Open the sapphire caseback of a well-made watch and you will see this discreet, almost electric sparkle: blued screws, planted like ink punctuations on rhodium-plated bridges. For amateurs, it's a workshop thrill; for laymen, simple coquetry. The truth? This blue is not makeup, but the tangible trace of an ancestral watchmaking gesture, both technical and poetic. In a watch movement, the blued screws do not only serve to flatter the eye: they tell of a way of doing things, a discipline of fire, a heritage of marine chronometers and master repairers.
The blue of the screws comes from a heat treatment on the polished steel. Carefully heated to around 290–310°C, the screw heads develop a thin layer of oxide (magnetite) which diffracts light giving this deep blue hue, sometimes tending towards purple depending on the angle. Before that, the craftsman polishes the screw to a mirror: the more perfect the surface, the purer and more homogeneous the blue. Traditionally, it is heated on a brass plate or on a bed of filings, sometimes with a flame, sometimes in the oven. A few degrees too much and the screw turns purple, then gray; a few degrees less and it remains straw. The precision of the gesture gives the nobility of the color.
Not all blue screws are created equal. To the trained eye, the “living” blue of a fire treatment differs from the “industrial” blue of a modern deposit. Here's how to get there:
Should we avoid PVD? Not necessarily. As always in watchmaking, the important thing is the honesty of the statement: a watch with an assumed industrial positioning can adopt a deposit blue. But if we talk to you about the art of fire and “master's blue”, we expect real thermal bluing.
In the 18th and 19th centuries, the fight against corrosion on board ships forged the standard for marine chronometers: blued steel was common, both for durability and readability. Abraham-Louis Breguet also popularized the use of blued elements – we think of the famous “blue hairspring”, the balance spring thermally blued to improve its stability and protect it from rust. In the Glashütte valley, the German school perpetuates this language: three-quarter platinum, screwed gold bezels, blued screws. A. Lange & Söhne, Glashütte Original, NOMOS and others have made it a cultural marker. On the Swiss side, many classic houses (Breguet, of course, but also independent artisans) still use this blue as a sign of fine work.
On a rhodium-plated movement with satin-finished Côtes de Genève, the blued screws create a graphic counterpoint. With tight beading, they prick the surface like stars on a winter sky. In the presence of screwed gold bezels, they trace a tri-chromic dialogue: cold gray, warm gold, deep blue. It's not just pretty — it's readable, orderly, almost musical. Watch photographers know: a movement without blue loses an octave.
Blued screws, by themselves, do not miraculously increase the precision of a watch movement. Their role is indirect: better protection of the steel, stability of the assemblies, perceived quality which, often, goes hand in hand with tighter quality control. Be careful not to confuse it with the timing screws of the old regulating organs: they could be gold, steel or blued, and participated in the inertia of the balance. Today, most modern calibers regulate differently (Microstella masses, serge screws, variable inertia balance wheels).
In a watch movement, the blued screws are witnesses: witnesses of a controlled fire, of a tamed steel, of a culture where technique combines with style. They protect, they structure, they guide the eye and the gesture. They tell of the continuity between marine chronometers, contemporary workshops and the undiminished pleasure of turning your watch over to contemplate its architecture. A little blue on a rhodium-plated bridge, and all watchmaking is reflected there.
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