Pliqué á jour (French). A technique of decoration in enameliing by which the design is outlined in metal and filled in with variously coloured transparent enamels but with no backing behind the enamel, so that the effect is similar to that of a stained-glass window. One process involved mixing a flux into the enamel so that it would have sufficient consistency not to run, but more often an openwork metal mount was attached to a sheet of copper foil, the enamel was filled in and allowed to harden, and the copper was dissolved by dipping the piece in acid, leaving only the mount and the enamel.
The work in jewelry was usually executed in a gold setting, but the technique has also been used with pierced porcelain. The method was discovered in the 15th century and used by Benvenuto Cellini and others during the Renaissance, and was reintroduced in France, c. 1900. The German term is Fensteremail (window enamel).
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson