Hesalite/Plexi Glass vs. Sapphire Glass and others
By Georg Platzöder, 27 January 2023
Until the 1920s, normal window glass was used for wristwatches and watches in general, leading to a need for a lockable metal cover. This remains to this day, although they now have a different type of glass.
At the end of the 1920s, a transparent substitute for glass, better known as acrylic, was created from the plastic polymethyl methacrylate (PMMA for short). To this day, acrylic is used in industry, handicrafts, medicine and much more. It is highly valued for its various properties, but we'll get into that later. Since then, by the way, acrylic has also been used as a watch glass under the names such as Hesalite or Plexiglass. The term "Hesalite glass" was then strongly coined by Omega in the 1960s with the development of the Moonwatch and is actually used almost exclusively in the watch industry.
Watches with Hesalite crystal have played a role in various moments of human history. Whether it was the Mount Everest expedition in 1953, during which Sir Edmund Hillary relied on a Rolex Oyster Perpetual with an acrylic crystal, or when an Omega Speedmaster Moonwatch with the reference number ST 105.012 became the first watch on the moon in 1969 on Buzz Aldrin's wrist. Hesalite glass has repeatedly proven its durability. Only for diving are watches with Hesalite glass rather unusual. With the right material thickness and construction, it can work; but, due to its general composition, it has a significantly higher water permeability than, for example, sapphire glass. Nevertheless, watches with Hesalite glass can be worn in everyday life without any problems!
What makes Hesalite glass?
Due to its significantly softer surface structure, compared to sapphire or mineral glass, Hesalite glass is considerably more susceptible to scratches and scuffs, but it is also more flexible, impact-resistant and overall significantly more resilient than its competitors. Due to its comparatively soft structure, Hesalite glass cannot break or splinter, but at most crack! And if the surface does get scratched, minor surface damage can be easily polished out.
OMEGA SEAMASTER 300 234.30.41.21.03.0
With proper care, Hesalite glass can last as long as any other glass and, if it does need to be replaced, it costs considerably less than sapphire glass, for example. Furthermore, the replacement of a Hesalite glass can be carried out by almost any watchmaker. Another characteristic of Hesalite glass is that, due to its material properties, it does not need to be anti-reflective like other types of glass. Therefore, the dial of a watch with Hesalite glass always appears clear and distinct! However, Hesalite glass always has a certain vintage look, which can be rather charming, but is a matter of taste and, to be honest, does not always fit in with the overall look of the watch.
What about Sapphire and Mineral glass?
Sapphire crystal is what the name promises: sapphire! This variant of the crystalline form of aluminium oxide is called corundum and occurs in nature in a number of colours, all of which, with one exception, are called sapphire. This exception, by the way, is red sapphires, which are called rubies instead.However, the corundum used for watch glass, just like the rubies used for watch movements, is a synthetic sapphire, i.e. a sapphire produced in the laboratory under artificial conditions. Its material hardness is 10 times stronger than that of mineral glass and 4 times stronger than that of acrylic! This makes corundum the third hardest transparent material in the world, after diamond and Moissanite!
This makes sapphire crystal an excellent choice for wristwatches, as its surface structure makes it extremely scratch-resistant and shatterproof. However, care should be taken to buy a watch with anti-reflective sapphire crystal! If you do not use anti-reflective sapphire crystal, the chemical structure of the corundum causes light to refract extremely strongly in the crystal, making it more difficult to read. Some sapphire crystals come with a characteristic curvature, which not only improves readability but also makes the structure even more stable. However, the curvature is not a must for an anti-reflective coating, but merely a small tip from our side regarding the durability of your new timepiece!
Sapphire glass is extremely robust due to the tight crystalline latticework of its surface structure, so that it can hardly be scratched in everyday life. Unless you are trying to do so with extreme pressure or diamonds. If your glass is still somehow "scratched", first try to remove these scratches with a simple eraser! These "scratches" may simply be aluminium residues that can be easily removed with the eraser.
Mineral glass, on the other hand, is silicon oxide, or simple crystal glass, which is closely related to rock crystal or smoky quartz due to its chemical structure SiO². Normally, mineral glass is very simple, normal flat glass, which is also used for window panes, for example. With an untreated surface, it has a hardness of about 400 HV (Vickers Hardness).
However, by adding various other oxides, the surface of mineral glass can be additionally chemically hardened! This not only gives the surface a hardness of about 900 HV, but also significantly reduces the formation of splinters in the event of breakage!
