Watch glass comes in a number of forms, each one offering particular properties and as always the type of glass used will depend on the primary function of the watch as well as the price point.
Poly(methyl methacrylate) (PMMA), also known as acrylic glass but you may have heard this referred to as Plexiglas, Acrylite, Lucite, or Perspex. This is a transparent thermoplastic often used in sheet form for its lightweight and shatter-resistant properties making it a good alternative to traditional glass.
Toughened or tempered glass is traditional glass that has undergone thermal treatment to change its chemical makeup to give it increased strength and safety glass features during the process called tempering. This process puts the outer surfaces into compression and the inner surfaces into tension, meaning if the glass was ever to be put under enough stresses to cause a fracture, the glass will crumble into small granular chunks rather than splintering into sharp shards.
Sapphire and scratch-free sapphire
Sapphire is a crystal form of aluminum oxide with a rating of 9 on the Mohs scale of hardness, 10 being diamond. Its properties of hardness and clarity make it a valuable and widely used material in place of glass as it offers great strength and resistance to scratching.
Mineral glass is standard tempered glass but made from silica. It’s more scratch resistant than Plexiglass which is a thermoplastic as we saw with Acrylic Glass which is also used for watch crystals. This is often used to produce scratch resistant crystals by combining mineral glass coated with synthetic sapphire.
An antireflective or anti-reflection (AR) coating is applied to the surface of lenses or other glass surfaces to reduce reflection or improve the efficiency of the material where less light is lost. It is used in complex systems like telescopes to reduce reflections and improve image contrast by elimination of stray light. In other applications such as watch glass, the primary benefit is the elimination of the reflection itself to give a clear view of the dial.
Flame Fusion Crystal
The Verneuil process or flame fusion, is the process of manufacturing synthetic gemstones that was developed back in 1902 by the French chemist Auguste Verneuil and is still widely used today. The process involves melting a powdered material using an oxyhydrogen flame and crystallizing the melted droplets to grow synthetic gemstones, ruby and sapphire varieties of corundum, together with diamond simulants rutile and strontium titanate on an industrial level.
Borosilicate Glass Capsules
Let’s start at the beginning, if you take a common drinking glass that is made of sand, sodium carbonate, and limestone, then add boric oxide you end up with borosilicate glass. So far so good?
Now your drinking glass has a high resistance to temperature changes, able to withstand rapid heating and cooling. For example, borosilicate glass is used in laboratory equipment, stage lights and oven dishes, and of course watch glass.
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