Mirror polishing

In watchmaking, there are at least three names for the same family of techniques, and confusing them is a costly mistake when buying or selling. Black polishing (or spéculaire), Sallaz polishing, and Zaratsu are all variants of mirror polishing on metal: the result is a surface that, at certain angles, reflects light without distortion, and at other angles appears almost black due to the absence of light scattering. Hence the name.

The term Zaratsu is simply the Japanese phonetic rendering of Sallaz, from the name of the German Gebrüder Sallaz machines imported to Japan in the second half of the twentieth century. The technical distinction that matters: Sallaz polishing uses the flat front of the rotating disc, not the rounded edge. This is what eliminates optical distortions and produces the perfect flatness that makes the Grand Seiko recognizable. When I read "Zaratsu" on a Grand Seiko technical sheet, I know the term indicates that specific finishing on case, bezel, and lugs, often in contrast with hairline or short-stroke brushed areas.

Black polishing applied to movements is another thing, though a close relative. You work on flat surfaces of steel components — plates, bridges, tourbillon cages — with progressive abrasive sequences that can include papers of various grits, boxwood, and diamond paste. The process is entirely manual. Greubel Forsey, in its Qualité Musée standard, combines it with anglage, rocking, circular graining, and straight graining: a complete program in which black polishing covers visible flat surfaces, while anglage treats the beveled edges. On Pietari Kupias's Katedraali or Gerald Genta's Grand Sonnerie, the black polishing on the tourbillon bridges requires working hours that explain a good portion of the price by themselves.

The quality of mirror polishing can be read by observing the edges: a truly flat surface must end with a sharp edge, not a rounded one. If the edge is rounded, the machine controlled the operator, not the other way around. At the workbench, I immediately distinguish successful mirror polishing from brilliant satin finishing pushed to its limit: the latter lacks the depth of black and the sharpness of edges. I cannot quantify the exact time needed for each piece in various manufactures, but I know that those who work correctly don't finish it in less than hours per component.