In 1914, Emile Fourcault developed a method of commercially producing large panes of sheet glass. Known as flat glass, its potential was realised by early modernist architects and a new breed of transparent architecture was born. In a similar vein, it was the advent of viable production methods and the affordability of sapphire crystal as of the 1990s that was the big factor in modern watchmaking’s move to transparency. In particular, much of the impetus for this move came from the introduction of one watch: the Corum Golden Bridge.

The Birth of Transparency

The idea of the Golden Bridge was born when Corum co-founder René Bannwart met Italian-born, self-taught master watchmaker – and later co-founder of the Académie Horlogère des Créateurs Indépendants (AHCI) – Vincent Calabrese. Calabrese encountered a turning point in his career when working as a boutique manager for a retail outlet in Crans-Montana, a well-known ski resort in the heart of the Swiss Alps. His wealthy clients often asked for personalised timepieces, which gave him the idea to create a movement that was easily customised while also being structural and airy. Simultaneously, this watch would have nothing to obscure the view of the movement within, i.e. no dial to hide the watchmaker’s work.

Vincent Calabrese (Image: Corum)
Vincent Calabrese (Image: Corum)

He took his single-bridge watch to Geneva’s Invention Salon, where it won a gold medal. Calabrese chose Corum as his industrial partner to turn the concept into reality, based on the fact that Corum, founded in 1955, was already so dynamic with its own designs and that Bannwart was as artistically minded as Calabrese; the pair got on famously. The Golden Bridge’s famous baguette-shaped movement was complicated to make serially, and complicating it even more was the fact that both Calabrese and Bannwart wanted to make it in 18-carat gold. “The reason for this choice was simple,” Calabrese remembers. “It was to be an exceptional movement. Up to then, there had never been a production wristwatch movement in gold.” And, of course, this special and unusual feature led to the very name of the new watch design.

The Golden Bridge was more than just a timepiece; it delivered its own messages. For Calabrese, the message was that neither the watchmaker nor his work should be hidden any longer. For Corum the message was that mechanics and art can coexist to make a beautiful, wearable object.

The Corum Golden Bridge launched in 1980
The Corum Golden Bridge launched in 1980

The Difficulty of Transparency

The original Golden Bridge’s serial case comprised two hand-faceted sapphire crystals cut in a difficult bombé-creusé shape that resembled a domed half of a cross-sectioned hexagon held together by two bases of yellow gold and four golden screws. Number 001 was presented to the Musée International d’Horlogerie (MIH) in La Chaux-de-Fonds on 25 September 1980, and it is still on display there.

To understand the novelty of this watch at the height of the Quartz Crisis, and the degree of difficulty of making it fully encased in sapphire crystal in 1979, it is helpful to understand something of the process of making synthetic sapphire crystal. The flame fusion used for this sees a single ingot of synthetic colourless corundum in the necessary size “created” in about 15 hours – something that nature needs about 100,000 years to accomplish.

Close-up of the Golden Bridge showing its bevelled sapphire case
Close-up of the Golden Bridge showing its bevelled sapphire case

Since diamond is the only thing harder than corundum, it is naturally the only element that can cut sapphire crystal. Initial cutting into slices takes five to eight hours. The blanks, as the corundum pieces are called after cutting, are ground to their desired shape within two-hundredths of a millimetre after which they go through a series of operations performed by highly skilled craftsmen: evening out the thickness, forming the exteriors, bevelling, faceting, spherical or cylindrical recessing and doming, polishing, cleaning, and quality control. Each sapphire crystal shape and size requires its own machinery, and in most cases the required machine tools can’t be purchased, but are developed by the supplier.

In 1979, sapphire crystal was not regularly used; Seitz AG’s ability to create such a precisely faceted crystal with a hole for the crown (drilled out using precision diamond-tipped tools and also made water-resistant with a gasket, all without compromising the transparency) was nothing short of a miracle. Reproducing the original crystal today would be much easier as cutting and polishing technologies have vastly improved since then.

Early versions of the Golden Bridge
Early versions of the Golden Bridge

The Golden Bridge was awarded seven patents in total: one, which Calabrese authored, was for the winding and setting device, which allowed the watch to be wound and set from the back, thus not disturbing the watch design with a visible crown. Another patent was awarded for the sapphire crystal case, and five more were issued for the design of various models including various wristwatch types, a pendant watch, and a table clock created by collaborating with Baccarat. Different variations on the theme in individual executions or small runs were made, all of which were unisex as the case was shaped to fit both the male and female wrist equally well.

