Sunray finish

The sunray finish is not a decoration applied on top of the dial: it is the dial itself, worked in depth. It is obtained by brushing or engraving the metal surface with striations that radiate from the center and open toward the perimeter, imitating the geometry of solar rays. The result is a surface that has no fixed color: from one angle it looks almost black, from another it lights up with the color of the base metal. This optical behavior is all that matters.

In the workshop we also call it soleil, a French term that has remained in use in the Swiss tradition. Technically, there is a distinction between sunray brushed, where the striations are traced with a radial brush on an already prepared base, and guilloché sunray, where the pattern is mechanically engraved with a rose engine lathe. The difference is not merely aesthetic: the engraved guilloché has markedly superior definition of the grooves, and the resulting play of light is sharper, almost metallurgical. Brushed sunray is more democratic, faster, and is found on mid-range watches as well as certain luxury sports watches. Guilloché sunray is rare, requires a specialized operator, and when executed by hand on a traditional lathe the manufacturing cost is significant.

An interesting variant is frosted sunray, where the radial striations are softened by an overlaid frosting finish: some of the play of light is lost, but a more uniform color depth is gained, useful on certain colors such as midnight blue or bottle green. Different again is flinqué, which technically belongs to enameling: Grand Feu enamel is poured onto an already guilloché dial, and the radial patterns show through the vitreous layer amplifying the tonal variations. I wouldn't call it sunray in the strict sense, even though the visual effect has something in common.

From the bench I have seen that the sunray finish is among the most sensitive to handling: a finger on a dirty dial, a wrong tool during movement disassembly, a loose lug that scratches. The radial grooves trap any residue and cleaning must be done with a very soft brush, never with abrasives. Restoring a deteriorated sunray requires taking the dial back to the lathe, an operation that not all workshops can perform and that makes economic sense only on pieces of a certain value. On inexpensive brass dials with varnish, the sunray is often stamped or screen-printed: it does not age well and has nothing to do with the mechanical processing described above.