Circula dive watch from the 1970s with a scratched hesalit glass and the new edition with sapphire

The glass or often crystal protects the interior of the watch, thus dial, hands and movement, against damage through dirt or moisture. In general, watch glasses can be classified in three groups: Sapphire glass, mineral glass and hesalite glass (also: plastic, acrylic, Perspex). They differ in particular in their hardness and the reflection behavior. But what are the differences, advantages and disadvantages of the different watch glass types? This and other topics are covered in this article.

Hesalit glass – mainly found in vintage watches

Hesalite glasses, which were widespread a few decades ago (e.g. in watches of the 50s), are only used very rarely today – but in some cases watch manufacturers still use domed hesalite glass to underline the retro character. Hesalite glass is about 50 times softer than sapphire glass and therefore very sensitive to scratching, but on the other hand it is very elastic and therefore very shatterproof or impact-resistant. Superficial scratches can be polished out by yourself with a polishing paste (e.g. Polywatch) and a little patience.

Mineral glass – mostly hardened

Chemically speaking, mineral glass comes very close to window glass – mineral glass is therefore naturally less shatterproof than hesalite glass. Today, most mineral glasses that are used in watches are made more resistant on their surface by special manufacturing processes (e.g. by heating or steaming). In product descriptions you often read about hardened mineral glass. The coating gives the glass a hardness of around 900 HV (Vickers hardness), but still remains much more susceptible to scratches than sapphire glass.

Sapphire crystal – the ultimate in scratch resistance

The so-called sapphire glass is the most resistant to scratches, although the name is somewhat misleading, since sapphire glass is not actually glass. Rather, it is a synthetic product, which is manufactured from high-purity synthetic aluminum oxide using the complex, so-called Verneuil process.

Sapphire glass has a hardness of around 2000 HV – twice as much as mineral glass. Sapphire glass can therefore only be scratched by a few materials, including real diamond with 4,500 to 10,000 HV. For comparison: stainless steel comes to about 220 HV.

A disadvantage of sapphire glass is the comparatively strong refraction of light, which can lead to reflections that can limit the readability of a watch. To avoid this, sapphire glasses from high-quality watches, as with Circula, are anti-reflective coated on both sides. More specifically, in Circula watches, the inside and outside of the sapphire glass are anti-reflective coated by vacuum evaporation in an oven. This method eliminates 99% of the reflexes perceived by the eye and greatly improves readability.

Scratches in mineral glass or sapphire glass cannot be removed without machines. The most sensible thing with these types of glass is mostly an exchange.

Domed (curved) sapphire crystal – complex to produce

Circula AquaSport II Automatic with domed and anti-reflective sapphire glass.

Flat sapphire glass is comparatively easy to produce, since it can be “cut” in slices directly with the help of diamond blades. In comparison, the production of curved or domed watch glasses is significantly more expensive. The reason: In order to be able to produce domed sapphire glass, the front and the back of the glass have to be processed by grinding.

For the Circula SuperSport, AquaSport and ProTrail, despite these significant additional costs, we install a domed sapphire crystal with anti-reflective coating – as a scratch-resistant tribute to the design of the great watch classics.

Easily recognize sapphire glass yourself

By the way, you can easily check whether your favorite watch comes with sapphire glass using the so-called water drop method: dab a drop of water on the surface of the watch glass and observe its behavior. If the drop remains spherical, it is sapphire crystal. If it spreads and flows, it is mineral glass.

Sapphire crystal vs. Mineral glass vs. Hesalite glass – differences in comparison

In summary, the advantages and disadvantages of the different watch glass types

Sapphire crystal Mineral glass Hesalite glass
Scratch resistance Very resistant
(ca. 2000 HV)
Relatively resistant
(ca. 900 HV)
Not very resistant
(ca. 40 HV)
Shatterproof Relatively shatterproof Shatterproof Very shatterproof
Reflection Strong reflection
(except when anti-reflective coated)
Little reflection Very little reflection
Care (removing of scratches) Not possible (only exchange) Not possible (only exchange) Polishable with Polywatch polishing paste
Cost Relatively expensive Relatively cheap Very cheap

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⌀ 40 mm