Polishing a metal surface so flat and smooth that it appears mirror-bright or black depending on the viewing angle.
Black polishing, poli noir in French, makes a metal surface perfectly flat by fine polishing with diamantine, so that it appears black or sparkling depending on the viewing angle. The black is an effect of reflected light, not a coating.
Flatness decides the tools. A ground tin block charged with abrasive paste lets flat surfaces be polished without deforming them, and polishing irons of tin, zinc or lignum vitae, used like files, are shaped for bevels, bridge sides and screw heads. A. Lange & Söhne reserves the technique for a few select parts, polished by hand on a tin plate with special abrasive pastes over as long as several days. It differs from anglage, which shapes an edge, though some workshops take a bevel through to a black polish.
In job listings
Black polishing is a technique, not a job title. It belongs to movement finishing and decoration workshops, where it falls to experienced watch finishers and angleurs alongside anglage and flat polishing. A listing that names it points to hand work on high-end movements. See current finisher roles.
