Sapphire crystal?! - which glass is the best for a watch?
Today we will talk about a component of a wristwatch that most people don't really notice. It is transparent and hardly noticeable in the overall appearance of a wristwatch. Of course, we are talking about the glass, which is built into the case and isolates the dial from the outside world.
A good watch glass must have other qualities than, for example, the material or the movement. Of course, it should be robust and scratch-resistant, but at the same time it should also allow good readability, for example, by being anti-reflective.
But which glasses are actually available for wristwatches? Everything you need to know about this topic follows in this article.
There are different materials for watch glasses. There is the plastic glass, which is robust, but easily scratched, mineral glass, which is scratch-resistant, but can still break and the sapphire glass, which is very hard and extremely scratch-resistant, but must be anti-reflective to guarantee good readability.
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The origins of watch glass
Once upon a time, when the watch world was taking its first steps, untempered mineral glass was used. This was colorless glass that was first blown into spheres. These spheres were then separated into spherical sections called calottes and then cut into watch cases. Therefore, the watch glasses always had a slight curvature.
Some time later, an advanced method was used by heating the domed glasses again in muffle furnaces and then shaping them into the correct shape with a wooden punch.
The constant development of mineral glass eventually led to the discovery of hardened mineral glass in the 1970s. This glass is extremely scratch-resistant and was considered an extraordinary innovation at the time.
In the 1930s, there was also an increased use of plastic glasses. Initially, these were cut from plastic panels. Some years later, progress allowed the glasses to be easily cast from injection molding and granules. This process allows plastic to be cheaply and lightly produced. At the same time, it is also shatterproof.
In turn, the 1980s saw the discovery of sapphire crystal. This is an even harder and scratch-resistant glass, which is made from an artificial sapphire.
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These glasses are available for wristwatches
However, as mentioned above, wristwatches have quite different glasses, all of which have their own advantages and disadvantages.
Synthetic glass
Plastic glass is also known as Plexiglass or Acrylic glass, as well as Hesalite glass. This glass is primarily characterized by its lightness and robustness. It is also cheap and easy to produce. Moreover, plastic glass allows particularly good readability because it hardly reflects any light, preventing any glare.
The only problem that comes with plastic glass is that it seems a bit cheap compared to glass, as it is made of plastic. Additionally, plastic glass is not particularly scratch-resistant due to its soft composition, and thus it quickly shows signs of wear.
In terms of price, this glass is also the most affordable option. To use a benchmark, it can be noted that one square meter of Plexiglas costs about 25 euros. If you consider how small a watch is compared to a square meter, you can imagine how cheap plastic glass for watches is in production.
Mineral glass
Mineral glass is not just window glass. Mineral glass is a kind of crystal glass. Because it consists of silicon oxide. Its chemical composition is related to that of rock crystal or smoky quartz. If other oxides are added to the glass, it becomes more resistant.
So, untreated mineral glass is about 20 times harder than watch glass made from plastic. Moreover, mineral glass in watches is often extra hardened, reducing shard formation should the glass break. The most significant risk with mineral glass is shard formation in the event of breakage.
The prices for mineral glass are many times higher than those for plastic glass. Of course, the prices depend on how well it is processed, how thick it is, etc.. Basically, however, you can say that you have to pay at least around 10 euros for a mineral crystal. At least, that's the data we can take from the marketplaces accessible to us.
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Sapphire glass
Sapphire glass is the king discipline among watch glasses, as it is indeed as hard as a sapphire. Only diamonds and a few materials such as tungsten carbide can damage sapphire glass. However, that does not mean that one should try to do so. Because one must not forget that the scratch resistance refers to normal use and not active force.
Sapphire is the third hardest transparent material in the world after diamond and moissanite. Technically, sapphire glass isn't even glass, but a synthetically produced crystal made of aluminium oxide.
Furthermore, you should absolutely make sure that the sapphire glass is anti-reflective. This is the only disadvantage of sapphire glass. Due to the unique structure of sapphire glass, light refracts heavily on it, which means that the dial can be very hard to read in direct light.
Often, both sides of the sapphire glass are anti-reflective. The anti-reflective coating on both sides can limit the resistance of the glass. This is also the reason why many manufacturers only use anti-reflective coating on the back of the sapphire crystal.
