Part I of our story on the evolution of hairspring materials covered temperature compensation along with the development of the first specialised balance spring alloy, Elinvar. The story brought us to the 1920s, when scientist and horologist Charles-Edouard Guillaume (1861-1938) finished his work on nickel-iron alloys and watchmakers begun embracing Elinvar springs paired with mono-metallic balances.

In this second part we turn to newer hairspring alloys, like the now-ubiquitous Nivarox. Then we look at today’s landscape and the future, touching on research done by the Swatch Group with alternative, niobium-based alloys and also the specialised but obscure Seiko SPRON 610 hairspring. Lastly we discuss silicon springs, which are growing more prevalent across a range of timepieces.

Elinvar’s weaknesses

Elinvar was by far the greatest breakthrough in self-compensating alloy hairsprings at the time. Guillaume considered Elinvar good enough and not needing further improvement — unsurprisingly since he was its inventor — but other watchmakers and engineers continued to experiment with iron-nickel compounds because Elinvar’s inherent properties made it a good, but imperfect, material.

Even though the alloy behaved predictably with temperature changes, its physical properties were not ideal to begin with. Elinvar was a soft metal, which posed its own suite of problems for spring applications.

The importance of softness in terms of hairspring performance is not related to outside interference, but instead linked to the internal molecular work and behaviour of iron-nickel alloys. As the hairspring vibrates, some energy is lost to internal friction due to the movement of molecules that make up the alloy.

The tourbillon regulator in the Daniels Grand Complication most probably uses an Elinvar spring. He was known to source new old-stock Hamilton springs, especially the 922 model.

While harmonic oscillator models, like a sprung balance, theoretically rely on a perfect potential-to-kinetic energy transformation, the reality is a little more complicated. A sprung balance is in fact a damped oscillator, which once started, tends to oscillate in increasingly smaller arcs, due to resistive forces of all kinds, until it stops when the energy has run out. This is why an escapement is required to force the balance to continue oscillating, restoring the lost energy at each impulse. 

One of the resistive forces is related to the internal molecular work of the spring, which absorbs potential energy. Compared to tempered steel springs, the iron-nickel exhibits a more pronounced molecular defect, which translates to an over-damped oscillator. This was first noticed first by watchmaker Paul Perret (1854-1904), a peer of Guillaume’s, in his early experiments with Invar-based springs.

Another inconvenience of iron-nickel alloys, apart from their inherent damping, is ferromagnetic behaviour. Although the springs themselves do not become magnetised, they are still attracted to magnetic fields. So a watch equipped with such a spring would run erratically in magnetic environments even if it wouldn’t remain magnetised.

However, this was considered a characteristic rather than a flaw at the time. The fact that regular timekeeping would resume after the removal of the magnetic field was considered satisfactory enough.

Dr. Straumann and the birth of Nivarox

Reinhard Straumann (1892-1967) was a Swiss engineer from Waldenburg in the canton of Basel who pursued practical watchmaking training but also earned an academic degree in mechanical engineering. 

Compared to Guillaume, Straumann was more in touch with actual watchmaking, having been a horological apprentice. After his studies concluded, Straumann joined Revue Thommen, a watch and instrument maker located in his native Waldenburg.

In the 1920s, Elinvar springs became widespread among many watch manufacturers and Straumann himself worked with them. He quickly became disappointed by the iron-nickel springs, which he discovered damped the oscillator too much and were susceptible to direct magnetic influence. Straumann also argued the soft metal was ultimately unsuitable for applications in smaller watches, due to its softness and deformability. 

While the engineer did not discard Elinvar completely, he felt it could be improved. Straumann even got in touch with Guillaume at some point to discuss the idea, but the latter didn’t show much interest.

Straumann didn’t let the matter rest, however, and looked outside horology for answers. A native of the German-speaking part of Switzerland, Straumann also had the advantage of easily communicating with engineers in neighbouring Germany — Waldenburg is today just a 30-minute drive away from the German border.

Straumann followed the latest developments in material science and metallurgy across the border, and became acquainted with beryllium alloys. A variety of companies added beryllium to metallic alloys to increase yield strength and Straumann believed adding traces of beryllium to Elinvar would solve the issue of softness. 

After early experimentation with some German companies, Straumann soon discovered adding beryllium to the iron-nickel concoction would in fact affect the thermoelastic behaviour of the resulting alloy. Subsequent experimentations with additional alloying materials such as chromium, molybdenum and tungsten proved very successful. Not only did these additional metals stabilise the thermoelastic behaviour of the beryllium-alloyed Elinvar, but it also made it easier to fine-tune the thermoelastic coefficient.

