As blue as cornflowers
“Blued screws” – the makers of watch movements are proud of this title and like to point out that their movements gleam blue in all the right places. Bluing is a traditional treatment that requires intense heat and a delicate touch.
The origin of the gleaming blue screws is rooted in a pragmatic concern. There is namely the perpetual question whether to bestow metal parts with a protective coating against corrosion and oxidation, or to temper them. Since the metal parts used in watches must possess extreme durability in order to stand up to the great mechanical demands of the watch movement, they are tempered. Traditionally, steel parts are tempered in a process whereby they are heated to a certain temperature and then “quenched” in water or oil. This compresses the molecular structure of the steel causing it to become hard and very brittle. The colorful radiance is more or less just a pretty side-effect of this process.
Watch hands and screws have been “blued” since at least the 16th century. Given modern methods, this treatment is nowadays no longer necessary to protect metal. Yet, the blued parts are still cherished as a high quality decorative element and as an expression of tradition.
Of course, not all that shines blue in watches has been blued in the traditional heating treatment. Modern chemistry and technology offer a plethora of alternative methods to give metal a bluish tone, thereby creating the impression of high quality. Yet, with such cases, one only need examine them a bit closer: even to the naked eye, they are easily identifiable by the bare slots on the screw heads. These are not blue, because of a peculiarity of the treatment used on the screws. The screws were given a nickel coating that was then removed from the heads. Subsequently the screws were dipped into some oxidizing or coloring chemical liquid, whereby the top parts of the screws acquire a shiny blue coating. However, the slots do not become blue, because they still retain the original nickel coating. “This coloring is not held in as high esteem as the heat bluing,” comments Rainer Langeleist, work shop supervisor at NOMOS in Glashütte. He is a defender of the traditional method of bluing through intense heat. This treatment has been practiced at NOMOS since 1998. All the blue screws required by NOMOS are “annealed” directly in house.
The principle behind this method is simple: The metal part is heated and then changes color in accordance with the temperature. This change begins already at 225º C when the part turns light yellow. Further heating causes it to turn dark yellow, reddish-brown, purple, violet and dark blue. At circa 310º the color turns to light blue and then eventually to light gray at around 325º. This wonderful transformation to blue and later to gray is caused by the presence of oxygen which initiates a process of oxidation of the surface of steel that has a carbon content of 0.4-1.6%.
This may sound very simple, but the difficulty is in the details. Uncleanliness and oil impair the homogenous oxidation of the steel surface and result in the appearance of gray specks. Sometimes streaks and irregular coloring cannot be explained. On this matter, Rainer Langeleist says, “NOMOS traveled a long path filled with much experimentation to achieve the consistently beautiful results that we now have. Nevertheless, there always remains the chance that the highly difficult procedure will go wrong somehow.”
At NOMOS, the path from a simple screw to this noble blue creation begins with a rough screw produced by a highly specialized manufacturer according to specifications from Glashütte. This is a standard practice for demanding producers, who take care that everything in their movements is made according to their conception.
NOMOS stipulates the height of the screw head, as well as the width and depth of the slot. They also request beveled edges on the screw heads. Those who prefer standard goods usually choose inexpensive fillister head screws, which have the advantage that inferior polishing and the occasional scratch are not immediately noticeable due to their domed surfaces. When dealing with flat surfaces, though, the polishing must be impeccable. Thus, in top quality movements you will find screws that have been grinded completely flat, with polished heads and beveled edges.
When the tempered, but not polished screws are delivered to NOMOS, the first step is to smooth them. They are sorted by hand into a polishing plate, where they are secured with lacquer and a magnet that has been fitted to the back of the plate. Then the discs are pre-grinded by hand in four stages each employing a finer grade of sandpaper. Then they are polished twice by machine: with grinding pressure, the polishing plate is pressed against a rotating tin disc coated with an abrasive. At the same time, the polishing plate is turned, because the direction of the polish must be diffuse in order to achieve a fine, brilliant effect. The polish is then closely examined and the screws are meticulously cleaned. The last remnants of grinding and polishing agents are removed using ultrasound and multiple cleansing and dissolution baths. The metal must be absolutely clean otherwise the bluing process will fail. Now the screws are piled onto a copper boiling-out pan and placed on a ceramic plate inside an oven pre-heated to 295º Celsius. From now on the temperature must remain constant – in order to achieve the NOMOS cornflower blue, the temperature cannot be allowed to fluctuate more than one degree. Depending on their size, the screws remain in the oven for three to five minutes before being removed and cooled on a stainless steel screen. They are then appraised using a high-power magnifying glass. The primary criteria are that the desired blue tone has been achieved and that the coloring is even. Once the examiner is satisfied with the results, the screws are brought to the assembly department for a final inspection, whereby a technician examines each one individually and decides if it is beautiful enough to be used in a NOMOS movement. He or she also groups them according to shared nuances, so that they conform to one another inside the movement. They also weed out any pieces with specks or smears. Despite all the experience with this process at NOMOS, such deformities are still a consistent occurrence.
The people at NOMOS have determined that humidity also has an influence on the end result. They do not do any bluing on days when the humidity is above 40%. Today NOMOS is proud of their perfect cornflower blue, so proud that up until several years ago they used to blue all the screws used in the movements. It was Walter Lange who first explained the old local watchmaking tradition to his colleagues: only those screws used to fasten structural parts such as wheels or bridges are blued. Consequently, the screws used in functioning parts now only glimmer with a simple fine polish. So, NOMOS now also maintains the Glashütte tradition in this regard.