Thought Experiment: How Transparent Can You Make A Watch?

Mystery minute repeater, anybody?

The idea of transparency in watch and clockmaking has existed for almost as long as there have been watches at all. Portable clocks started to become small enough to be functionally if not nominally watches around the beginning of the 16th century, and by the 17th century watchmakers were using rock crystal for watch cases, although not for actual watch mechanisms. Practically speaking, a really transparent watch is difficult – I’d go so far as to say, next to impossible – to make; the American Waltham Watch Company made pocket watches with transparent rock crystal plates in the late 19th century, but as far as I know, no one has ever made a watch in which the moving parts of the movement were entirely transparent. The closest anyone has ever come that I can recall was, of all people/corporations, Cartier, which produced escape wheels and levers from what they called “carbon crystal” (a type of synthetic diamond) in the ID One and ID Two concept watches but the transparency for mobile elements didn’t go any further back in the going train than the escape wheel.

If you want to make a transparent watch there are historically two basic options. The first is skeletonized watches and indeed, those can be almost unbelievably transparent; the amount of metal that can be removed and still leave you with a functioning watch is incredible. These however are obviously still made with conventional watch materials (with some caveats; Patek’s openworked caliber 17-170 has plates and bridges in 14k gold).

Image, Phillips; Patek ref. 899

The other option, and this is one found in clockmaking as well, is the category of mechanical timepieces known as mystery watches and clocks. The best known are of course from Cartier, and they create the illusion that the hands of the timepiece are moving with no physical connection to any driving mechanism. The basic trick is simple – the hands are connected to transparent glass (or synthetic sapphire) disks, which are driven by gear teeth on their edges, hidden under the dial or somewhere in the case. Over the years Cartier has done some ingenious things with the basic principle, including a mystery tourbillon, in which the tourbillon cage is on a transparent disk, driven by teeth hidden on its periphery, allowing the escape wheel pinion to gear to a fixed fourth wheel in the usual way.

All this naturally raises the question of just how far you can push transparency in watch design, especially since skeletonized and to some extent, mystery watches and clocks are pretty transparent as it is. The only way to take transparency further would be to start using transparent materials for, not just plates, bridges, and cases, but also for the moving components of a watch as well. Ideally, for a watch to be completely transparent, you would need to find a way to use transparent or at least translucent materials for the gears, including the lever and escape wheel; as well the balance, balance spring, gear pivots and pinions, jeweled bearings, the mainspring and mainspring barrel, ratchet wheel and crown wheel, and the keyless works.

The Girard Perregaux Quasar Light Tourbillon up top is a good example of how far you can take transparency in modern watchmaking – not only the case, but also the three movement bridges are in synthetic sapphire, and the movement plate has been openworked, so it’s sort of a combination of the two currently available strategies for making a mechanical watch as transparent as possible. The only company as far as I can recall to use transparent materials for actual mobile elements of a movement is, as mentioned, Cartier, which used synthetic diamond for the lever and escape wheel of the ID One and Two concept watches, and Zerodur ceramic glass for the balance spring.

Could you use synthetic diamond for other moving elements – other gears in the gear train, for instance? You could try but the problem with diamond is that while hard, it’s also brittle and the further back you go in the gear train, the slower the gears are turning and the higher the side load is on balance pivots and on gear teeth and under a constant load, gear teeth and pinion leaves might tend to crack or shear off. Synthetic diamond makes sense for the lever and escape wheel as the hardness and smoothness of the material means (or should mean) that you can dispense with lubrication, and the mechanical forces involved are low, but for gears and pinions under heavier load, an alternative material would have to be found. The logical second choice for transparent gears and pinions would be synthetic sapphire, which while not as hard as diamond, is more resistant to chipping or fracturing from shear forces, although it’s still not as good, purely from an engineering standpoint, as conventional watchmaking metals (brass for gears and steel for pinion leaves).

Once you get back to the mainspring barrel, you have to figure out if you can how to make the mainspring out of something other than conventional steel mainspring alloys. Cartier used fiberglass for the mainsprings of ID Two, which isn’t transparent of course but it does show you can use materials other than metal alloys. Transparent springs are currently being manufactured from polycarbonate but it’s a long road between making springs from the material in general, and making mainsprings, which have to tolerate the stresses associated with multiple turns of the mainspring barrel and repeated tensing and relaxing and which have to store enough energy to power the going train. Still, the fact that you can make springs out of transparent plastics shows that the possibility is at least open. The mainspring barrel, ratchet wheel, and crown wheel could also be made of synthetic sapphire or even a suitable polymer although given how extravagantly expensive a watch with a diamond and sapphire gear train would be, I wonder if the market would accept such materials – plastics in watch movements generally get the side-eye from enthusiasts even when they make sense mechanically (the nylon chronograph brake in the Speedmaster caliber 1861 bothered some people).

The keyless works would present a major challenge as well, for all of the reasons outlined above for other transparent components under mechanical load, and one of the biggest challenges would be the gear pivots. Conventional pivots of hardened steel turning in synthetic sapphire/ruby jewels are extremely low friction and reasonably resistant to shock; they are also very thin and making them out of synthetic sapphire or diamond seems off the table. Another possible problem would be the fact that if you used a mix of synthetic sapphire and diamond for gears and pinions, at some point you might have a sapphire gear driving a diamond pinion and whether or not that would present any challenges in terms of wear or friction, I have no idea; certainly, diamond is harder than sapphire.

If however, you could make a watch with most or all of the moving elements out of transparent material, you could do all sorts of things from a design standpoint that can’t be done in modern watchmaking with conventional, or even relatively new materials, like silicon. The whole question of making a watch with moving parts made of transparent materials is something I’ve been thinking about pretty much since Cartier launched the ID One in 2009 and my first thought was that if it were possible, you could do mystery, or at least mystery-esque, versions of complications, where only the components of the complication were in conventional metal materials. What I thought of then, and still wonder about, is whether or not it would be possible to do a mystery minute repeater, with only the repeater components in steel – they would seem to float freely in the movement, with the base caliber components supporting them all unseen, like invisible but friendly domestic spirits.

The illusion would not be perfect of course, since diamond and sapphire while transparent, don’t have the same refractive index as air and would therefore be visible to some degree, just as the sapphire bridges in the GP Quasar Light Tourbillon are visible … but it would still be cool, no?. At some point, and pretty early on in the development process you would have to start the laborious and costly process of prototyping gears, pinions, and other components in materials seldom if ever used for active movement parts but as Cartier showed with the ID One and Two, there is at least no theoretical obstacle to such a thing. I shudder to think what the service costs would be if you broke a tooth off a synthetic diamond gear, though. Still, it’s surprising how many apparently impossible mechanical problems can be solved if you throw enough money at them.