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Joël Pynson
June 2023
Update : February 2025, August 2025
"The idea didn't come from an opinion poll. We sought to give our watches an added value, an extra advantage that the others didn't have. Observing, for example, that the case of a gold watch scratches, our designers at the time had the idea of a garment that would not age, a product that would always retain its original beauty, hence the scratch-resistant watch [1]."
The Rado Diastar watch turned 60 in 2022, and is still present, virtually unchanged, in the Biel-based company's product ranges. Its presentation in 1962 came as a shock to the Swiss watchmaking industry, not only because of its highly unusual design, but above all because of its physical characteristics, since it was then presented as the first "scratchproof" watch. In fact, Rado was the first watch company to master the complex technique of manufacturing watch cases in "hard metal," a technique that would go on to become the company's success story.
A Story That Began in 1917
According to the official history, Rado was founded in 1917 by brothers Fritz, Ernst and Werner Schlup in Lengnau, near Biel [2]. Little is known about the watches made by the company in its early days, as they were subcontracted to other brands. In 1937, Schlup & Cie became a public limited company, and in 1947, Paul Lüthi, Doctor of Economics and son-in-law of Werner Schlup, joined the company. Paul Lüthi remained with the company for over 40 years, helping to establish Rado as one of Switzerland's leading watch manufacturers.
From the early 1950s, Schlup & Cie launched its own watches on the market, under the names Rado, Exacto and Conway, Rado being, it seems, the company's top-of-the-range model. The watches were of good quality, often automatic, with particular attention paid to water resistance.
1950
At the end of the 1950s, only the Rado brand was promoted, and specific models appeared such as the Rado Jubilar, and the 56-H automatic chronometer in 1958, which featured a small mobile anchor-shaped symbol on the dial. This device, patented in 1958 [3], animates the dial, and can give an indication to the wearer: lubricated with the same oils as the movement, it indicates that servicing is required if it freezes. It was to become one of the symbols of the Rado brand.
The mobile anchor, a RADO symbol
Patent from 1958
From 1959 onwards, Rado focused its communications on the water resistance of its watches, using a construction called "Water-Sealed," comprising a Piquerez Compressor case with bayonet closure on one side and a Plexiglas (PMMA) glass with tension ring on the other. Water-Sealed" watches have a symbol engraved on the case back, representing two seahorses facing each other.
Ad from 1960
The first of these models, in 1959, was the Starliner, water-resistant to 220 meters, followed in 1961 by the Green Horse, water-resistant to 120 meters. A variant of these watches, equipped with a rotating bezel, designed for diving and christened Captain Cook, was launched around 1962.
A Surprise at the Basel Fair
At the 1962 Basel Fair, the Rado stand featured two watches unlike any others: an ovoid men's watch and a square ladies' watch. Their oversized bezels had curious shiny, bluish reflections, and the watches were advertised as "scratchproof". Indeed, they were the first watches to use "hard metal," the term used in Switzerland at the time to refer to tungsten carbide.
Hard Metals
Since the beginning of the 20th century, it had been known that the addition of chromium, tungsten and vanadium to steel made it possible to produce very hard "high-speed steel" tools for machining steel and cast iron [4]. We owe these developments to Frederic Winslow Taylor, the man who left his name to "Taylorism [5]".
Tungsten is in fact a very hard metal - around 8 on the Mohs scale, which ranges from 1 for talc to 10 for diamond. It is insensitive to corrosion, but has an extremely high melting point (over 3,400°C), which means that it is preferentially used in powder form in combination with other metals.
The tungsten carbide first appeared in the United States in the late 1920s. It could be used to make tools 25 to 75 times more resistant than high-speed steel tools [6]. In the 1950s, tungsten carbide was produced in Switzerland by Wolfram & Molybdene, in Nyon, using a complex process [7]: tungsten ore is first purified to obtain a fine yellow powder, then reduced in a furnace under a flow of hydrogen to obtain pure tungsten. The tungsten is then carburized by adding carbon and heated to around 1400-1800°C in a protective atmosphere. The result is small, hard, brittle pieces of tungsten carbide.
In this form, tungsten carbide is not usable. It must then be mixed with a binder, cobalt, for several hours in ball mills, to obtain an extremely fine powder. This powder is then pressed at pressures of the order of one tonne/cm2 to obtain bars which have the consistency of chalk, but which can be machined to give them the shape of the desired part.
