Why is enamel heated in a muffle furnace rather than directly in a flame?

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Samson killing Philistines — Enamelwork Standing dish depicting Samson crushing the Philistines with the jawbone of an ass, enamel on copper by Pierre Courteys, c. 1580; in the Taft Museum of Art, Cincinnati, Ohio. Height 9.8 cm, diameter 22.9 cm.

Enamel is heated in a muffle furnace because the furnace provides a compartment that is specifically protected from the flame.1

Using a muffle furnace ensures that the enamel is heated in an environment shielded from direct contact with the fire, which allows for a more controlled and even fusion of the glass to its metal base. This protection is essential because the brilliance and quality of the finished enamel depend on maintaining an equal temperature throughout the fusion process. Direct exposure to a flame could lead to uneven heating or contamination, potentially compromising the integrity and appearance of the enamelwork.1