Blued screws are steel screws whose heads have been heated until a thin oxide film turns them blue. The film is magnetite, and it once helped protect the steel from rust. Modern alloys and electroplating now do that job better. Bluing is kept mostly for its looks and to keep an old craft alive.
How are screws heat-blued?
Thermal bluing needs no chemicals. Heat alone makes a blue oxide film grow on the steel. The hotter the steel gets, the thicker the film grows, and its thickness sets the colour you see. As the steel warms, it turns yellow, then gold, then purple, and finally blue. In the traditional method, the small parts are heated over a flame on a layer of brass shavings in a copper tray.
Where it comes from
Watchmakers were already heat-bluing steel parts in the 1500s to protect them from rust. The Pomander Watch, sometimes called perhaps the first watch ever made, has one blued hand. In the 18th and 19th centuries, marine chronometers used blued steel to resist corrosion at sea. Abraham-Louis Breguet helped make blued parts popular, including his well-known blue hairspring. In Germany, Glashütte watchmaking kept the tradition going. There, blued screws appear alongside three-quarter plates and screwed gold chatons.
Why do watchmakers still use them?
Rust protection is no longer the main reason they are used. Blue screws stand out against rhodium-plated bridges and gold or brass chatons. This helps watchmakers find the fixing points more easily during assembly. The craft is not limited to watches. Bluing also colours steel parts in fine clocks and other high-quality metalwork.
Not to be confused with
Not every blue screw is heat-blued. Some are coloured by chemical treatment, and others by PVD coating. Both kinds look evenly coloured and flat. Heat-blued screws do not have that same look. Thermal bluing uses no chemicals at all. The blue is a thin film of magnetite oxide that heat alone grows on the steel.
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