Black Polishing: The Perfection of the Surface in Fine Watchmaking
Among the most refined techniques of artisanal watchmaking, mirror polishing — also known as black polishing, polimento negro, poli noir, poli-bloqué or spéculaire — stands out as a symbol of absolute perfection. Applied by hand to visible components, this finish not only reflects light with optical precision but also reveals the watchmaker's mastery and respect for the most demanding traditions of the craft. In this article, we explore the tools, methods and secrets behind this meticulous art, which remains untouched by mass production.
What Is Mirror Polishing?
In high-end watchmaking, finishes do not serve aesthetic purposes alone — they are a statement of technical excellence. One of the most prestigious methods is mirror or black polishing. This technique is applied to the visible surfaces of parts such as repeater hammers, bridges and screws, with the aim of obtaining an absolutely flat, optical finish.
Black polishing represents the highest level of horological finishing. The resulting surface is completely free of scratches, marks or imperfections, and reflects light so evenly that, at certain angles, it appears black. This optical effect is not merely decorative: it is the result of rigorous craftsmanship, in which even the smallest flaw compromises the final result.
Black polishing represents perfection in horological finishing.
The resulting surface is free of scratches, marks or defects, and reflects light so evenly that, at certain angles, it appears black.
This optical effect is the result of meticulous work, in which even the slightest imperfection would compromise the final result.
How Does Black Polishing Work?
The process begins by wearing down the surface with extremely fine tools and abrasives: files, burnishers, polishing wheels or very fine-grit abrasives. The aim is not to smooth the surface completely, but rather to reduce its microscopic roughness, giving it an intense shine. The crystalline peaks of the metal surface are partly torn away or pushed into the microcracks by friction. The result is more an optical shine than a polish in the traditional sense.
Tools typically used
Tin plate
Tripod for holding parts
Fine-grit diamond paste
Glass and steel plates as a base
Lantern chuck (for screws)
Soft woods (pine, balsa) for burnishing
Abrasive papers of various grits
Degussit stones or fine diamond files
Hollow drills for chamfering
Loupes or a microscope for quality control
Isopropyl alcohol and cleaning cloths
These tools are essential to ensure absolute control over flatness, sharpness of edges and perfection of shine.
Practical Example: Hand-Polishing Screws
Polishing a simple screw involves several manual stages:
Preparing the Head – The end of the screw is smoothed with abrasive paper resting on a flexible base, to ensure flatness without damaging the edges.
Chamfering the Edge – A hollow drill is used to create an even chamfer around the head. Pinewood and polishing paste refine the finish.
Chamfering the Slot – Using Degussit stones or diamond files, the grooves of the screw slot are polished.
Polishing the Surface – The head is first polished on fine abrasive paper to remove deep scratches, and then on the tin plate with abrasive paste to achieve the black shine.
Final Cleaning – The screw is cleaned with isopropyl alcohol to remove residues and reveal the finish.
This process can take from 30 minutes to an hour per part — a requirement rarely found outside fine watchmaking.
Mirror Polishing Methods
Black polishing can be achieved by several methods, each with its advantages and limitations. Here are the main ones:
Modern Mechanical Methods
These methods consist of rubbing the part by hand on abrasive paper placed on a rigid plate or polishing disc. The part is fixed to a holder and the abrasive paste penetrates the grooves of the surface.
However, this process tends not to be precise enough for black polishing:
The geometry of the part is not always respected.
The abrasion is too coarse for the level of perfection required.
The edges are not left sharp enough.
Even so, it is a useful technique once you have mastered the use of abrasive discs with different levels of wear.
Lapping (Precision Fitting with an Abrasive Plate)
This process makes it possible to bring the part to the correct shape, thanks to the controlled geometry of the polishing plate and the abrasive. The roughness obtained is constant and predictable, which makes it suitable for precision polishing. This method is effective, but it does not guarantee perfect optical surfaces.
Zinc Plate
This traditional method uses a soft zinc plate lubricated with abrasive paste. When the part is perfectly flat, rubbing it by hand on the zinc plate makes it possible to achieve an absolutely even mirror finish.
Disadvantages:
With use, hard particles can become embedded in the plate and scratch the part.
The process is slow and requires great manual dexterity.
It is not suitable for parts with angles, reliefs or complex shapes.
The Artisanal Method
This is the most demanding — and the most effective. The part is worked by hand with abrasive papers and plates, with particular care given to flatness and edges.
To obtain a perfect finish:
The abrasive must be lubricated to avoid scratches.
Every surface must be carefully worked by hand, especially those visible under a loupe.
The method guarantees sharp edges and absolutely flat surfaces.
Limitations of Modern Methods
Although modern polishing techniques are sufficient for series production, they rarely produce surfaces of optical perfection. The parts end up needing to be retouched by hand, especially in a fine watchmaking context.
"Modern polishing methods are satisfactory for series production. However, the perfect optical effect can only be achieved with black polishing."
Demonstration: Mirror Polishing in Practice
To clearly understand the precision and rigour that black polishing demands, nothing replaces watching the technique first-hand. The video below, published by the Watch Repair Channel, shows in exemplary fashion the artisanal process of mirror polishing a metal surface — from preparing the part to the final shine.
Throughout the video, you can see the watchmaker using:
a tin plate,
diamond abrasive paste,
slow, controlled circular movements,
regular checks of the flatness of the surface.
The result is a perfectly polished part which, under direct light, shows the characteristic black effect, the sign of a truly specular finish.
Authentic Demonstration: Black Polishing Filmed in Real Time
The video published by the channel DHVJ SA presents a precise demonstration of the black polishing process (specular polishing) applied to a flat metal surface. It is a professional execution, filmed in real time and without narration, that reveals the purity of the technical gesture and the quality of the preparation.
During the video, you can observe:
the controlled application of abrasive paste on a tin plate,
the exact position of the part on the surface,
regular, gentle linear movements,
the progressive shine of the surface until it becomes reflective and, finally, black under oblique light.
This silent, direct recording is a faithful testimony to watchmaking know-how in its purest form. The simplicity of the presentation highlights the complexity and demands of precision handwork.
NOTE: This article was written based on information from Horological Finishing and Decoration, High-end by Audemars-Piguet, by Audemars-Piguet, which we highly recommend.
Comments