The final stage is sintering: the parts are subjected to a protective atmosphere at temperatures ranging from 1400 to 1800°C for several hours, and the cobalt acts as a binder: as it melts, it coats the tungsten carbide grains and crystallizes with them as it cools. After sintering, the parts obtained are too hard to be machined: they can only be polished with a diamond wheel. They have in fact become harder than steel or quartz: their hardness on the Mohs scale is 8.5, higher than steel (4.5) or quartz (7.0), and equivalent to sapphire (8.6). However, this process has one drawback: during sintering, there is significant shrinkage, of the order of 25%, which can be a source of deformation. This has to be taken into account, and parts made with increased dimensions before sintering. Along with the fact that sintering makes it difficult to produce large parts, it is the shrinkage that has discouraged can manufacturers from taking an interest in tungsten carbide. But not at Rado.
RADO DiaStar patent
On October 5, 1960, Schlup & Cie registered Swiss patent CH 517 963, relating to a "watch case", with Marc Lederrey, then head of the company's Research & Development team [8], named as the inventor. The objective of the patent was clear: "To create a watch case in which the visible parts of the outer surface exposed to contact with foreign bodies, for example the upper faces of the case in the case of a wristwatch, have an unalterable appearance, i.e., cannot be scratched in contact with such bodies, nor oxidize or tarnish in contact with air, so that it is possible to give these parts of the case a fine, neat appearance."
Tungsten carbide is perfectly suited to this objective, especially as it is clearly specified that only "the upper faces of the can are concerned." The inventiveness here stems from the fact that tungsten carbide is not used to manufacture the entire can, which would be impossible due to machining difficulties, but to produce a cap covering the upper part of the case, which can be produced in a conventional way, in steel, for example. There remains the problem of shrinkage. The solution is also described in the patent: "To avoid distortion during sintering, it is sufficient to rest the said parts on one of their faces in a crucible, for example made of graphite, and to place a relatively low load on them to sinter them." Once produced, the hard metal cap is force-fitted, or preferably bonded, to the steel case.
These photos show the tungsten carbide cap covering the steel case.
To contribute to the overall hardness, the glass is sapphire. Rado had mastered its use, since the same Marc Lederrey filed a patent in 1961 for the heating of a sapphire crystal in a case [9].
Rado DiaStar faceted sapphire crystal
The patent for the Rado Diastar took many years to be validated by the Swiss Federal Office of Intellectual Property. Filed in 1960, it was only granted 12 years later, in 1972, after much back and forth with the Federal Office. The Office contested the inventiveness of the use of tungsten carbide, which is logical, since this compound already existed and was known to the "man skilled in the art," a term used by patent specialists to judge inventiveness in a particular field, in this case watchmaking. Tungsten carbide tools already existed and were used in the watchmaking industry. The invention concerned the partial use of hard metal for the visible parts of a watch case. However, there was already a patent [10] for tungsten carbide rings to fix glasses in watch cases. Strictly speaking, however, this did not concern the case itself, and this was undoubtedly the subject of debate for several years.
Launch of the DiaStar
Apart from its tungsten carbide cap, the men's DiaStar model had a steel case, was water-resistant to 200 meters and featured a proven automatic caliber supplied by Ébauches SA.
The Rado DiaStar was immediately launched in Asia, where it enjoyed exceptional growth: "We first chose the Asian market, because at the time, competition there was less fierce than in Europe and the U.S. (...) RADO popularized the automatic watch in Japan, and very quickly demand was high. After 10 years, our brand already accounted for 30% of Swiss watch exports to Japan [11].”
After Asia, the DiaStar was launched with similar success in the Middle East in the early 1970s [12], when Rado joined the General Watch Holding, created by ASUAG to promote the concentration of companies within the group. The success of this concentration was mixed: created in 1971, it included up to 16 watch manufacturers (Certina, Edox, Longines, Mido, Oris, Eterna, Rado, Rotary, Technos, ARSA, etc.). But 12 years later, many companies were in difficulty and were sold: Oris in 1982, Technos and Roamer in 1983, Eterna and Atlantic in 1984. If Rado was able to get through the difficulties of the "quartz years" without too much damage, it was certainly due to the success of its models in its markets, and to the stability of its management, Paul Lüthi only handing over the reins in 1986, after ASUAG had merged with SSIH (Omega-Tissot) to form SMH, which later became the Swatch Group [13].
