sapphire is broken. Nothing. It's not expensive anyway. But the sad thing is that when the sapphire mirror is broken, it will produce small debris, which can easily scrape the dial and needle, causing irreparable damage. The dial and needle of a watch are probably the most expensive parts besides the machine core. Sometimes, even a dial costs more than a few machine cores.