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Good evening, everyone!
This column discusses mechanical watches from an engineer's perspective. For the 12th installment, we will be talking about:
"About Watch Crystals - Their Respective Characteristics"
We'll be discussing this topic.
The glass (crystal) of a wristwatch refers to a component designed to "prevent adverse effects caused by wind," and it is an important part that protects the dial.
There are three main types of glass used in wristwatches:
"Plastic (acrylic resin)"
"Mineral glass"
"Sapphire crystal"
This time, we will explain the differences between these types of glass, so please read until the end.
"Plastic Crystal"
Plastic crystals are made of "plastic (acrylic resin)" and are sometimes called plexiglass. Among the three materials, it is the lightest and softest, and its biggest characteristic is that it is easy to process.
Plastic crystals were mainly used in watches from the 1940s to the 1970s. While they are still used in复刻 models and watches like the [OMEGA] 'Speedmaster', they are generally associated with vintage watches.
Furthermore, plastic crystals used in watches are very often "domed" in shape, giving them a completely different feel from sapphire crystal glass, which is generally flat, resulting in a warmer design.
The reason plastic crystals are domed is to increase their "strength." Unlike glass, plastic crystals are susceptible to impact and are prone to cracking. Therefore, by curving the shape, the likelihood of cracking is reduced.
However, because the crystal is raised, it's common for scratches to appear without you noticing. Combined with the inherent softness of the material, scratches are a concern. This makes it a particularly suitable material for those who can appreciate minor scratches and the aging of a watch as part of its character.
Another point about plastic crystals is that scratches can be polished out. This is not possible with sapphire crystal or mineral glass. You can ask a watch repair shop to polish it, or it's not too difficult to polish it yourself using a polish designed for plastic.
Incidentally, if you want to remove scratches from sapphire crystal or mineral glass, polishing is difficult, so the glass usually needs to be replaced, incurring a high cost.
Mineral Glass
Mineral glass refers to common glass and is a material widely used in familiar items such as window panes and drinking glasses. A characteristic is that it has a slight color, in contrast to the colorless and transparent sapphire crystal.
Mineral glass is harder than plastic but not as hard as sapphire crystal, with a Mohs hardness rating of at most "6," and as low as "3." (※Mohs hardness: A scale for mineral hardness, expressed as an integer from 1 (softest) to 10 (hardest).)
Therefore, it has drawbacks such as being prone to cracking and chipping. However, because it is a low-cost and easy-to-process material, it is also widely used in many watches.
Mineral glass is mainly used in casual watches priced under 50,000 yen, and is rarely seen in luxury watches.
Although it was adopted in some [Tudor] models from the 1990s, if glass replacement is necessary during manufacturer maintenance, it seems they switch to sapphire crystal.
Sapphire Crystal
Sapphire crystal is the most commonly used glass in luxury watches. It has the same hardness as natural sapphire gemstones and is resistant to scratches.
With a top-class Mohs hardness rating of "9," this material won't scratch easily even with a light rub, nor will it be easily damaged by scratching with hard metal. Its ability to maintain its original brilliance indefinitely makes it an incredibly popular glass.
However, due to its superior properties, the cost is also higher, and despite being an ideal material for watch glass, it is primarily adopted in luxury watches.
It doesn't scratch like plastic, nor is it as prone to cracking as mineral glass. Moreover, its high purity ensures excellent visibility. Beyond its practicality, its aesthetic beauty as glass is outstanding, quickly making it a prominent choice for luxury watch crystals.
Although it is difficult to process due to its hardness, it is an exciting material for the future, with brands like [Hublot] using sapphire crystal for their cases, and unique variations such as the green sapphire crystal in [Rolex]'s 'Milgauss' emerging.
Summary
How was it?
There are three types of watch crystals: plastic (acrylic resin), mineral glass, and sapphire crystal, each with its own appealing characteristics.
Just as you choose a watch based on its design or functionality—such as sapphire crystal for durability or plastic for a retro feel—it might be interesting to choose a watch based on its crystal type.
I hope this article provided valuable information and sparked some interest in luxury watches! Also, please feel free to ask any questions directly; we will be happy to answer them thoroughly. We look forward to your visit and inquiries.
Stay tuned for the next installment! See you then!
