The watch glass also known as ice shows , is the transparent part which allows both to read the signs and protects its constituent parts. Not to be confused with watch glass : laboratory equipment .
Initially, made of glass and then of composite materials , either plastic , for example Plexiglass , or glass-based, developments in the techniques of processing synthetic sapphire ( monocrystalline corundum ) allowed its use for the majority of Quality watches, combining impact resistance with scratch resistance, as well as a quasi non-existent coefficient of expansion , ensuring high stability and durability.
The sapphire crystal
Sapphire glass was introduced in the late 1980s for leading brands and gradually became widespread in the 1990s for all high-end brands.
The sapphire crystal is obtained from a “core” of synthetic sapphire monocrystal made from an alumina melt using an oxyhydrogen torch at over 2000 ° C. The process was invented by the Frenchman Auguste Victor Louis Verneuil in 1902. It is still current and serves the cause of watchmaking both for glasses, but also for the rubies used in calibres.
Sapphire glasses were used very early by watchmakers and especially for female models whose ice were not very large. Originally, synthetic sapphire “stalactites” were not large and their cutting required relatively small sizes.
The watch glass is contained in a notch of ice , housing hollowed either in the bezel of the case or directly in its caseband . In waterproof watches , an ice seal ensures the necessary elasticity .
Today, it takes between 7 and 22 operations to cut a watch sapphire glass and shape it. This explains its high price, especially since this material can be worked only by itself or diamond because of its high hardness (scale of 9 on the scale of Mohs )
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