« The crystals watchmakers rely on »
It protects against the elements, offers a view inside, and is something of a science in its own right: the watch crystal. There are three types of this indispensable component, and every enthusiast should know their strengths and weaknesses. We explain what sets acrylic, mineral, and sapphire crystal apart, look at their history, and share useful care tips.
Affordable but prone to scratches: acrylic crystal
To understand how acrylic crystals found their way into the watch industry, we need to look at the situation in the 1920s: until then, timepieces were mostly fitted with ordinary window glass, whose tendency to shatter and scratch became a problem with the increasingly popular wristwatch – which, unlike the pocket watch, had no protective lid –. The solution arrived in the early 1930s, when the first watch crystals were milled from plastic sheets and given different names depending on the manufacturer or material: acrylic glass, Plexiglas, and Hesalite are still common terms today, and all of them refer to plastic.
By the 1950s at the latest, plastic had taken over the entire industry, dominating the luxury segment as well. Today, acrylic crystals are mostly found in entry-level watches, though they occasionally appear in high-end models such as the Omega Speedmaster Moonwatch Professional. Their advantages are obvious: not only are they shatterproof and inexpensive to produce, they also produce very little glare. On the downside, their scratch resistance is extremely low at 40 HV (Vickers hardness), so ordinary everyday wear leaves marks even without any mishaps. On the other hand, these scratches are very easy to remove from the crystal with a polishing paste such as PolyWatch. Arguably the greatest advantage of Plexiglas, however, is its domed shape, which lends it a certain warmth and vintage appeal compared to other crystals.
The next step: hardened mineral crystal
By the early 1970s, Hesalite was drawing growing criticism for its poor scratch resistance, which led to the development and widespread adoption of mineral crystal. This material is very similar to window glass; untreated, it measures around 400 HV, making it roughly ten times as scratch-resistant as its plastic counterparts.
The key trick with this type of crystal lies in a chemical treatment that more than doubles the material's hardness through heating and vapor deposition – typically reaching 900 HV.
Compared to modern sapphire crystals, that figure isn't impressive, but at the time it marked a considerable leap over plastic. Yet the greater scratch resistance comes at a price: mineral crystal is less shatter-resistant, reflects incoming light more strongly than plastic, and scratches cannot be removed. If it gets damaged – and given its low hardness compared to sapphire, that likelihood shouldn't be underestimated – the only option is replacement. Fortunately, that's fairly inexpensive; an average mineral watch crystal costs around €30.
All in all, we're not big fans of this crystal, since it offers neither the visual appeal and high shatter resistance of plastic nor the exemplary scratch resistance of sapphire. Only on entry-level models in the low three-figure price range are these drawbacks acceptable.
Tough and scratch-resistant: modern sapphire crystal
Contrary to what the name suggests, the "queen of watch crystals" isn't made from the gemstone of the same name; it's produced from aluminum oxide using a complex chemical process (the so-called Verneuil process). In other words, it's a synthetic material that has little to do with glass. When it comes to hardness, though, sapphire crystal lives up to its name: a hefty 2,000 HV guarantees excellent protection against scratches, with only the 10,000 HV of pure diamond to fear. This sets a benchmark that has been standard practice in the luxury segment since the early 1990s at the latest and has been steadily making its way into ever more affordable price ranges.
Tissot, for example, uses the prestigious material even in its base models priced at around €300. As scratch-resistant and tough as they may be – sapphire crystals, too, have their critics. As with mineral crystal, any damage means the crystal has to be replaced (which, as you'd expect, is more expensive here), and its shatter resistance falls short of Plexiglas. On top of that, it has the strongest glare of all three crystal types, which can make reading the time in direct sunlight noticeably harder. Even so, well over 90% of the luxury watches in the Uhrinstinkt collection are fitted with sapphire crystal.
Upgrades: anti-reflective coating and domed crystals
In practice, however, that last issue hardly matters, because most sapphire crystals are anti-reflective coated on one or both sides. Using vacuum evaporation, the crystal is coated with a thin layer of a different, ideally color-neutral material to eliminate reflections. The underlying physical principle is known as "destructive interference": different portions of the light are reflected at the surfaces of the crystal and the anti-reflective layer, and these cancel each other out as the waves overlap.
Reflections can even be eliminated entirely underwater, as the HYDRO technology from Sinn Spezialuhren demonstrates: the dial, hands, and movement are immersed in a special oil whose refractive index is matched to that of the sapphire crystal. Another popular upgrade to the synthetic material concerns its shape: manufacturers such as Breitling use a domed rather than a flat crystal in many of their models.
Just as shatter-resistant as flat sapphire, it combines the hardness of the modern material with the three-dimensional charm of classic Hesalite. Complex grinding processes make production considerably more difficult, but the effort pays off: domed, coated sapphire crystal finally offers a solution with no weaknesses from either an aesthetic or a practical standpoint. Queen of watch crystals? Yes, but only in this form.
