Guilloché

The Victoria and Albert Museum defines the word this way: "Guilloché is a highly decorative technique used in metalwork, which combines mechanical engraving and translucent enamels to create surface embellishment." The museum dates its spread to the mid-eighteenth century: "around the 1750s silversmiths began to use the rose engine lathe to cut metal surfaces with intricate patterns — such as waves and ripples, staggered lines, or sunburst rays — at an even depth and with unprecedented precision."

The machine survives in public collections. The Science Museum Group holds a rose engine lathe of about 1740 (object number 1912-53), recorded as able to "cut shapes using a set of 'rosettes'" and fitted with "a cross slide rest which can move axially and radially". The rosettes are the shaped cams that make the cutter rise and fall as the work turns, producing the repeating wave or ray pattern. The V&A notes that the lathe allowed "delicate, intricate lines or precise geometric forms such as ellipses" to be cut "at a faster speed than engraving by hand".

Watch usage is broader than the V&A's, because it applies the word to the engraved metal alone with no enamel over it. Breguet, describing its own dials, refers to "hand turning the gold dials on rose engine" and states that "all of Breguet's guilloche dials, without exception, are created in this fashion respecting the finest hand craft traditions", and that "around 1786, Breguet began fitting his watches with engine-turned silver or gold dials of his own design". Those are the company's statements about its own work.

The open question is certification. A pattern cut on a rose engine and one pressed or stamped into a dial can look similar, and this Codex found no body that certifies the difference. ISO/TC 114, the ISO committee for horology, holds eight standards directly, none of them concerning dial decoration or how it was produced. A claim that a dial is hand-guilloché rests on the maker's own statement.