Firing is the process that transforms enamel powder into a vitreous surface: without it, there is no enamel, only pigment. In Grand Feu — great fire, as we say in French — the kiln temperature exceeds 750–850 °C, sometimes reaching 900 °C; at these temperatures the silica powder and metal oxides melt and bond to the base. A single cycle is not enough. The dial returns to the kiln five to fifteen times, often more: the Moritz Grossmann laboratory reaches ninety passes distributed over several days. Each return to the kiln adds a layer, consolidates the color, and deepens the transparency.
The base must be compatible with the enamel's thermal expansion. Gold is the preferred material — yellow gold for classic white enamel, 18k white gold when the project requires it — because its coefficient of thermal expansion remains close enough to that of glass to limit internal stress. Steel exhibits different expansion rates and makes the process less stable. Porcelain follows its own rules: firing reaches 1,300–1,400 °C, as in the Meissen dial of the Senator or in the Goldfeather Imari Nabeshima, which undergoes five firing phases and is reduced from a solid piece to a domed form one millimeter thick.
The risks are real and permanent. The differential contraction between glass and metal during cooling can produce microcracks, air bubbles, or delamination. Brown colors and certain reds behave unpredictably: Kong Lingjun's laboratory in Beijing spent months stabilizing the sand-chocolate hue of the Yáo dial. When inscriptions — numerals, ornaments, signatures — are hand-painted with liquid enamel and then re-fired, each firing is an additional risk for the entire already-completed dial.
The final result is a surface that does not become opaque over time. The visual depth achieved — that sensation of looking into the dial rather than at it — depends on the accumulation of overlapping translucent layers, each fired separately. No lacquer or varnish replicates this effect, because the physics is different: there you apply a film, here glass layers fuse together. From the bench I recognize authentic Grand Feu by the millimetric irregularity of the surface seen edge-on: it is never perfectly flat, and that is precisely what makes it alive.