Golden Bridge showcasing incredible gem-setting
Golden Bridge showcasing incredible gem-setting

Reimagining Transparency

Severin Wunderman founded Severin Montres in 1972 in Switzerland. The company’s best-selling product line was licensed Gucci watches, allowing Wunderman to practise his special brand of entrepreneurial and artistic creativity using the Gucci name. Corum going up for sale in the late-1990s when Bannwart retired from daily business coincided with Gucci taking control of its own watch brand, and Wunderman (who now had capacity) was smitten – especially by the Golden Bridge.

Severin Wunderman
Severin Wunderman

Wunderman wanted to re-launch the Golden Bridge in time for Corum’s 50-year celebration in 2005. So, he undertook the amazingly large project of modifying the movement to make it more reliable for the modern era while maintaining the horological work of art’s integrity.

Additionally, Wunderman was of the opinion that the modern age demanded distinct male and female case sizing as the era produced larger and larger wristwatches for men. The redesigned case contained four crystals instead of two: one on the back, one on the front and one on each lateral side. In between each crystal ran a shaft of gold or platinum. The new design strengthened the case, making it much more able to withstand shock and accentuating the straight line of the movement. And thanks to his artistic sensibilities, Wunderman launched limited edition after limited edition in many precious metals and gem-set variations.

The 50th Anniversary Golden Bridge – released to celebrate the company’s half-century – is the perfect showcase for the in-line baguette movement
The 50th Anniversary Golden Bridge – released to celebrate the company’s half-century – is the perfect showcase for the in-line baguette movement

Strengthening Transparency: Calibre CO 113

Corum’s delicate Calibre 13 became the sturdier Calibre CO 113 in time for the 50-year celebration thanks to a collaboration with Vaucher Manufacture in Fleurier, Switzerland. While the original version had an excellent, successful design, it had a few technical problems that, for one, made it somewhat unreliable and, for another, impossible to add to – with, for example, a tourbillon. So Corum’s biggest goal was to modernise and reinforce the movement.

Close-up of a fully diamond-set Golden Bridge
Close-up of a fully diamond-set Golden Bridge

Perhaps the biggest change was to move the winding stem from the back of the watch to the 6 o’clock position, which alleviated a lot of repair problems and made winding easier. Adding pillars to the movement also improved shock resistance while maintaining the integrity of its sandwich-style construction. Also, the frequency of the movement was upped from 3 Hz to 4 Hz, ensuring more precision.

The reinforced movement also made new mechanical iterations possible, the first of which arrived in 2009: the Ti-Bridge, which saw parts of the Golden Bridge now crafted in titanium (including bridges and case) and the movement now turned on its side. In 2010, Vaucher’s engineers were able to finally add a largely silicon-manufactured tourbillon housed in what Corum claimed was the world’s smallest tourbillon cage at the time, coming in at 8.5mm in diameter, resulting in a limited edition of 33 pieces of Calibre CO213. Beating at 19,200 vph, its plate and bridges were crafted in hand-engraved 18-carat gold.

An extension of the Bridges collection, the futuristic Ti-Bridge models feature a horizontal, linear movement
An extension of the Bridges collection, the futuristic Ti-Bridge models feature a horizontal, linear movement
Golden Bridge Tourbillon driven by Calibre CO213 movement featuring the world’s smallest tourbillon cage
Golden Bridge Tourbillon driven by Calibre CO213 movement featuring the world’s smallest tourbillon cage

An interesting automatic version of the Golden Bridge arrived in 2011, while 2014 saw the addition of some very artistic renditions of the watch that added either a hand-engraved, pearl-holding Dragon or a Phoenix. The latest iterations see Corum’s designers beginning to play more with the shape of the Golden Bridge’s case by making round versions called Golden Bridge Ronde (2016) and angular versions called Golden Bridge Stream and Golden Bridge Rectangle (2017).

While the Golden Bridge’s future keeps shifting shape, one thing does remain a constant: this pellucid watch’s amazing ability to continue showcasing the watchmaker’s art as its original architects intended.

Golden Bridge Dragon
Golden Bridge Dragon
Golden Bridge Round
Golden Bridge Round
Golden Bridge Stream
Golden Bridge Stream
Golden Bridge Rectangle
Golden Bridge Rectangle