An overview of the properties of Hesalite and Sapphire glass compared to various other types of glass
Which glass you personally prefer for your watches is something you will have to decide for yourself. Either way, all three types of glass have their advantages and disadvantages. So that you can see everything at a glance, we have compared the properties of Hesalite, sapphire and mineral glass in the following table.
| Features | Hesalite | glass | Mineral glass |
|---|---|---|---|
| Scratch-resistant? | Not very resistant (approx. 40 HV) | Very resistant (approx. 2000 HV) | Relatively resistant (approx. 900 HV) |
| Unbreakable? | Very shatterproof | Relatively shatterproof | Shatterproof |
| Reflection? | Very low | High (except with anti-reflection coating) | Low |
| Maintenance? (possible removal of scratches)strong> | Polishable | Not possible (replacement only) | Not possible (replacement only) |
| Cost? | Very cheap | Relatively expensive | Relatively cheap |
Watches with Hesalite glass
- The classic piece with hesalite glass, the Omega Speedmaster Professional Moonwatch (Ref. 310.30.42.50.01.001), is also as much of a classic in the luxury watch market as the latest new edition! Designed to assist the pilots of the first successful moon mission in 1969, it not only features a precise hand-wound calibre that could withstand the weightlessness of space, but also a shatterproof acrylic or Hesalite glass. In the event of damage as a result of unforeseen pressure differences, for example, the glass would crack at most and not shatter, therefore not exposing the astronauts to an increased risk of sharp, flying glass splinters.
- The 35 mm Longines Conquest Heritage (Ref. L16114522) also comes with a built-in Hesalite crystal as a throwback to various vintage watches from the manufacturer Longines. This model combines classic watchmaking craftsmanship with a trendy retro design and unbeatable value for money. With a black dial, an automatic movement, a date window at 12 o'clock, rose gold hands and indices, and a solid stainless steel case, this model can be found second-hand for less than £1,000 and is therefore one of the cheapest ways to wear a luxury watch with Plexi-glass.
- Alternatively, you can also grab the top dog among the luxury watches and opt for one of the rare Rolex watches with Plexiglas! The Rolex Oyster Precision (Ref. 6427) dates from the 1970s and still comes with built-in plexiglass, for just over £3,000, you get a watch that is interesting not only for those who want to save money, but also for collectors, since it was also a Rolex with a Plexi-glass that sat on Sir Edmund Hillary's wrist during the first ascent of Mount Everest.
- In reality, mineral glass is rarely used in wristwatches, especially in the luxury segment. Nevertheless, manufacturers such as Bruno Söhnle Glashütte from Saxon Switzerland use mineral glass in models such as the Rondomat III Stahl Automatik (Ref. 17-12149-741) to make the case back transparent and to allow a view of the complex workings of the automatic movement. A good example of the use of mineral glass in the rather low-priced luxury range.
- A more modern sister of the Moonwatch, the Omega Speedmaster 57 Chronograph (Ref. 331.10.42.51.01.002), focuses purely visually on vintage charm, but does not use the Hesalite glass found in vintage watches for its watch glass, but rather the comparatively modern sapphire glass. In addition to a water resistance of 100 metres and a precise automatic movement, this gives the Speedmaster, which is more suitable for everyday use, a certain scratch resistance that watches with Plexiglass are less capable of. This at a price of just under £5,300.
- Rolex models have now also stopped using Plexi-glass. Descendants of the Rolex Datejust Oysterquartz, which was worn by Reinhold Messner during his ascent of Mount Everest with Peter Haberlein, are now equipped entirely with sapphire crystal. Regardless of whether it is the Datejust (Ref. 116233), the current 36mm Rolex Explorer (Ref. 124270) or the Explorer II (Ref. 226570) including a second time zone or 24-hour display.
Watches with Sapphire or Mineral glass
Which glass is better?
Which glass is ultimately "better" is probably a question of personal taste. Or is it? What is certain is that watches with Hesalite glass are absolutely suitable for everyday use and robust enough to withstand almost anything. Thanks to some of its properties, such as its resistance to breakage or the extremely low light reflection, acrylic glass even has something over sapphire or mineral glass in some cases! It is also much cheaper to buy and maintain. But is that enough to hold its own against the excellent, anti-reflective and almost completely scratch-resistant sapphire crystal? Make up your own mind!