Excursus - anti-reflective coating for sapphire glass
Sapphire glass is best treated with an anti-reflective coating to allow optimal readability, thus offsetting the only disadvantage of sapphire glass. The anti-reflective coating is a very thin layer that allows a clear view of the dial. Such anti-reflective coatings are also used for glasses or camera lenses, but also for sapphire glass in watches.
This effect is actually of a physical nature. This is because the incident light is reflected several times within this coating, which means that the light rays then cancel each other out and are no longer reflected back to the viewer. This effect is particularly strong when applied to both the top and bottom of the glass.
Curved sapphire glass is even more scratch-resistant due to its shape and also aesthetically pleasing. With sapphire glass, it is virtually impossible for it to scratch under normal use. If you do notice wear and tear after some time, these are usually not scratches, but natural abrasion, meaning that softer metals have left traces on the glass. You should be able to remove this abrasion easily with an ordinary eraser.
Sapphire glass is considerably more expensive due to its complex manufacturing process. When it was first discovered, it was initially only used in a few wristwatches due to its high price.
Meanwhile, sapphire crystal is extremely established and indispensable in luxury watches. Nevertheless, it is about three to four times more expensive than ordinary mineral glass. This becomes clear when you look at the prices on Ebay, for example. On Ebay, a mineral crystal for a watch costs about 10 euros, while the price of a sapphire crystal starts at about 35 euros. Whereby it must be said that we have only used this as a standard of comparison. There is certainly much more expensive sapphire glass for watches.
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Omega: Hesalite glass vs. sapphire glass
You would think that sapphire crystal is the undisputed leader in watch crystal. This is true for most people, but many prefer Hesalite glass, especially for the Omega brand, and this has its own reasons.
Hesalite glass is simply made from the plastic polymethyl methacrylate. This plastic is still used in industry and crafts under the name acrylic. In essence, Hesalite glass is simply another term for acrylic or plastic glass. The brand Omega, in particular, coined the term Hesalite glass with its Speedmaster.
Actually, it is obvious that sapphire crystal is the best glass for wristwatches. However, this view is different for the Omega Speedmaster Moonwatch. .
With its chronograph, the Speedmaster was actually intended for the racetrack.
Contrary to all expectations, it outperformed other manufacturers in a NASA test for astronaut watches. Thus, it first traveled to space on the Mecury 7 Sigma mission. The Speedmaster was also a faithful companion during the moon landing on July 21, 1969. It became a permanent part of the astronaut's equipment, and each crew member received a Speedmaster, earning it the legendary status of "Moonwatch".
When sapphire glass became popular in the 1980s, NASA preferred to stick with Hesalite glass, fearing that in case of breakage, dangerous shards could jeopardize the astronauts' mission in space.
For example, it is often common for die-hard Omega fans to prefer the hesalite crystal over the sapphire crystal. Even if they will never fly into space with their Moonwatch, they prefer it because of its iconic history.
Moreover, the hesalite crystal does have its advantages. Namely, it is extremely robust and the scratches can be easily removed, and also a replacement of the complete glass costs only about 230 euros directly from Omega. At the same time, many want a scratched glass, because it is precisely this distinctive character of the watch draws.
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Conclusion
In the end, it can be said that all watch glasses have their advantages and disadvantages.
Plastic glass is sturdy and cheap, but prone to scratches.
Mineral glass much more shatterproof than normal glass, but causes problems when it breaks, and it needs to be anti-reflective.
Sapphire glass isn't even real glass, it's actually crystal. It is extremely hard, shatterproof and scratch-resistant. However, it first has to be anti-reflective so that it can be read well.
However, to draw a conclusion, it can be said that sapphire glass is the best watch glass for everyday use. With it, you get almost no scratches, it's extremely hard to break, and if properly anti-reflective, you can read the displays under the glass beautifully.
Scource cover image: Rolex
About the author
Alexander Weinberger
For me, the most fascinating thing about watches is the interplay between precise craftsmanship and artistic development. In a watch movement, hundreds of small parts have to be put together precisely so that they form a large whole.
