====== Sapphire crystal ======
[[Watch glass]]

===== Strength and hardness =====
Glass made ​​of artificially produced [[sapphire]] crystal (Al<sub>2</sub>O<sub>3</sub> = monocrystalline aluminum oxide). With a hardness of 9 on the [[Mohs scale]] it has a considerably higher scratch resistance and a resistance to fracture than [[mineral glass]]. With these properties, it is only by exceeded the [[diamond]] (Mohs hardness of 10).

===== Anti-reflective treatment =====
Today sapphire crystal is the material of choice as watch glass of high quality watches. In most cases, these glasses are [[anti-glare treatment|anti-glare treated]]. There is unilateral or bilateral [[anti-glare treatment|anti-reflective coating]].

===== Forms =====
There are flat, arched and [[domed]] sapphire crystals. The difference in production methods is sometimes considerable and is reflected in correspondingly different costs 

  * **Flat** or plane sapphire crystals are relatively easy to produce, since they can be sliced ​​directly from the cylinder.

  * Simple curved (**[[domed]]**) sapphire glasses are expensive to produce because they are milled from a block and must be grinded into the rounded form.

  * For certain types of watches also highly curved [[acrylic glass]] can be suitable. Because this can be easily scratched, there is also an alternative **high-domed sapphire crystal**. It differs from simple curved by the higher number of radii of curvature and the greater thickness of the blank to be worked. To achieve the different radii of curvature, as a rule, several grinding tools are needed. Also, the polishing causes considerable additional expense.


{{tag>Terms Watch_materials}}