The clear cover of a watch's face is usually referred to as the crystal - the crystal is the eye of the watch and through it one might see its soul! This element is designed to protect the dial and inner movement of the watch and needs to resist scratching and rupture.

The first portable watches like the ones manufactured by Peter Henlein at beginning of XVI-century were not provided of crystal. Those early watches were, instead, using a metal element or cage to protect the hands and the watch movement. The crystal has been introduced only at a later stage and sometimes still in combination with an external metal cage (e.g. Waltham WWI wristwatch).

Since then many different materials have been explored to protect the watch phase. Sapphire crystal has been introduced around 1930 but broadly adopted by luxury timepieces only over the last 30 years (driven by Rolex).  The main reason for the slow adoption was the manufacturing complexity and the cost associated. 

 

In this chapter we cover the different types of watch crystals as well as the manufacturing process of sapphire crystal.

 

TYPE OF MATERIALS

 

Crystals can be made of different materials but they all need to be transparent and protect the watch from external agents (dirt, water, scratches…). Nowadays only few materials dominate the market: plexiglass (often called plastic) or hesalite (acrylic glass), mineral glass and sapphire crystal as well as crystals based on a combination of the previous ones. Plastic and mineral glasses are the most affordable and flexible and therefore are used in a wide range of watches. In fact, depending on the cost of the watch, or the specific usage occasion the choice of the crystal might be quite different —influencing the price and value of the watch.

 

Sapphire Crystal

Sapphire crystal is the lead choice for premium and luxury watches. It is made of an extremely durable natural or most commonly synthetic material. The synthetic form of sapphire has exactly the same chemical composition of the natural one but it is much more affordable though still expensive compared to other materials. Sapphire crystal is the best and it pushes up the price of high end watches due to its intrinsic value and partly also to the manufacturing process.

Sapphire crystals are appreciated essentially for two reasons: resistance to scratch and transparency properties. Sapphire, part of the corundum family, is extremely strong and scratch resistant. Sapphire is, in fact, the third hardest mineral with a 9 on Mohs scale (measures of relative hardness of all materials) after diamond (10) and moissanite (9.25). This means that it is very scratch resistant though not scratchproof. Diamond and moissanite, being harder, can actually scratch sapphire crystal and derivates of those materials might sometimes be used in home/office surfaces or in appliances and devices. Hence despite this crystal being very scratch resistant it is always good to pay attention and try to prevent scratches.

Notably the harder is the material the more fragile it is. This is valid also for sapphire and while this material resist very well to scratches, it might still chip or shatter if impacted (very rare though). If this happens, microscopic particles of sapphire might get into the movement and damage it. For this reason the manufacturers of most classic watches tend to use sapphire while military watch manufacturers tend to use plexiglass / perspex or hardened mineral glass.

The sapphire crystal is also highly transparent and it lets light with wavelengths between 150nm (UV) and 5500nm (IR) go through dramatically improving watch face readability compared to other materials. At the same time it is important to prevent glare. For this reason sapphire crystals might be coated with an anti-reflective film that reduces glare and further improves legibility of the dial – the best crystal receive a double coating treatment (internal and external - double AR coating).

From a cost and value perspective, not all sapphire crystals are created equal and the cost of this element might be quite different depending on whether the sapphire is natural or synthetic, its thickness, the shape (e.g. flat vs domed) and other features (e.g. date magnify).

 

Mineral Glass

Mineral glass is generally found on mid-priced watches. It is made by using heat or chemicals to treat ordinary glass in order to make it more scratch resistant (i.e. better than acrylic glass). Unfortunately the treatment also has the effect of making the glass more prone to shatter. Overall the properties of the mineral glass are in between the sapphire crystal and the plastic/acrylic one. Interestingly also the cost of those crystals is in between which make them the most common choice of watch manufacturers.

 

Plastic / Acrylic Glass

Those crystals are the least expensive and for this reason they are the most commonly found on lower-priced wristwatches. Plastic crystals are often referred to as plexiglas and acrylic (or hesalite) and were commonly used on watches until the late 1970’s. Plastic glass is tough, flexible and transparent. It is not likely to shatter but it may scratch easily – though, with a special treatment, the scratches can be removed. Children’s' watches often have an acrylic glass as they are cheap but especially because they are unlikely to break (shatter resistant). For the very same reason those crystal are popular also with some military manufacturers such as CWC watches. Another advantage of plastic/ acrylic crystals is that they can be molded into elaborate shapes that sapphire and mineral crystals cannot.

 

Sandwich Crystal

In order to get the best from the classic materials, some companies have developed multi-layer crystals sometimes called sandwich crystals. Those are generally made of a combination of mineral and sapphire glass. In particular the idea is about combining the scratch resistance of the sapphire with the brittleness of the mineral glass. For this reason as well as for manufacturing reason a layer of sapphire is let grow on top of a layer of mineral glass. The most common sandwich crystals are “sapphlex” used by Seiko. This crystal is made of two layers: “hardlex” (especially hardened mineral glass) covered by sapphire. Unfortunately some of those crystals might suffer from delamination, a process where mineral and sapphire crystal layers separated.

There are also other techniques that can give the material properties similar to the ones of sandwiches crystal like the method used by Invicta which seems to be a variation of the “Flame Fusion” technique. The resulting material is not a laminated glass, it is in fact mineral glass and sapphire crystal fused together into a single material that cannot delaminate.

