We hear lots of people talk about the crystal, whether it is domed, double domed or just flat. But then there is the added complication of the material the crystal is made from. The three main materials are sapphire crystal, mineral crystal and acrylic. It is a lot of terminology so I thought we should have a look at each of these in detail and understand the advantages of each and how they impact the enjoyment of your watch.
Generally in the watch industry the glass on the front or the back of the watch is referred to as the “crystal” but do be careful as the term crystal does have a specific technical meaning that defines the atomic structure of the solid. A crystalline structure is one that shows regular geometric patterns at an atomic level. We will try and bring a bit of clarity to this whole situation!
It’s Not Just Glass
So let’s start with the easy one. Acrylic. This is plastic. We do not like plastics at all for a huge number of reasons, a subject which I should perhaps write a whole blog post on. For a watch owner the biggest issue with acrylic watch glass is that it will scratch incredibly easily in comparison to glass or sapphire crystal. This will reduce your enjoyment of your watch as you have to look past the inevitable scratches to read the time – not to mention pollute the earth.
That said, there are those who are keen on the acrylic solution as scratches can be polished out easily or, if the damage is sufficient, the acrylic crystal can be replaced easily and inexpensively. I have also seen it said that an acrylic crystal does not shatter – it will only split or rip. The key point for me is that it will damage with a lot less force than a well made glass or sapphire crystal and given how polluting plastic is to our environment, if there is a good alternative we should always choose the least harmful to our planet.
It Looks Like Glass – Most Of The Time It Is Glass
Next in line is a mineral glass crystal. Mineral glass is made from silica (SiO2) and referred to as quartz when it is in its crystalline form. This is exactly the same chemical compound and crystalline structure that is used in a quartz movement. Silica melts at over 1,700oC and then cooled very slowly to create the crystalline structure. or Quartz. The glass we see every day is not the crystalline form of silica, it is an amorphous structure, which means it has no regularly defined crystal structure at the atomic level.
The reason for this is to reduce the melting point of the glass. This is done by the addition of what are called fluxes. These are introduced in specific quantities in order to reduce the melting point of the glass. Usually salts of Sodium, Calcium or Magnesium are added and can reduce the melting point of the glass to around 800oC. Crucially though, the addition of these fluxes also reduces the hardness of the resulting glass.
Usually referred to as mineral glass, the “mineral” in the name refers to the various fluxes introduced to the silica to reduce the melting point of the glass. The “glass” refers to the silica which is the primary constituent in all glass. A typical mineral glass has a Mohs hardness of approximately 6.5, where diamond is the hardest with Mohs hardness of 10.
Hard, But Not Quite Hard Enough
The result is a transparent glass but it is not particularly scratch resistant. It is sufficiently scratch resistant to be used in a window which is unlikely to be subject to rough treatment but when it is on your wrist it will pick up scratches remarkably quickly. Sometimes mineral glass is coated with an anti scratch coating which is referred to as a sapphire coating. This is sometimes referred to as “Sapphire Glass”. Do be careful not to get this mixed up with sapphire crystal, as sapphire crystal is a completely different material, as we will see below.
To address the lack of hardness of mineral glass, a proprietary glass called K1 glass has been developed. It is a treated glass that has higher scratch resistance than regular mineral glass but still is prone to scratches. The glass is also prone to breakage if it falls or hits a hard object. Once a piece of mineral glass is scratched or breaks there is not much you can do to remedy it other than replace the glass.
Sapphire crystal
Sapphire crystal is both made from the main chemical compound of sapphire, aluminum oxide (Al2O3) and is used in its crystal structure, hence the term sapphire crystal. To produce the sapphire crystal pure aluminum oxide is heated until molten (over 1,700oC) and then cooled in a very controlled fashion so that the aluminum oxide forms into its crystalline structure – sapphire.
