Watch crystals are rarely referred to in the pages of Europa Star, except perhaps for the occasional mention in a watch caption, but the art of crafting sapphire crystal is a fascinating affair, as our editor Sophie Furley discovered during her visit to Stettler Sapphire in Lyss, Switzerland.
Windows on watches
Watch crystals were first placed on pocket watches to protect the dial and underlying movement from water, dust and knocks. They have been an integral part of watchmaking ever since.
Watch glass or crystal comes in all shapes and forms—circles, squares, rectangles, ovals, curved, domed and often with numerous facets. Some have built in magnifiers, a blue or green tinge, anti-scratch coatings or anti-reflective treatments that make them almost invisible to the naked eye. Perhaps because of its unobtrusive nature, the watch crystal isn’t a component that commands much interest, that is, until you know how it is made.
There are three main materials that are used as watch windows: Plexiglass, mineral glass and sapphire crystal. Diving and sports watches are sometimes fitted with Plexiglass, an extremely hardwearing and inexpensive material that is also used in the fabrication of motorcycle helmet visors and aquariums. Although Plexiglass is almost indestructible, the downside for watches is that it scratches relatively easily and reflects a great deal of light.
Glass, or mineral glass as it is often called, is mostly found in mid-range watches; it also reflects the light and scratches easily, but the advantage is that the scratches can be polished away.
For high-end watches, most watch brands prefer to use sapphire crystal, a material that is so hard that it will only scratch if it comes into contact with a diamond or another crystal. But it isn’t only the durability of the surface that attracts watchmakers. Anyone familiar with fine timepieces will have noticed how transparent watch crystals are. Sometimes, under certain light, we even question whether the watchmaker has forgotten to fit the crystal at all, and our fingers become possessed as they move towards the dial for a little poke to reassure ourselves that everything is in order! But where does sapphire crystal come from?
In its natural form, sapphire crystal was formed under the Earth’s crust millions of years ago. Crystals appear when the liquid in the Earth consolidates and the temperature drops, or when water passes through clefts under the ground, dispensing minerals in between the rocks. However, it is also possible to make synthetic crystal, which is quicker to produce, has far fewer impurities and doesn’t require any mining!
A trio of processes
For industrial crystal production, there are three main techniques – the Verneuil, EFG and Kyropoulos processes. The Verneuil method, a flame fusion growth technique, was invented by the French chemist Auguste Verneuil in 1902. The crystal starts life as an aluminum oxide, Al2O3, that is grown by placing a sapphire crystal seed into a kiln. The aluminium oxide is fed into the kiln with a hydrogen and oxygen flame (at temperatures exceeding 2,000 degrees centigrade) and the material gradually settles on the seed, forming a carrot-shaped crystal rod. These rods are perfect for producing round watch crystals.
For square or rectangular crystals, the EFG method, developed by the Polish scientist Jan Czochralski in 1916, is preferred as the crystals grow in a rectangular form. A tiny sapphire seed crystal is placed into a tungsten mould containing molten aluminium oxide, and then slowly withdrawn upward resulting in flat crystal plates.
For much larger crystals, the Kyropoulos process (invented by Spyro Kyropoulos in 1926 at the Physical Institute in Gottingen, Germany) is mostly used as the crystals are grown in large melts where the crystal forms in a large, single block.
EFG process and Crystal polishing
Stettler Sapphire has been supplying the watch industry since 1881 when Hans Stettler set up the family business making semi-finished jewels for watch movements. In the 1950s the company started developing watch crystals and this has since become its core business. Five generations later, under the direction of Martin Stettler, watch crystals now represent around 80 per cent of the company’s production, with the remaining 20 per cent of its output dedicated to applications such as photo, laser, medical, optic and sensor technologies. The company’s ultramodern manufacturing processes can produce anything from a handful of crystals for a limited series of wristwatches to the industrial production for many of Switzerland’s traditional brands.
Stettler Sapphire grows a small percentage of its own crystals at its second facility in Mauritius, but it also has partnerships with suppliers of raw crystal all over the world. Stettler’s expertise lies in the transformation of these raw crystals into some of the most incredibly shaped watch crystals on the market today.
