Heat Bluing: What It Is, How It’s Done, And Why It Matters

Every watch enthusiast with any time in the game has seen them — heat-blued hands, screws, and other steel and titanium components, add incredible depth and color to both the dial and the movement side of a watch. There are few decorative techniques as old. One of the oldest known watches, made for the Protestant reformer, Phillip Melancthon, and completed in about 1530, <a href="https://art.thewalters.org/detail/27471/spherical-table-watch-melanchthons-watch/" target="_blank" rel="noopener">has a single hand for the hours</a>, which after almost five centuries is still a vibrant, deep cornflower blue.
Heat-bluing is, as the Brits like to say, just what it says on the tin. If you take a piece of steel and start to heat it, it’ll begin to change color as the temperature of the metal rises and it will go through a predictable sequence of colors as the temperature rises. Starting at 480Fº, the surface of the steel will turn brown and as the temperature goes up, the colors will cycle through purple, blue, grey, a range of incandescent rainbow colors, and then brighter and brighter shades of red to yellow before the steel (depending on the alloy) starts to melt, at over 2000ºF.
The colors that immediately precede melting are due to the radiant temperature of the metal, but the lower temperature colors — including every watch fan’s favorite, cornflower blue — don’t happen because the metal is hot enough to glow. Instead, they occur because the heat encourages the formation of a layer of oxide on the surface of the metal and depending on the thickness and chemical properties of the oxidation layer, light hitting it will produce different colors.
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The best known type of oxidation for iron and steel is plain old rust, which is the enemy of anyone who works with those metals, whether they’re swordsmiths, gunsmiths, or watchmakers. Rust is chemically similar to the oxide layer that forms during heat bluing but it’s not identical and in comparison with heat-bluing, rust, if exposed to moisture, will expand and flake off, leaving the underlying metal exposed…lather, rinse, repeat. If unchecked, rust can eat completely through, and totally destroy, steel or iron objects of any size, sooner or later.
Heat-bluing, on the other hand, actually discourages corrosion, thanks to a phenomenon called passivation. The name refers to a process in which the surface layer of oxide actually protects the underlying metal and prevents any further oxidation. Titanium is a great example. It oxidizes readily, but the oxide layer forms so quickly and evenly that it shields the underlying metal.
For a process so basic and important to steelwork, it’s funny that the history of bluing steel is so obscure — nobody seems to have a definite idea as to who was the first to notice and use it, but it’s been used in watchmaking, as far as anyone can tell, right from the beginning. There are two reasons for using it — one is that the oxide layer that gives blued steel its beautiful color helps prevent corrosion and rusting, and the other is that it looks great. Heat-bluing is not the only way to blue steel, by the way — the oxide layer can be produced by chemical treatment as well. However, watch enthusiasts tend to regard heat-bluing as the most prestigious (as well as traditional) way to blue steel.
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Heat bluing a steel part is simple in principle but very difficult in practice. The apparatus is simple — a small alcohol lamp (also called a spirit lamp) and a copper plate for holding the object you want to blue. You put the component on the plate and hold it carefully over the flame until the steel starts to color, and then, when you hit the right shade, you take the copper plate off the heat.
The basic problem is that the desired color happens within a fairly narrow temperature range and as you heat a steel component, you have to watch it very closely and make sure you take it off the heat neither too soon, nor too late. Steel begins to turn blue at about 575ºF, but the best temperature range for heat-bluing is pretty narrow. Blue is preceded by brown (about 480ºF) and then purple (500º to about 540º) so it’s quite easy to overshoot or undershoot.
In either case the color will be off, and the trick is that even once you take a screw or steel hand off the flame, there will still be residual heat which you have to account for. Heat bluing traditionally is done by putting the part in question on a copper plate, which is then held over the flame, and the artisan watches closely to make sure the part — a screw, a hand, or, as in the case of De Bethune, a spherical moonphase display — turns the correct color.
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Heat blued hands are a feature of some of watchmaking’s most beloved timepieces — Grand Seiko, for instance, has made a name for itself for many reasons but not the least of them is that it makes lavish use of <a href="../shop/4742243.html">heat-blued seconds hands in its watches</a> (which are blued over a flame in a way that would be instantly recognizable to anyone who’s learned the craft, or seen it done, in the Vallée de Joux or Geneva.) And occasionally, you can find watches with heat-colored components in other shades — Moritz Grossmann, for instance, uses strikingly beautiful heat-purpled <a href="../shop/4752408.html">hands on some of its watches</a>.
One of the most interesting uses of blued steel, and heat-bluing in general, is at De Bethune; the company not only heat-blues steel components but also those made of titanium. Titanium can be heat-blued in the same way as steel but the temperature regime is different — titanium doesn’t start to oxidize blue until it hits about 930ºF (500ºC). Like steel, titanium will cycle through various colors at varying temperatures, starting with a pale straw-gold at 385ºC (725ºF) and then going through purple, deep purple, red purple, brownish grey, and then a greenish-blue at 925ºC (almost 1700ºF). De Bethune also uses yellow-gold heat oxidized titanium components (most recently, in the lug inserts for the new DB27 Titan Hawk JPS). For better control and more even heating, De Bethune heat-blues its <a href="https://www.debethune.ch/en/inside-manufacture/innovations" target="_blank" rel="noopener">larger titanium components in a kiln</a>.
Heat-coloring metal components for obvious reasons, usually involves working with a part made of a single alloy, but you can get around this if you combine the right metal alloys in a single part. A case in point (and the only one I’m aware of in watchmaking) is De Bethune’s spherical moonphase complication. The spherical moonphase has a couple of advantages over a traditional moonphase, the most important of which is that it’s a more accurate representation of the actual appearance of the Moon as seen from Earth. De Bethune’s Moon sphere is made of two materials — one hemisphere is made of steel, and the other is made from palladium, a platinum-group metal. The sphere is assembled, and then heat-blued by hand over a spirit lamp flame in the traditional way. The steel turns blue, but the palladium stays silver-white in color because palladium doesn’t begin to color until much higher temperatures than those needed to heat-blue steel.
As we mentioned, you can create a blue oxide layer on steel through chemical means as well as heating, but there is something undeniably romantic about the heat-bluing process. It requires very close attention over a very short period of time, and very close manual control of the entire process in a way that chemical bluing does not — a little piece of usually unseen, but utterly fascinating performance art, in traditional watchmaking.

De Bethune Grand Seiko

Written By

Jack Forster

WatchBox Global Editorial Director Jack Forster has been fascinated by every aspect of watches, clocks, and watchmaking for his entire life. He began systematically studying and learning about watches over 20 years ago, and during the first explosion of watch enthusiast forums, he became a moderator on thepurists.com. Jack became a full-time watch writer in 2007, when he joined Revolution Magazine, first as group technical and features editor, and then as editor in chief of Revolution Magazine USA. He joined the enthusiast website Hodinkee in 2015 as managing editor and then as Editor in Chief. In 2022, Jack joined WatchBox as the Global Editorial Director. He is the author of Cartier Time Art: Mechanics of Passion, and has written about watches, and other subjects, for many other venues including CNN, American Express Departures, and Vanity Fair.

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