Once the crystal is grown it is then processed through a number of steps, cleaving, grinding and polishing to achieve the finished piece. The interesting point of a sapphire crystal on a watch is that it is a single crystal so there will be specific atomic planes along which refraction will occur and highlights will be seen in the crystal. It is for this reason that it is important to also apply an anti-reflective coating to a sapphire crystal if it is going to be used as a viewing aperture.
As a material to be used on a watch as the transparent window there is nothing better than sapphire crystal, it is exceptionally hard with a Mohs hardness of 9 so it can only really be scratched by diamond. This combined with its excellent optical properties means it really makes the watch face pop!
But What About The Geometry
We touched on the principal geometries used for watches above, these are flat, domed and double domed. A flat crystal is the most common and is generally used for cheaper watches (a simple cross section is shown to the left). A flat crystal is easily manufactured from a single large sheet of the material of choice and each one cut from it. This is the process used in most mass produced watches.
A Single Domed Crystal
Single domed crystals are generally flat on the base (closest to the watch face) and domed on top. Depending on how the exact geometry is implemented it can distort the view of the watch face. This can be used to good effect to magnify the view and can be useful in dive watches. There are limits though and the dome curvature needs to be very low.
There are examples of fashion watches that play with the distortion a high curvature domed crystal introduces. For normal use it is wise to remember that the greater the curvature of the dome the more limited the viewing angle. Generally single domed crystals are restricted to specialist applications because of the distortion they introduce.
Double Domed
There are a number of real benefits of a double domed crystal and that is why they are particularly popular on many high end and luxury watches.
Just for clarity though, let’s take a little step back, and look at the cross section of the double domed crystal. The key to the double domed crystal is that it is the same thickness along the whole arc of its curvature. This results in a very wide viewing angle with minimal distortion.
Double domed sapphire crystals are immediately appealing aesthetically because of their enhanced translucency which is only enhanced with a liberal application of anti reflective coating.
More Than Just The Optics
There are not only optical benefits to the double dome, there are practical benefits for the designer too. The softness a dome can bring to the design can help complete an overall design, especially the design line of the case in profile. A further benefit for the watch designer is that a double domed crystal creates additional space within the watch between the dial and the underside of the crystal. This can help create a slimmer case as the bezel or the side of the case can be reduced in height and additional space gained by the domed crystal.
A more practical benefit of double domed crystals is that the dome shape is structurally stronger than an ordinary flat surface, especially when higher pressures are exerted, for example underwater. This is a second reason why double domed crystal is prevalent in dive watches. A double domed crystal will handle the depths of oceanic diving with a thinner cross section than a flat crystal ensuring the watch is not only more appealing to look at but also lighter on the wrist.
The Best Underwater
The double domed shape also helps reduce the visible distortion of water while diving. Being able to read the watch face at odd angles is an important feature for divers, professional and amateur alike. The performance out of the water is also superior for the double domed crystal, especially from odd angles, although for all watches they become unreadable when the angle of view approaches 90 degrees.
But if we boil it all down the most important aspect of a delicately domed crystal is the way the light dances across the surface and plays with the eyes of an appreciating watch owner, yet allows for amazing pictures (please see above) that every watch collector and watch maker craves from their favorite timepieces.
But There Is A Price To Pay
All this does not come for free, the cost of manufacture of a double dome crystal is significantly more than a flat mineral glass, predominantly because of the high cost of manufacture. Making a double-domed crystal is exponentially more difficult as both the interior and exterior surface need to be exactly parallel across the whole dome or distortions will be very evident. Each sapphire crystal is cut from a block with the dome ground out with diamond tools and all the ground out material is just wastage. The price for a sapphire crystal is driven by its water-resistance – the higher the depth of resistance the thicker the crystal. The degree of dome can also impact the cost – the more domed the crystal the higher the cost.
Most entry level watch brands shy away from using a double domed crystal for this reason. But those who are more knowledgeable understand the value of the investment and for their favorite horological masterpieces, all of the added benefits far outweigh the slight increase in cost.
Want to Know More?
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