Raw crystal to watch crystal
When the crystal is ready to be machined, the raw material is glued onto a support with an adhesive putty. It is then loaded into a machine that slices through it to create rough looking discs or squares. At this stage the crystals are opaque (it is only when they are polished that they become transparent). The next stage is to load them into a large milling machine that will grind away at the crystals until they are the required thickness. This is a long operation as crystal registers nine out of ten on the Mohs scale (the second hardest material after diamonds, which register a maximum 10 Mohs). All the operations needed to cut, grind, bevel or polish the crystals require diamond tools or diamond paste, which is one of the reasons why sapphire crystals are so much more expensive than other materials. The next step is polishing, which is a huge part of the operation, and Stettler has a myriad of machines (many of which they have designed themselves) to polish every single facet of a watch crystal, and when that is not possible, they have a team of hand polishers who will meticulously polish where the machines cannot go. Not all crystals have flat surfaces; there are convex crystals, box crystals, crystals with apertures, loupes and so forth, that all need to be polished to perfection. “If you polish away too much material on one side of a crystal, it will be evident to the wearer once the crystal is mounted, even if it is a hundredth of a millimetre off,” explains Beat Allemann, Stettler’s Head of Sales and Marketing.
The process of cutting, grinding and polishing sounds simple in words, but there are a number of challenges when working with this special material. One of the main problems is that there is always a risk of impurities in the form of bubbles or lines. The only time these can be detected is after the crystal is fully polished. So it is during the final quality control process when any blemishes will become disappointingly apparent. When some crystals cost as much as $15,000 to produce and hundreds of hours of production, an impurity can be a major set back. “Working with crystal is the biggest challenge as it is a living material,” shares Martin Stettler. “One crystal differs from the next, one supplier differs from another; it is impossible to manage 100 per cent.”
But it isn’t only impurities that can cause problems; the only two materials that can scratch crystal are other crystals and diamonds, and both are present along the production line. It is essential to be as meticulous as possible when handling the crystals. The Swiss watch industry is well known for its high quality standards and there is no room for compromise.
Being a family business also presents its own trials, as many retailers will be able to sympathise with. “To act as an independent, family business is tough,” notes Stettler “There are only a few big groups in the watch industry and their market power is much greater than ours.”
But let’s return to the crystal’s production as the journey doesn’t end here. After the crystals pass quality control, they will then go to other partners who specialise in the application of different treatments and coatings. The most common is the anti-reflective coating, which is a film that is applied to the crystal to reduce the reflection of light and improve the view of the dial. Developed by the Ukrainian physicist Alexander Smakula in 1935 while he was working for the Carl Zeiss optics company in Germany, the technology was a German military secret during World War II.
A compound of aluminium oxide (Al2O3) magnesium fluoride (MgF2) and hafnium dioxide (HfO2) is heated to 1,800 degrees Fahrenheit in a vacuum. The compound melts and then evaporates into a gas and disperses throughout the chamber. When the gas comes into contact with the crystal it condenses on the surface. The process is basically the same as if you stand over a pan of boiling water with your glasses on; the water evaporates and condenses on the lenses. The only difference is that the steam disappears at room temperature, where the anti-reflective coating fixes at room temperature. The same process is used for adding the green or blue tint that is popular on some sapphire crystals, or an additional anti-scratch coating. Although the objective of the watch crystal is to provide an unobstructed view on the dial, most brands like to be able to see the crystal just slightly, this is why the coloured tinges are so popular. They don’t impede the visibility, but they are noticeable at certain angles.
The crystal trend
The use of sapphire crystal is not only reserved for protecting a dial. More and more watch companies are experimenting with crystal for other parts of the watch too. Century uses a selection of hand-faceted sapphire crystal cases and bezels in both its men’s and women’s lines. Louis Vuitton’s Tambour Mystérieuse features a mysterious movement in which the hands, mounted on crystal discs, seem to float unattached to its watch movement. Ulysse Nardin uses a clear crystal baseplate and blue crystal bridges in its Royal Blue Tourbillon to allow a transparent view of its movement; Corum also uses grey-tinged crystal plates and bridges in a recent version of its Golden Tourbillon Panoramique. Breitling, among others, has coated the crystals of its timepieces with anti-reflective coatings on both sides to optimize clarity. And MB&F has just launched its new Horological Machine No. 4, the Thunderbolt, whose production takes the watch crystal to a whole new level. In an industry where the workmanship, details and finishing of a timepiece are, for the most part, on full show, the beauty of the sapphire crystal is in its invisibility, its discreetness and its service to the rest of the timepiece. It is a noble component that deserves to be noticed and appreciated, even if it isn’t supposed to be seen!
Timepieces featured in this article are examples of watch crystals only and are not necessarily products of Stettler Sapphire.
Source: Europa Star August - September 2010 Magazine Issue