Black polishing is not an applied color: it is an optical effect created by surface geometry. A perfectly flat plane, free of any microscopic scratches, reflects light only perpendicularly. Observed from any other angle, it absorbs everything and appears deep black. This is the paradox that makes the technique so demanding: you work to create a surface that, under most lighting conditions, disappears.
Execution is entirely manual. Work is done on flat plates — traditionally tin, copper, or boxwood — with progressively finer abrasive pastes, until every imperfection below the micron is eliminated. At the bench, you constantly control with eye and touch. A single foreign particle, one imprecise gesture, and you start over. On soft materials like brass and gold the risk is even greater: the surface yields with less control. This is why black polishing a tourbillon bridge can require several days of work on a single component.
In Haute Horlogerie you find it on movement screws, bridges, plates, wheels. Rexhep Rexhepi applied black polish to the seconds reset hammer of his Chronomètre Antimagnétique — a first for him — combining it with a bercé finish on the central pinion: two treatments that dialogue through contrast. Guillaume Melin executes it for Van Bricht on bridges, hands and tourbillon cage with different tools depending on the piece. Jean-Claude Biver, in the JCB-003, extended it to all polished surfaces of the movement as a unifying finishing criterion.
The term black polish is used imprecisely to also indicate black surface coatings such as PVD (Physical Vapor Deposition) and DLC (Diamond-Like Carbon). These are different things. PVD deposits a thin layer of material physically onto case, caseback or bracelet; DLC adds a layer of amorphous carbon with hardness close to diamond. Both protect the metal and give it a black, matte or polished appearance depending on the final finish, but they have nothing to do with mirror polishing performed on caliber components. The 1972 Porsche Design Chronograph 1 was the first commercial watch with an entirely black PVD case — a radical choice then, common today on tactical and military watches.
Those who work seriously on movements can tell the two apart at a glance. Black polishing has a depth that no coating can imitate.