Understanding Watch Components: Why Blued Screws Matter in Horology

Usually located in the movement of a timepiece, blued screws hold parts like plates and bridges in place. Their application is most noticeable in high-end mechanical watches with transparent casebacks that showcase the elaborate mechanism. 

Essentially, steel screws are subjected to a certain kind of heat treatment or bluing in order to increase their hardness. In traditional watchmaking, bluing is primarily synonymous with tempering of steel, which is carried out after carbon steel has been hardened. However, because the initial hardening process makes the part overly hard and hence brittle, watchmakers must temper that part further to soften it. 

History and Science of Blued Screws

Heat bluing was employed by watchmakers as early as the 16th century to protect the steel components of their watches — hands, screws, and, in some cases, dials — from oxidation and rusting. The heat bluing procedure performed two functions: it protected a watch's steel components and strengthened these tiny parts to keep them from becoming brittle. This was especially crucial for screws. 

When exposed to tremendous heat, steel creates a protective oxide coating, which turns deep blue when the temperature is exactly perfect. Traditionally, timepiece makers get the desired results when the process is performed over an open flame, wherein the small components are put on a small bed of brass shavings on top of a tray that provides even heat distribution. It is imperative for each component to be clean and free from any oil that might cause imperfections during the bluing process. For this, watchmakers must ensure that the material is evenly thick across the board; thinner parts are likely to turn blue faster.

The Process of Bluing

Since carbon steel hardening will produce hardened steel that is too brittle for most applications, tempering of steel is executed after carbon steel hardening. The steel component concerned is heated to a specific temperature during the tempering process, maintained for a while, and then cooled. The steel component becomes more ductile as the temperature rises, while the hardness decreases. As a result, the temperature at which the steel is tempered can vary based on the application. A spring, for instance, would undergo higher-temperature tempering, compared with a screw, because the steel in a spring needs to have significantly greater flexibility and resistance to deformation. Once the proper temperature is reached, heating a screw only takes a few seconds each time.

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Modern Scenario

Corrosion-resistant technology has come a long way since the 16th century. These days, watches rely on alternative techniques to prevent corrosion. Regarding the blue hue, it is often accomplished via spray painting or electroplating. Because of this, only upscale watchmakers continue to insist on employing manual flame bluing for each hand and screw, and watches opting for this traditional technique are increasingly in demand.

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Brands Using Blue Screws in Their Movements

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Frederique Constant Manufacture Classic Moonphase Date

Frederique Constant’s Manufacture Classic Moonphase Date (Ref. FC-716GR3H6) has a round 40 mm stainless steel case, a wide polished bezel, an onion crown at 3 o’clock, and straight lugs. The timepiece also has steel-coloured hour, minute hands, along with hour markers. 

Powering the watch is the new Calibre FC-716; with a new barrel, it provides a power reserve of 72 hours. Turning the watch over showcases the movement with the oscillating weight. The movement is secured by screws that are made from steel and are tempered or heated so that they become blue; in other words, these screws are not painted in blue.

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Jaeger-LeCoultre Duometre Chronograph Moon

This 42.5 mm platinum timepiece has a copper-coloured opaline dial. At 3 o'clock on the chronograph hours and minutes subdial, the moon phase display is merged; at 9 o'clock on the time subdial, the night and day display is integrated. Positioned on a subdial at 6 o'clock, the seconde foudroyante (flying second) display is flanked by two openworked sections that expose parts of the mechanism. 

The two power reserves are visible on the arc-shaped bridge on either side of the subdial. Powering the watch is a manually wound Jaeger-LeCoultre Calibre 391 with 50 hours each for the barrel and the gear train. ‍Blued screws can be found on the caseback, especially the chrono counter, the hours, minutes dial, the bridges, and the dual wing. 

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Breguet Souscription Caliber (1796)

In a 1797 brochure he had produced, Abraham-Louis Breguet explained the meaning of the watch's name. ‘Souscription’ is French for ‘subscription'. A souscription or subscription calibre is distinguished by a huge central barrel and a running train symmetrically disposed on either side of the barrel; it powers a single hand that reads off the hours and minutes. Blued screws can be found on the caseback, especially on the bridges and the mainspring. 

Its simple yet surprisingly edgy design looks striking even today. Breguet announced a 36-hour power reserve and detailed the regulating components before stating that the watches would measure a respectable 25 lignes (61 mm) in diameter.

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A. Lange & Söhne DATOGRAPH HANDWERKSKUNST

This 41 mm timepiece gets a yellow gold case, crown, and two chronograph pushers at 2 o’clock and 4 o’clock. The tachymeter scale has been crafted from gold and the main dial is created from black rhodium with rhodiumed, light-grey totalisers. The hour, minute hands are crafted from gold, while steel- and yellow gold-plating have been applied to seconds, minute counter hands, and the central chronograph hand. At 4 o’clock is a 30-minute counter, at 8 o’clock rests a 60-minute counter, and at 12 o’clock, a large rectangle date window is found.

Powering this 25-piece limited-edition timepiece is Lange manufacture Calibre L951.8, a manually wound movement with a 60-hour power reserve. Turning the watch over showcases the sapphire caseback, while the movement is secured by blued screws on the outer periphery. The watch is finished on a hand-stitched alligator leather strap with a dark brown-grey seam and a deployant buckle in yellow gold. 

Historical F.P. Journe Tourbillon

Limited to 99 pieces only, this model is a reinterpretation of the original pocket watch in the form of a wristwatch. This 40 mm 18 ct gold and silver guilloché watch has a grained silver dial with blued steel Abraham-Louis Breguet hands. 

The traditional structure of the tourbillon movement on the caseback is constructed from grained and gilded brass, and is similar to the 1983 original; it is visible through the transparent sapphire. The original detent escapement has been replaced with a lateral pallet escapement and the 3 o'clock crown has taken the place of the winding and keyed time-setting. Blued screws have been incorporated into the tourbillon and the bridges as well, and on the dial side at 3:30 and 8:30.