Grand feu, literally "great fire," is the highest-temperature enameling technique used in watchmaking: firings occur between 800 and 900 °C, sometimes exceeding that. The distinction from petit feu—used for overlaid details like numerals or minute markers, fired at significantly lower temperatures—is more than just terminology: the two processes produce chemically different surfaces with incomparable hardness and color depth.
The base of grand feu enamel is a mixture of silica, red lead, potassium, and soda, ground into the finest powder and colored with metal oxides. The powder is applied wet to the support—brass, silver, copper, or ceramic depending on the design—and then fired. A single pass is never enough: the process repeats five to twelve times, sometimes up to thirty for certain deep blacks where every imperfection is immediately visible. Between firings, the piece is hand-polished. The final result is a glassy, extremely hard surface with color depth that no printing or varnishing can replicate. The color doesn't fade in the next decade or the next: this temporal stability is why great nineteenth-century examples still have legible dials without filters.
From the bench, after forty years on historical and contemporary pieces, I'll add a fact that technical sheets tend to omit: the rejection rate is brutal. It depends on the base material, the thickness of the layer, the humidity that day, the furnace's age. Some manufactures declare reject rates of 30-40 percent; on difficult colors—saturated black, certain burgundy reds, translucent emerald green over guillochage—you can go considerably higher. This is the true hidden cost of grand feu, not the hours of labor.
The technique combines with other processes: cloisonné adds gold wires soldered to create partitions filled with colored enamel; flinqué applies translucent enamel over a guilloched motif that remains visible in transparency; the fumé effect is achieved with overlapping translucent layers that vary in density to create gradients. Aventurine, which is not pure enamel but crushed glass mixed in, is sometimes called grand feu aventurine but is a separate category. On exactly how a grand feu from an in-house manufacture differs chemically from one created by an external enameler—like Brodbeck Guillochage in Fleurier or Les Emailleurs de la Cité in Geneva—I have no reliable technical data: I can read the surface, not the upstream process.