WATCH CRYSTALS

The earliest wristwatches often did not have crystals. A fully metallic case would open directly to the dial and hands (or one hand). As soon as watch crystals appeared, they were made of mineral glass. Back crystals, on the other hand, emerged between the late 18th and early 19th centuries, well before the invention of winding and time-setting mechanisms. Like clock dials, back crystals were then pierced with two holes to allow the passage of a key for winding and setting the watch. The evolution of tools and technologies constantly improved the quality of these crystals (beveling, quality of polishing).

It wasn’t until the 20th century that plastic and sapphire crystals, as well as surface treatments (anti-reflective coatings, hydrophobic treatments, etc.), became commonplace.

The usual material for producing plastic crystals is polymethyl methacrylate (PMMA), an acrylic material (plexiglass). This material is appreciated for its strength, excellent optical properties, and UV resistance.

Two production methods are used depending on the type of crystal to be manufactured. To make flat crystals, slices are cut from a cylinder of material. The resulting discs are then diamond-cut (polished) to achieve the final crystal thickness and complete transparency.

To create volumetric crystals (box crystals), the method involves injecting the material. The cost of the mould does not encourage small-scale production, but this method offers the advantage of producing crystals with complex shapes, limited only by their optical properties. Polishing steps, more or less complex depending on the crystal’s shape, are carried out after injection. Numerous surface treatments have emerged and continue to develop, addressing issues such as reflections, scratches, UV protection, and more.

While plastic crystals are the most susceptible to scratches, they offer the advantage of being impact-resistant and easily restorable through polishing.

The glass was the sole material used in watch crystal production until the middle of the 20th century. The rise of plastics in the 1960s and 1970s and then sapphire led to glass becoming a rarity. Glass can produce flat, domed, and shaped crystals. In all cases, the raw material is machined before being polished and receiving any necessary surface treatments (anti-reflective coatings, scratch-resistant coatings, UV protection, etc.). Like sapphire, glass can be metallized for decorative or indicative purposes.

The first process for synthetic corundum manufacturing dates back to the early 20th century (the Verneuil process, also called flame fusion) and is still in use today. It involves obtaining corundum “boules” (known as balls) by fusing aluminium oxide particles. The resulting single crystal possesses characteristics and properties identical to natural corundums (sapphires and rubies), without their “flaws” such as inclusions, for example.

This innovation led to the replacement of natural rubies in watch movements with synthetic stones in the 1930s, as they were easier to mass-produce, more durable, and ultimately less expensive.

The Verneuil method’s evolution and reliability quickly enabled the production of watch crystals. However, the material obtained through the Verneuil process loses its homogeneity beyond 40mm in diameter, limiting the size of the largest components that can be produced using this method to that dimension.

Another process, EFG (Edge-defined Film-fed Growth), has allowed for very large crystals to be obtained since the 1970s, generating a base material in the form of large plates. Sapphire crystals are then cut, ground, and finally polished from the raw material. Corundum (9 on the Mohs scale) is the second hardest material after diamond, so only diamond can be used to machine or polish it.