Watch glass comparison – Pros and cons

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: plastic glasses or acryl, mineral or sapphire glasses. They mainly differ in their hardness.

Plastic glasses largely disappeared

Acryl glasses, widespread with watches in the 50’s, are nowadays hardly used anymore as they are in comparison to sapphire crystals 50 times softer and are thus more sensitive to scratches. Sapphire glasses are the most scratch-resistant glasses there are, whereas the name can be misleading as they are no glass. They are rather a synthetic product which is produced in a complex Verneuil process from ultrapure aluminum oxide. The name rather refers to their hardness, which is comparable to a sapphire.

Sapphire glasses have higher scratch-resistance, mineral glasses less light refraction

By contrast, mineral glass is a crystal glass that is structurally similar to quartz. Being extra hardened through special production processes (e.g. heating or vaporization), most modern mineral crystals have a more resistant surface than a few years ago. The glasses thus achieve a hardness of about 900 Vickers, but still remains more susceptible to scratching than sapphire crystal. This reaches a hardness of around 2000 Vickers or 9 Mohs. It can therefore only be scratched by very few materials, as for example a diamond with a hardness of 10 Mohs. A disadvantage of sapphire crystal is the comparatively strong light refraction, which can cause reflections on the dial. In order to avoid this effect, many luxury watches, like the ones from Circula, have a sapphire glass with a double anti-reflex coating. That is, a chemical layer is vaporized on the inside and outside of the glass, so that the light is refracted less.

Domed glasses are much more expensive

The production of domed watch glasses is again significantly more complex and expensive compared to flat glasses. Especially in sapphire crystal, a strong curvature is still very expensive at the moment, since the manufacturing process is relatively new and costly. Despite the considerable additional costs for the Circula Classic and Heritage, we use a domed, double anti-reflex coated sapphire crystal as a scratch-proof tribute to the design of the great watch classics.

How to recognize sapphire crystal at home?

The water drop method allows relative safe detection. To do this, 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.

Text: Cornelius Huber