As Straumann describes in his patent filed in 1932, one could make the coefficient negative, positive, or null, just by adjusting the alloy mixture. The new alloy kept the quirky thermal proprieties of Elinvar, but with the added strength of beryllium.

Habring² movement with a Triovis regulator and modern Carl Haas hairspring

The early work on the new alloy springs was done by Straumann in conjunction with other German engineers and manufactured by Carl Haas, the German metal-wire specialist that today supplies hairsprings to Habring² amongst others.

These new springs contained 0.1-3.0% beryllium and a mix of chromium, molybdenum and tungsten that together amounted to 5-30% of the material, with the remainder being iron-nickel alloy. When rolled and heat tempered, the new springs were as stiff as hardened steel, did not over-damp the oscillator, and had a sensibly linear thermoplastic behaviour in the -50°C to +50°C range. 

Straumann’s observation about tailoring the thermoelastic coefficient to function was important, since the then-new monometallic, one-piece balance wheels still deformed slightly with temperature. Made from brass or German silver, the balance wheels would still affect the running rate with temperature changes — so having a tailor-made spring to counteract these effects was advantageous. 

The new material developed by Straumann and his collaborators was named Nivarox, a contraction of nicht variabel oxydfest — “non-variable [and] non-oxidising”. The name is slightly misleading since it doesn’t convey the full extent of the new material’s characteristics. Nivarox does not oxidise or rust (compared to Elinvar that was less oxidation-resistant), but it is also harder and stiffer. Nivarox was still susceptible to magnetic influence, but to a lesser extent compared to Elinvar.

Early Daniel Roth tourbillon, with a movement jointly developed with Lemania during his time at Breguet. Just like most timepieces of the era, it uses a Nivarox hairspring.

Straumann left Revue Thommen and founded Nivarox in 1934 in Saint-Imier. There he produced Nivarox hairsprings and worked on other metallurgy projects. In 1954 he set up the Dr. Ing. Reinhard Straumann Institute in Waldenburg, an entity separate from Nivarox that would carry out research and development work.

The institute was reorganised in 1990 and renamed Straumann, a firm that’s the world’s largest maker of dental implants. Straumann’s grandson, Thomas Straumann, is a billionaire thanks to the substantial stake in the company.

Notably, the younger Straumann bankrolled the establishment of H. Moser & Cie. in 2006, along with its sister company, hairspring maker Precision Engineering AG. The subsequent financial crisis forced Mr Straumann to sell his watchmaking investments, but Precision Engineering is today one of the leading suppliers of hairsprings to independent watch brands.

As for Nivarox, it is today the largest maker of hairsprings in Switzerland. In 1984, Nivarox merged with Le Locle-based les Fabriques d’Assortiments Réunies (FAR), the result of a 1932 combination of various small producers of escapement parts.  This gave birth to Nivarox-FAR, which was eventually acquired by today’s Swatch Group but left to operate under its original name. Nivarox is now the prevalent material for hairsprings, with its maker the biggest supplier of them.

For a long time, Nivarox seemed to be the final chapter in the long evolution of hairspring alloys. 

A Precision Engineering hairspring inside a Moser movement 

Nivachron

Nivarox-FAR remains a key company of the Swatch Group, delivering hairsprings and escapement parts to all the companies owned by the conglomerate, ranging from Omega to Longines. Nivarox-FAR was also instrumental in adapting George Daniels’ Co-axial escapement for mass production. While the first Omega Co-axial movements were made in ETA facilities, the escapement parts were supplied by Nivarox-FAR. 

Its parent allowed Nivarox-FAR a fairly large degree of freedom in terms of research. One of Nivarox-FAR’s more successful exploits in hairspring metallurgy came about in 2018 when the laboratory announced a new hairspring material, Nivachron, developed in collaboration with Audemars Piguet.

Nivachron hairspring in a Sistem51 movement.

Nivachron promised 10-20 times more magnetism resistance compared to Nivarox, not to greater shock resistance while maintaining the same thermal insensitivity. The name itself was odd, with “Nivachron” possibly being a word play on “Nivarox” and “chronometric”.

The new material was described as a “titanium-based alloy”, which upon further inspection of patents filed looks to be a titanium-niobium alloy. Patents for the material filed between 2017 and 2018 point to a bi-phased composition of 45-48% titanium and a balanced remainder of niobium. The patents suggest traces of other elements may be found, but in any case under 0.3%.

