Grand feu means literally «great fire», and the temperature is precisely the point where this technique separates itself from everything else in watch enamelling. The enamel is fired in an oven at temperatures exceeding 800°C — often between 800 and 820°C — and the cycle repeats from five to ten times, sometimes up to thirty for certain deep blacks, with each pass consolidating the previous layers and building the optical depth of the colour.
The base composition is as simple as it is ancient: silica, red lead, potassium and soda, ground into the finest powder and coloured with metallic oxides. It is the firing that turns it into ceramics, not paint. This is why grand feu enamel does not fade: the colour is part of the material, not a coating over it. Polymeric paints yield within decades; grand feu enamel does not.
The process is radically artisanal. Each layer is applied by hand, dried, inspected and then fired. The oven offers no forgiveness: a single bubble, a microscopic crack, an irregularity in thickness and the dial is discarded. Rejection rates in production are high, even among experienced artisans. This applies to smooth white backgrounds as much as to the most elaborate executions: in grand feu cloisonné, the artisan solders fine gold wires onto the metal support to create partitions — the cloisons — that contain pastes of enamel in different colours, then fired together. Patek Philippe has documented cases where a single cloisonné piece required over twenty metres of gold wire and forty-eight distinct enamel colours.
There is also grand feu fumé, where translucent layers overlapped create colour gradations — from light to dark — without a sharp boundary between the tones. And grand feu on a guillochée base, called flinqué, where the engraved pattern beneath the translucent enamel remains visible through the layers, multiplying the visual depth.
Grand feu distinguishes itself from petit feu by temperature and result: petit feu fires at lower temperatures and allows more detailed finishes on overlaid elements — numerals, decorations — that would not withstand the heat of grand feu without deforming. In the Breguet Classique Souscription 2025, for example, the white background is grand feu and the black numerals are petit feu, applied in a second phase at reduced temperature.
From my bench I have seen grand feu dials from the nineteen-fifties still perfect, with colours that no lacquered dial from that era would have preserved. It is a technique that ages better than the time it measures.