Polissage
This is a surface treatment of steel components, which according to watchmaking tradition, is reserved for the rarest and finest haut de gamme timepieces. There are actually several names in use to describe this surface treatment: poli mirroir, poli noir, poli bloc. It is interesting that two of the names are descriptions of the appearance and the third of the method for achieving it.
Poli mirroir captures the look of the surface when light is reflected off it at an angle, for the component shines much like a mirror. Poli noir, or “black polish” accurately describes the appearance when the surface is viewed from a directly perpendicular position, as the absence of reflected light from that angle transforms the look to that of highly polished black onyx. The name poli bloc connects with the traditional method of achieving the finish, which is painstaking hand polishing upon a zinc surface, lightly coated with fine abrasive.
At Breguet, poli mirroir is the finish treatment for the hammers of sonneries such as the minute repeaters, the Réveil du Tsar and the bridges of the Tourbillons. In order to achieve the perfect optical effects, mirror shine or black onyx, depending on the angle of light, a poli mirroir finish requires a radiantly smooth, completely flat surface, utterly free of the slightest trace of blemishes or scratches.
Today still, Breguet craftsmen continue to use engine-turning lathes designed and built over a century ago. With a precision of a tenth of a millimeter, they engrave intricate patterns reflecting their uncommon virtuosity. From start to finish, engine-turning depends essentially on the craftsman’s sharp eye and steady hand, of which the lathe is but an extension. Once the dial plate has been meticulously engine-turned by hand, it is silver coated using techniques developed over two centuries ago: powdered silver is delicately brushed on the plate with circular or linear movements, depending on the type of satin-like finish desired.
Today, the same kind of engine-turned guilloché work engraved on gold Breguet dials is also executed on delicate and brittle plates of mother-of-pearl – a truly impressive achievement in its own right.