The DiaStar made Rado one of the first Swiss watch brands: five years after its launch, Rado claimed to produce almost a million watches a year [14].
An inimitable design widely imitated
"At the time of the DiaStar's launch, the fashion was for flat, non-waterproof cases with fairly slim bezels. So we came up with a line that was diametrically opposed to the trends. We distanced ourselves from all the other collections. The line of the model was dictated by the raw material itself, since, on the one hand, the brilliance of the case required generous surfaces to be perfectly enhanced, and, on the other hand, we could afford large glasses, since metal is scratchproof [15]."
The Rado DiaStar, in fact, was unlike any other at the time of its launch. In the years that followed, it came in a round shape for men and a square shape for men, always with oversized bezels. New case colors were introduced in 1973 with the first electronic tuning fork movements, and the following year with the first quartz, analog or LCD display versions. On the dial side, countless variations existed: colored dials in different materials, indexes embellished with semi-precious stones, day-date versions, and so on.
This highly distinctive design inspired other Swiss manufacturers: let's mention Enicar with its Sherpa Star model in 1966, Zenith with a round watch, ref. A6620 in 1967, Zodiac with the Astrographic in 1970, or Bulova in 1971 with certain versions of the Accutron.
The Rado Diastar inspired many other watch manufacturers
Other manufacturers also produced watches with carbide caps in the 1970s, such as Technos with its Borazon models, or Solvil & Titus. Technos was part of the same group (General Watch Holding) at the time.
Ad from 1974
With technological progress, it became possible to treat the surface of steel cases to coat them with a thin layer of hard ceramic. This is known as vapor-phase deposition (PVD). This process was used to deposit titanium carbide on cases by Omega in 1970 [16], for a rare model of the Seamaster chronograph, then later by Oris in 1985, and by Tissot, Raymond Weil and Zenith in 1987.
Omega Seamaster chronograph with titanium carbide cap
After the DiaStar
Rado launched several watch ranges after the DiaStar, such as the Manhattan, a square waterproof watch in steel in 1967, or the NCC (new conception & construction) range in 1971, but none of these enjoyed the success of the DiaStar. In 1982, Rado launched the Diastar V2200, made entirely of sapphire, a material even harder than tungsten carbide [17].
In the 1980s, other manufacturers turned their attention to hard metals. In 1983, for example, Longines offered watches in the XL24 alloy, featuring tungsten carbide, nickel and chromium [18].
Finally, the development of "technical" ceramics enabled IWC, for example, to launch its Da Vinci chronograph in zirconium dioxide in 1986, and was taken advantage of by many watch manufacturers, including Rado with the Ceramica range in 1990 and the Sintra range three years later.
DiaStar vintage
Vintage DiaStars are a collector's delight. Very easy to find in Asia, a little less so in Europe, they are generally in excellent condition due to the very characteristics of their case and extremely resistant glass. It is important, however, to check that there are no chips on the cap, and to check the movement, especially if the watch comes from a country known for its humid periods.
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1- Interview with Mr Paul Lüthi-Schlup, CEO and Chairman of the Board of Directors of RADO, in Journal Suisse d'Horlogerie, 1981, 1, p. 5 ff
2- La Suisse Horlogère, Édition Hebdomadaire, 1967, p. 1462
3- Patent CH 345602
4- Revue Internationale d'Horlogerie, 1907, p. 1089
5- Revue Internationale d'Horlogerie, 1930, 14, p. 33 et seq.
6- Revue Internationale d'Horlogerie, 1929, 9, p. 34
7- Revue Internationale d'Horlogerie, 1955, 6, p. 27 et seq.
8- Thanks to Léo Desseaux, Sales and Marketing Coordinator at Rado for this clarification
9- Patent CH 379 407
10- German patent 866 626 registered in 1952
11- Journal Suisse d'Horlogerie, 1981, 1, p. 5 et seq.
12- The Horological Journal, April 1981, p. 32
13- Europa Star, 1986, 6, p. 20
14- La Suisse Horlogère, Édition Hebdomadaire, 1967,
15- Journal Suisse d'Horlogerie, 1981, 1, p. 5
16- Marco Richon, Omega, Voyage à travers le temps, edited by Omega, 2007, p. 546
17- La Suisse Horlogère, Édition Hebdomadaire, 1982,
18- Revue de la Fédération Horlogère, 1983, 7, p. 26-27
Notes :
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