Usually when buying a wristwatch in a watchmaking, the choice of the type of glass that is mounted on the case should be almost as important as the choice of model or style for example, although many overlook this fundamental aspect. And they evaluate only other aspects. The main purpose of a glass is clearly to protect the dial of a watch from damage caused by ingress of water and dust. But above all from scratches or accidental blows.
The glasses of the clocks are certainly a very complicated subject. In the construction of clocks, different types of glass are used, and each of them has advantages and disadvantages, but there are some general properties that apply to all glass, and that somehow unite them in the watchmaking world.
1) Plexiglas glass (hexalite or acrylic) is known to be less expensive than other glass on the market, but has some great qualities that protect your watch. This type of glass is not easily shattered and is resistant to daily domestic shocks. In addition, if the face of your watch accidentally gets scratched, the Plexiglas glass can be easily polished, making your dial brilliant. As a result, do not worry if you are a bit clumsy because, even if it is not scratch-resistant, Plexiglas glass provides your watch with important protection. It is well known that brands such as Rolex and Omega use Plexiglas glass because of its specific properties. When the Plexiglas glass breaks, it does not break apart and that is why this material has been used for most of the Rolex sports line. Omega even chose this material for his Speedmaster watch that went to the moon, to prevent glass splinters from floating in space if the watch broke.
The advantages of Plexiglas glass are the economy, the lightness, the ease of processing (building a curved glass is much more expensive), the elasticity (very appreciated property in the diving clocks) that allows you to take advantage of the pressure of the water, creating a sort of “seal” effect to the advantage of watertight and also high transparency.
The disadvantages, however, are that it is more “tender” compared to mineral glass, and therefore it is easily scratched; however, removing lines and very modest scratches is not as problematic as what happens in mineral glass. Of course, the operation is possible only for very few times: the polishing process takes material and thins the thickness of the glass, and this is also a function of how deep the scales / lines are. It deforms when exposed to heat. Aging, or crystallization, which manifests itself with micro-fractures in the structure, visible even to the naked eye and yellowing in sunlight.
2) Mineral glass, which is seven times harder than Plexiglas, is found in most mid-to-high-range designer watches. These glasses have been heated or chemically treated to make them more robust and resist scratches. Even if they are not the best anti-scratch products on the market, mineral glass to break up must be kept in extreme conditions. Despite this, apart from the price, there are no big differences between the mineral glass and the most expensive glass on the market. Even if your mineral glass dial has taken a bad blow, a replacement should be quite affordable. No wonder many designers choose this material to protect their watches, as they offer scratch resistance and repair at affordable prices. Mineral glasses can be flat, domed or optical.
The advantages of mineral glass are economy, ease of production and good transparency.
3) Sapphire glass is the absolute best and it is easy to understand why. Sapphire glasses are three times more resistant than mineral glass and, in the scale that measures the hardness of the material, are just behind the diamonds. Since sapphire glass is found in most luxury watches, it is not surprising that this is the most expensive glass on the market, but we can say that in this case “the game is worth the candle”. The sapphire crystal is scratch-resistant and resistant to cracks and breakages. In fact, the only material capable of scratching it would be another sapphire or a diamond. The term “sapphire” could be misleading: sapphire glasses are not formed from a natural sapphire, but rather from a synthetic compound with the same properties, which is what gives the glass its strength properties. Although the sapphire promises to protect, a replacement would cost more than a new mineral glass, but with this level of hardness the breakages are truly unlikely.
The advantages of sapphire glass is that it represents the hardest among the glasses used for watches. Moreover, compared to mineral glass and Plexiglas glass, it is completely transparent, maintains its shine and being very resistant, it can be thinner, thus contributing to the subtlety of the watch itself.
Clearly there are glasses of very particular shapes (curved, ect.) That are not normally found in the supply, and that must be made specifically, beyond the type of material used. Be it mineral, Plexiglas or Sapphire.
The workshop of watchmaker’s technician in watch store Capri Kronos by Mario Ruocco is able to replace or reconstruct every kind of glass for your wristwatch!