When comparing watches to purchase you may have come across the term ‘sapphire glass’ and noticed that watches with a window made out of the sapphire crystal are slightly more expensive than watches that don’t. Now you must be wondering, what is sapphire glass? And is sapphire glass worth the money? These are questions we will be answering as we are going to explain to you what sapphire glass is and how it compares to other types of glass and glass-like materials.
A wristwatch’s glass has to be able to take a punch. You can imagine that while wearing a watch, this is the area that gets hit most often in everyday life. You’ll hit the glass on a door you were trying to open, or on the side of the table you pass. The strength of the glass is of great importance to the quality of a good wristwatch.
There are roughly four types of watch window materials. Let’s divide them up in the following categories, each with their own advantages and disadvantages.
The Types of Watch Glass Crystals
1. Plastic or Acrylic Glass
Back in the days these babies used to be the most commonly used variant or watch glass. It is certainly the least expensive to use. Plastic is tough, hard to break and very light. The biggest con is that it scratches extremely easily and is a good sign of a cheaply made watch. Children’s watches often sport plastic or acrylic glass, since these watch glasses are quite hard to break or shatter. Scratches on plastic can easily be removed using a polishing machine. In short, plastic is not a very high quality watchmaking material.
2. Mineral Glass
You’ll find mineral glass on a lot of mid-range watches as they are slightly more expensive than plastic watch glasses, but more scratch resistant. These days almost all mineral glass is what they call “tempered”, which means it’s treated with intense heat to make the glass slightly more scratch resistant. This also reduces the chance that the glass will splinter as the mineral glass gets damaged, scratched or chipped. If that happens, mineral glass cannot be polished and to be replaced. If the glass cracks deep enough for the glass to splinter on the inside, glass may even damage the dial or get stuck in the movement.
2. (Synthetic) Sapphire Crystal
Sapphire glass is probably the most sought after of all the watch glasses, but also the most expensive. Sapphire glass is found on higher-end watches. It is constructed of either genuine or synthetic sapphire crystal. The synthetic version of sapphire crystal is not actually glass. It is made at high temperatures by crystallizing aluminum oxide. Both synthetic sapphire glass and real sapphire glass are classified as one of the hardest materials in the world. Sapphire crystal rates a full 9 on the Moh scale.
The Moh scale is used to measure the relative mineral hardness of materials, with diamond being a 10 on the Moh scale as one of the hardest materials. Of all the watch glass types, sapphire glass is the least likely to break, chip or get scratched. Diamond and man-made materials such as silicon carbide with a higher rating on the Moh scale can scratch sapphire glass, but most other materials can’t, making this glass the perfect fit for a high quality watch.
At pOrtahl we have chosen to incorporate sapphire glass into all of our watches. We do this for two reasons. First of all, a watch that you are going to be wearing every day should be able to take a beating. We’re certain you will be bumping your watch against all types of surfaces as you go about your day. It’s human nature and it’s something we don’t want you to worry about. Secondly, the longevity of a watch is one of our key design elements when it comes to manufacturing high quality watches. We aim to create watches that you will be able to pass down from generation to generation, therefore we should use the very best materials.
How hard is sapphire crystal and how does it compare?
So, sapphire crystal is known for its incredible hardness, rated a full 9 on the Mohs scale, with diamond being the hardest material and rated a 10 on the Mohs scale. With a rating of 9, sapphire crystal is only one step behind diamond, making it incredibly durable and resistant to scratches.
In comparison, plastic and acrylic glass have a Mohs rating of around 3, while mineral glass has a rating of around 5 to 7. This means that sapphire crystal is significantly harder and more scratch-resistant than other commonly used watch glass materials.
While sapphire crystal is not completely indestructible, it is highly unlikely to break, chip or get scratched with normal use, making it an ideal choice for those who want a high-quality, long-lasting watch.
The con of sapphire glass
There is a downside to sapphire glass that should be addressed. Sapphire crystal is reflective, because it has a higher refraction index than for instance mineral crystals. This means light bounces off it’s surface possibly making it hard to read the time through untreated sapphire crystal on a sunny day.
Therefore sapphire crystal needs to be treated with an anti-reflective coating. This will lower the refraction index of the crystal significantly, allowing you to read time from any angle. Scratches could appear on treated sapphire glass, but note that these scratches are most likely scratches in the coating, not in the sapphire crystal. This could be prevented by applying the AR coating on the inside of the glass as opposed to on the outside.
pOrtahl watches are treated with five layers of AR coating on the inside of the sapphire glass, allowing for optimal transparency.
The best type of crystal for your watch glassIf you are an active person or if you know yourself to be a little clumsy at times, sapphire crystal is definitely the way to go. It is virtually impossible to scratch as its hardness is rated a 9 on the Moh scale – only one point behind diamond at 10. The downside is that sapphire crystal is reflective, therefore if you do opt to get it – make sure it is treated with an anti-reflective coating. If you know yourself to be careful with your watch and you would rather save some money, go for the mineral glass.
Thank you for reading our blog on sapphire glass. If you would like to find out more about the pOrtahl project and our watches, click here.