Notably, the use of niobium in hairsprings goes much further back, to the early 2000s with the Rolex Parachrom hairspring, an alloy of niobium and zirconium. 

A Nivachron hairspring installed in a Tissot movement (that is in turn derived from the Valjoux 7750)

In metallurgy, a bi-phased composition has the components present in different microstructures at a given state (temperature, composition ratio). For Nivachron, a binary alloy, the two constituents appear in different crystal conformations, namely body-centred cubic for niobium and compact hexagonal for titanium.  

The result is an interesting alloy that in certain compositions exhibits Elinvar-like thermoelastic behaviour. Moreover, niobium-titanium alloys have high magnetic resistance, making them far superior to classic Nivarox in that respect.

Swatch Group also explains that Nivachron hairsprings are stronger, as the alloy has a higher yield strength than Nivarox, but it doesn’t seem that characteristic makes much of a practical difference in horological applications. When a mechanical watch is subjected to strong shocks, either the balance pivots or the escapement will break before the hairspring deforms, so the wearer does not necessarily profit from a stronger hairspring. 

A Blued Nivachron hairspring

At its launch, Nivachron was touted as the next big step forward in mass-produced hairsprings, but it slowly faded over the years. Nivachron springs were installed in Swatch System51 movements and in some Tissot and Certina models, but it was clear the Swatch Group was moving away from alloy springs and into silicon manufacturing. As it turned out, the alloy was not as revolutionary as it seemed to be at its debut.

Earlier this year Breguet presented the Classique Souscription, which features a blued Nivachron spring. Here the blue hue looks like the result of classic heat tempering, unlike the more artificial colour treatment of Rolex Parachrom. Other than the latest Breguet, the Swatch Group has mostly kept quiet about its use of Nivachron springs, with even less heard about it from Audemars Piguet. 

The Japanese connection

The Japanese watchmaker Seiko also had a lot to gain from the research on hairspring alloys carried out in Switzerland. As Nivarox became the standard, Seiko made their own alterations to Elinvar and experimented with adding cobalt to the mix. 

Little is known about the exact contents of the cobalt-Elinvar alloy Seiko used for a long time, but it was reliable enough that the watchmaker employed such springs for more than 50 years. Then, in the early 2000s, Seiko went about further perfecting its hairsprings, which the brand did in collaboration with Tohoku University. 

In 2009 SPRON 610 hairsprings were launched. The new in-house springs were manufactured entirely by Seiko, or more accurately one of the companies in the vast Seiko Group, from ingot to wire, but were reserved for the more exclusive Grand Seiko timepieces. 

Again, the exact composition of SPRON 610 remains elusive. It looks to be an optimisation of the cobalt-enriched Elinvar, making SPRON an alloy of cobalt-nickel-iron. Seiko claims a high magnetic resistance (higher than basic Elinvar in any case) and a high tensile strength. 

The addition of cobalt should have a similar effect that the addition of beryllium had on Elinvar in making Nivarox. Namely, SPRON must be a hard and resistant alloy, both to mechanical strain but also to corrosion. Just like Nivarox preserved Elinvar’s thermoelastic proprieties, so should in theory SPRON. 

Cobalt-nickel alloys are generally given aerospace uses, with them exhibiting excellent durability and fatigue resistance. Assuming that these qualities were preserved for cobalt-nickel-iron alloys, SPRON should be a very suitable material for hairsprings, with similar proprieties to Nivarox. 

While Seiko is secretive about the exact nature of their SPRON alloys and how performant their springs are, newer Grand Seiko models are adjusted to a -3/+5 seconds per day deviation, two seconds tighter than the C.O.S.C. tolerance of -4/+6 seconds a day. This suggests the hairsprings are chronometrically-potent and on par with those in modern Swiss precision timepieces. 

The silicon route

Apart from the more recent Nivachron venture from the Swatch Group, the most intensive research into hairspring materials was done in the early 2000s. Rolex with its Parachrom, Seiko with SPRON 610, and a consortium of brands working on alternative silicon oscillators. 

As explained in our past article, Ulysse Nardin Freak: The Saga of a Scientific Timepiece, Part III, the first watchmaker to experiment with silicon hairsprings was none other than Dr Ludwig Oechslin with his work for Ulysse Nardin. The Ulysse Nardin Freak became the first-ever watch with a silicon escapement when it was launched in 2001. Soon after, CSEM reached out to Dr Oechslin in order to inquire whether silicon could be applied to other movement parts as well. 