 

CRYSTAL SHAPES

Most watch crystals have a simple circular and flat shape. However, some and generally the mid-high end of the range have more sophisticated shapes. In another post we have discussed about the different watch shapes and also underlined the fact that generally the crystal tends to fit that shape though not always. Depending on the shape and the specific material the manufacturing process might become very complex and it can dramatically drive up the cost of the crystal.

From a two-dimensional perspective the most common glasses are round, oval, tonneau and square or rectangles with or without cut corners. However, there is also a third dimension that complicates the shape of the crystals. They could, in fact, be flat, raised (like a cake), domed, totally curved or faceted. The dome shaped crystals are sometimes referred to as "bombé", "chevé", and "boule". The word "dome" comes from latin "domus" meaning house and it was then used to define a rounded vault in certain structures like a "duomo" (Italian Cathedral Church) - the specific structure of a duomo is a surface curved both inside and outside. Some vendors, however tend to differentiate between single domed (surface curved on one side and flat on the other) and double domed or fully curved (surface curved both sides). 

The flat crystals tend to be the cheaper ones as they might easily be cut from the same cylinder of material while the others require more complex manufacturing techniques.

Another element which might complicate the shape and increase the cost of the crystal is the cyclop or the magnifying glass generally positioned above the date indicator on the watches. The first watches with Cyclops use to have it positioned on top of the crystal but some of the new models have it inside – this prevent scratches and make it more subtle from an aesthetic point of view.

 

SAPPHIRE CRYSTAL MANUFACTURE

Sapphire started being adopted more broadly as material for wristwatch glasses only with the introduction of synthetic crystals. The synthetic sapphire can be manufactured (or grown) in different ways, however, the most popular methods can be categorized into two main types: i) production from melting and then crystallizing and production from a solution that is then let crystallize. Verneuil's fusion (published 1902) and Czochralski's pulled crystal (created 1916 – used on sapphire 1960) are the most commonly used melting techniques, while the flux growth and hydrothermal growth are the most popular versions of solution processes.

Production from melt material is generally the less expensive hence used to manufacture sapphire for the low end of the market. Conversely the solution process (flux growth in particular) is generally used for the fine jewelry sapphire.

The principle of the Verneuil’s process (known also as flame fusion) is relatively simple and still today is the most commonly used for manufacturing sapphire crystal for wristwatches. A fine powder of aluminum oxides (alumina) is slowly dispensed through a mesh into a pipe and flows through it also thanks to a stream of oxygen. The powder will then end up in a bigger pipe with a counter-current of hydrogen and a flame at the bottom. The flame melts the fine powder into small droplets which fall into a rod and start crystallizing to form a column of crystallized sapphire. Eventually, the long cylindrical piece of sapphire is then sliced (with diamond-tipped cutters) and polished to become a watch crystal. In order to get colored sapphire other oxides might be added into the process – for example chromium oxide would lead to the production of red ruby (also used in wristwatch movements), or ferric oxide and titanium to blue sapphire.

The Czochralski process is a manufacturing method used to develop crystals for different applications: semiconductors, metals, salts and synthetic gemstones. One of the key advantages of this process is the possibility to growth large cylindrical ingots that can then be sliced into discs. As for the Verneuil’s process the main material is melted but instead of letting it deposit drop by drop for the crystallization, a “seed” is placed into the melted material to initiate the crystallization process. In particular a small crystal (so called seed) is placed on top of the melted material and then slowly raised. The melted material covers the seed and as it gets pulled away together with the seed it crystallizes making the seed bigger. This process continues as the seed gets further raised and will lead to the development of a large cylinder of sapphire which can then be sliced into discs. There exists a series of variation of this process like the gradient-freeze (1964) and the Kyropoulos method in which instead of pulling out the seed the melted material is cooled in different ways.

The hydrothermal process is based on a principle very similar to the one used in the Czochralski process. In both cases a seed is used to initiate the crystallization of the synthetic sapphire. The main difference is that in the Czochralski method the seed is in contact with melted material at very high temperature while in the hydrothermal process it is possible to use a solution at much lower temperature (less than 500°C vs about 2000°C). In order to make this possible, however, it is necessary to leverage high pressure reactors.

The flux-growth process is the method that requires the longest lead time (up to a year) and investment (expensive equipments) and that generally leads to the best results. For this reason this tends to be used only for the production of high end synthetic jewels. This method is based on the principle that some specific melted material (so called flux) can then dissolve a second material. During the gradual dissolution synthetic crystals start forming sometimes initiated by the use of “seeds”. The key benefit is that this method allows a crystallization process below the melting point of the aluminum oxide and the formation of better crystals.    

Once the sapphire is produced with any of the above methods, it still needs to be worked in order to give the final shape and finish needed for being mounted on a wristwatch. There are essentially two operations that need to be accomplished: cut and polish. The flat crystals are the easiest to manufacture and generally both cut and polishing are done by machines. On the other hand the more asymmetrical and domed shapes are the most difficult and might require treatments by hand.

By now the crystal is ready to be used in the wristwatch. However, in certain cases the crystals get some more treatments: anti-reflective layer coating, metallization, silk screen printing, laser engraving or diamond layer coating. Among those the anti-reflective treatment is the most common (either single or double-AR coating).

sapphire crystal manufacturing process

 

Summarizing most of the high-end watches tend to use the sapphire crystal with some exceptions like the army/sport watches which might benefit more from mineral glass. Plastic/acrylic glass can still be found on some vintage watches but especially in the low-end and kids watches. Notably not all sapphire crystals are the same and shape, dimensions and manufacture process might drive price up or down. Todd & Marlon choice is definitely for sapphire crystal and this is what has been used in the very first collection. 

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