As you know, Grand Feu is a centuries-old technique of enamelling that enables one to achieve an enamel that is both extremely pure and particularly resistant. The perilous firing operations at extremely high temperatures make it possible to show incredibly precise details. However, as M. Emch has pointed out on many occassions, it also comes with extremely high reject rates
The great French enamalist (of British origin), Taxile Doat (1851-1939) is considered as the master of grand feu ceramics. His book "Grand Feu Ceramics: A practical treatise on the making of fine porcelain and greÌs" (1905) is probably a seminal read on the subject
Regarding the detailed technical process, here's what I found: Grand feu refers to high fire faience. High temperature kilns were used to fire the body and the glaze of porcelain or faience, between 1100 to 1450 degrees. The traditional polychrome decor which was brilliantly developed in Renaissance Italy consists of applying colors to an unfired tin glaze base or faience. Subsequent firing of the piece at 1000 degrees centigrade restrains the decor to the few colors that can take such a high temperature: blue, violet, green, yellow, orange. The unfired faience glaze constitutes a pulverulent layer that absorbs the pigment, allowing no mistakes or chance for correction by the painter. Upon firing the pigments blend with the base.
The best examples in horology that I can find are presented by J*D and Jaeger Le Coultre.
Now, I'm off to find me a copy of Doat's treatise