e first got interested in silicon because it’s a non-magnetic material that is non-deforming,” explains Jean-Pierre Musy, an expert acknowledged throughout the profession who until recently led the Advanced Research programme at Patek Philippe.

In actual fact silicon is also elastic while being non-deforming; in other words, when knocked, it moves and immediately returns to its initial shape. Moreover, being non-magnetic, its coils run no risk of sticking together. On the downside, it is brittle! Another problem that needed solving was that it is sensitive to temperature variations.

But research was about to enable this dual handicap of a material that was outstanding from all other points of view to be overcome. The arrival of silicon in fine watchmaking was made possible thanks to a technology that enabled it to be sliced into wafers.

2006

Initially, Patek Philippe joined forces with a microtechnology research centre, IMT at Neuchâtel University, with the precise objective of improving this siliconslicing technology.

The first step consisted of creating a silicon escapement. There was no obstacle to creating this in pure silicon, as its sensitiveness to temperature fluctuations has no impact on its function. And incidentally, this material made it possible to dispense with any lubrication at the points of contact between the wheel and the sapphire pallets of the lever. No mean advantage.

In parallel to this research conducted by Patek Philippe, other entities were also exploring the promising potential of silicon. At that point, a consortium was set up consisting of Patek Philippe, Rolex and the Swatch Group to conduct joint research at CSEM (Swiss Electronic and Microtechnology Research Centre), based in Neuchâtel.

2011

“Our great fear was that despite all its qualities silicon would turn out to be too brittle and too sensitive to temperature variations to be used to produce balance springs,” Jean- Pierre Musy confesses.

“But we found a solution: the silicon was oxidised, producing something like a fine layer of bark that made it more rigid and insensitive to temperature fluctuations. And tests have proved it: it no longer varies with changes of temperature, and when subjected to 5,000G shocks (equivalent to falling from a height of one metre onto a hard floor), it does not break.

Having said that, its length, the number of coils and its geometry also help make it more resistant. The thermal coefficient of the oxide’s modulus of elasticity has the opposite sign to that of silicon. This being the case, like Charles Édouard Guillaume but 82 years later, we set out to minimise the effect of temperature.”

The horological silicon developed by the three partners in this technological joint venture was named Silinvar®, a contraction of “silicon” and “invariable”, in deference to the famous Invar by Charles-Edouard Guillaume. They jointly hold an exclusive licence.

2016

Learning from history

This Silinvar®, a real technological leap forward, greatly advanced balance spring technology. Even so, a number of little “defects” remained. But the watch adjusters at Patek Philippe (the "stars” of watchmaking as Jean-Pierre Musy calls them) also have a long memory and a sense of history.

“When Patek Philippe began making balance wheels in silicon, we went back and pored over the theory of the Michel brothers, a theory dating from the 1800s and abandoned because of the technological limits of the time,” Jean-Pierre Musy explains.

“We applied it to silicon, making reinforcements at intervals in the mass. The effect is the same as that of the Breguet terminal curve, the balance spring remains in the plane of oscillation while the centre of gravity returns to the centre. But it has the advantage of being flat.” The patent was granted, and it became the Patek Philippe terminal curve: a flat balance spring in silicon with a totally concentric development. The first Spiromax®.

However, this first generation did not solve the problem of the influence of the escapement on the balance spring, which creates delays at small amplitudes. To compensate for this usually calls for giving the balance spring a bit of an “advance”, rather than making it completely concentric. That way, the escape wheel and the balance spring offset one another.

With the Spiromax®, by “simply” changing the position of the thickened region, or boss, on the silicon balance spring, the watchmakers at Patek Philippe succeeded in fully offsetting the escape wheel and balance spring against one another, thereby correcting the delay generated at small amplitudes. That was the second-generation Spiromax®.

The third Spiromax® generation saw the advent of a second boss, this time at the centre of the balance spring, whereas the first was positioned at the end. Its function was to “achieve tiny differences in speed between the different vertical positions of the oscillator, and thus to improve the precision of the timepiece even further”.

The final improvement, the fourth Spiromax® generation, optimises the position of the spring in relation to the unbalance (wobble) of the balance, so that their respective curves offset one another in the vertical position as they always cross at the same place.

As we can see, the conquest of precision is a matter of step-by-step progress, microdetail after microdetail. “All the problems were known. Balance spring theory has historically been the same since Huygens; it’s the solutions that are radically different. That said, it’s still taken 14 years for Patek Philippe to achieve a near-perfect result.” The “near-perfect” that escapes Jean-Pierre Musy’s lips says it all.

It looks as if the final word on the balance spring has not yet been said.