The first silicon hairspring. Image – ‘Silicon and Watchmaking, report of trials with silicon hairsprings at the Musee International d’Horlogerie’ by Dr Ludwig Oechslin, MIH

Dr Oechslin thought of hairsprings as an ideal application, exploiting silicon’s elastic capability and even built the first prototypes in collaboration with CSEM. By installing raw silicon hairspring prototypes into simple Unitas movements, Dr Oechslin reported a 106-second daily error over a temperature variation range of 31°C.

Unitas movement equipped with an experimental silicon hairspring. Image – ‘Silicon and Watchmaking, report of trials with silicon hairsprings at the Musee International d’Horlogerie’ by Dr Ludwig Oechslin, MIH

In its pure state, silicon exhibits a strong variation of Young’s modulus with temperature, meaning the springs’ elasticity was dependent on temperature changes. While diamagnetic (impervious to magnetic influence) and fatigue-resistant, silicon springs seemed to suffer from the same issue as the earliest iron hairsprings: no temperature compensation. 

The gross daily deviation of the prototype silicon springs was due to silicon’s negative thermoelastic coefficient, which softens the spring at higher temperatures, making the movements run noticeably slower.

Silicon hairspring in the Breitling-derived Tudor MT5813 movement.

The solution to this issue was found shortly, and was surprisingly straightforward: a coating was required for the pure silicon core spring. A suitable coating an oxide of silicon itself, SiO₂, which has an advantageous opposite thermoelastic coefficient value to that of pure silicon. 

The proposed SiO₂ coating has a positive thermoelastic coefficient, which means the coating becomes more rigid at higher temperatures. Since in the direction of spring deformation there are two layers of SiO₂ coating bounding the pure silicon core, the resulting hairspring, when engineered correctly, has a near-zero thermoelastic coefficient. 

The oxide coating has about 6% of the silicon core’s thickness, meaning it does not affect the overall hairspring dimensions. Compared to alloy springs which are rolled, coiled and heat treated, the silicon springs are etched from a large wafer and thus don’t suffer from eventual molecular shifts. 

Syloxi hairspring, Rolex’ own take on the silicon spring

The result of coating silicon with silicon dioxide was christened as silicium, a material that was the subject of the original patent regarding silicon hairsprings: patent EP1422436B1 filed in 2002 concerned a silicon hairspring that was self-compensating for temperature changes, without mandating any specific spring shape.

The research into silicium was backed by CSEM, Rolex, Patek Philippe, Swatch Group, and Ulysse Nardin, allowing those brands exclusively to manufacture silicon springs under the patent guidelines, but each implemented its own coil geometries. The patent has expired in 2022, leaving the for open for other companies to explore silicon manufacturing. 

The adoption of silicon has been widespread and it is more than likely that several million mechanical watches with silicon-equipped movements have been sold since the material made its debut.

That said, silicon is not the perfect material. Silicon is not isomorphous, so its elasticity depends on the molecules’ orientation — which is to say it is advantageously elastic in the coils’ radial direction, but not so much when strained in other directions. In practice, this means silicon hairsprings need more care when the movements are either put together or serviced, since the coils can be rather brittle when pulled vertically.

Closing thoughts

This two-part story on the evolution of hairspring materials in mechanical watches was meant to be an overview of the landmark advancements in spring compositions since its invention 350 years ago. The account covered to an extent both the historical and scientific aspects of the important steps watchmakers and engineers took towards more accurate mechanical timekeepers.

The story intentionally overlooked some smaller-scale advancements, like TAG Heuer’s prototype carbon-composite high-frequency hairspring or the recent mono-block silicon oscillators. The story didn’t explore specific hairspring coatings or boutique manufacturers (like Precision Engineering and its PE 5000 alloy) but rather focused on those inventions which were truly revolutionary, reaching mass production and acceptance across a large range of timepieces. 

MB&F movement with a PE 5000 alloy hairspring supplied by Precision Engineering AG.

First came Invar, which paved the way for specialised spring alloys. Elinvar followed, with the first truly temperature-impervious hairsprings. Nivarox closed the chapter, with the most widespread use across millions of modern timepieces. 

Other notable advancements, like the rather obscure SPRON 610 from Seiko or Swatch Group’s own Nivachron speak of the modern metallurgy and research capabilities of large brands, although their development is not as groundbreaking as the Invar family of alloys. 

Finally the use of silicon parts in timepieces is perhaps the most interesting and laudable feat of 21^st century watchmaking. Silicon hairsprings have been more and more prevalent lately, although classic brands tend to steer clear of them. The latest micro-mechanical developments also point to a rise in mono-block high frequency oscillators, so the 350 year old hairspring might be soon overtaken by events.