Black polish is a mirror finish on steel watch parts. It looks deep black from some angles and shows a bright, flawless shine when light strikes it directly. It is widely used in haute horlogerie, most often on movement screws. It also does a practical job: it helps lower the risk of oxidation.
Why is it called black polish?
The name describes what the eye sees. A black-polished surface looks black at certain viewing angles. It only lights up when light strikes it directly, and then it shows a bright, flawless shine. So the same surface shows either a deep black or a bright shine, depending on the angle. A finished screw head can look black from one side and bright from another.
How is black polishing done?
The traditional method is handwork. The polisher rubs a steel part on a flat metal plate coated with a very fine abrasive paste. The part moves across the plate in straight lines, circles or figure-eights. The work goes through a series of diamond pastes. Each one is finer than the last, starting at about 400 grit and ending at around 20,000 grit. The result is the dark, mirror-like surface the finish is named for.
Where it comes from
The technique goes back to fine metalwork and the decorative arts of the 1700s and 1800s. Watchmakers took it up for movement parts. Early examples include screws, swan-neck regulators and cap plates. Today it is most often found on movement screws. It sometimes appears on bridges and other parts as well. It is widely used in haute horlogerie, or high-end watchmaking.
Not to be confused with
Zaratsu polishing, used by Grand Seiko, is not the same hand process as black polish. Zaratsu is an industrial offshoot of traditional black polishing. Grand Seiko’s method started with a polishing machine bought from the German company Gebrüder Sallaz in the 1950s. Black polish in the traditional sense means a steel part rubbed by hand on a flat plate with finer and finer pastes.
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