Frosting is a dull, matte finish applied to movement parts such as bridges, and also to dials. It is made by blasting with media or by engraving. The result is a very fine texture that does not reflect light like polished metal, and it also helps the surface resist corrosion to some extent.
How is frosting made?
Frosting can be made by hand, with chemicals or by machine. In the hand method, called tremblage, the craftsman uses a burin to cover the surface with evenly spaced pin-prick marks. In the traditional chemical method, plates of brass, gold or silver were heated and then put into strong acid. Today the finish is often made by bead blasting. It uses air pressure to fire a fine abrasive at the surface.
Which parts are frosted?
In a watch movement, the parts most often frosted are the bridges, the main plates and the barrel bridges. Dials can be frosted too. The finish was used on clocks as well, where it has been described as a grainy, matted surface put on brass parts as a step before gilding. Wherever it appears, the surface is made up of a huge number of tiny irregularities. Together they give the metal its matte, decorative look.
Where it comes from
Frosted surfaces were already widespread in watchmaking in the 18th century, especially in England and Switzerland. The old ways have not disappeared. Top makers such as Roger Smith and Greubel Forsey still frost their parts using traditional methods. Lang & Heyne frost the main plates of its movements by silver grinding. A mixture of salt and silver is brushed onto the brass, in the manner of early German watches.
Not to be confused with
Frosting is not the same as sandblasting, though both can involve firing an abrasive at metal. Sandblasting uses coarser abrasive particles and gives a more industrial look. Frosting gives the metal a very fine texture. When it is made by blasting today, the usual method is bead blasting, which uses a fine abrasive. Frosting need not involve blasting at all. It can also be made by engraving, or by hand with a burin.
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