What does the term plique-a-jour refer to in enameling?

The term plique-a-jour specifically refers to a technique in enameling where transparent enamel is poured into open cells, creating a window-like effect that allows light to shine through. This method is akin to stained glass, resulting in vibrant hues and luminous qualities in the finished piece. The open cells, typically formed by a metal framework, are crucial because they allow the enamel to be visible from both sides, enhancing the effect of transparency. This artistic technique showcases the beauty of the enamel while also demonstrating the skill involved in creating intricate designs.

In comparison, the other options involve different techniques or methods in enameling but do not represent what plique-a-jour is. For instance, adding gilded foil pertains to a decorative element rather than the manipulation of enamel. Creating walls to contain the enamel suggests a different style of enameling that focuses on the containment rather than the transparency effect. Finishing the enamel over a detailed metal pattern does not capture the essence of the open structure of plique-a-jour, as it instead implies working with a solid backing. Therefore, option C accurately represents the unique characteristics of the plique-a-jour technique.