====== Watch glass ======
**Watch glasses** have to be robust against scratching and rupture.

Scratch resistance is measured on the [[Mohs|Mohs hardness scale]] in steps of 1-10. The following types of glass are used with watches 

  * [[Plexiglass|Plexi / acrylic glass]] \\ (polymethyl methacrylate (PMMA)); 3-4 Mohs, little scratch resistance, but unbreakable. Scratches can be polished with special acrylic polishing paste. From this material extremely arched ([[domed]]) watch glasses can be produced.

  * [[Mineral glass]] \\ 5 Mohs. Mineral with hardened surface 6-7 Mohs (harder than [[stainless steel]]).

  * [[Sapphire crystal]] \\  is an artificially produced crystal 9 Mohs (close to [[diamond]], extremely scratch resistant and break-resistant).


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