What is Plique-a-Jour in Jewelry?

First a little word from Wikipedia:

Plique-à-jour (French for "letting in daylight") is a vitreous enamelling technique where the enamel is applied in cells, similar to cloisonné, but with no backing in the final product, so light can shine through the transparent or translucent enamel. It is in effect a miniature version of stained-glass and is considered very challenging technically: high time consumption (up to 4 months per item), with a high failure rate. The technique is similar to that of cloisonné, but using a temporary backing that after firing is dissolved by acid or rubbed away.

This technique is found in very high karat and high-end, unique and handmade pieces though the process is mainly found in French jewelry from the turn of the century.

The technique isn't used so much anymore because it is technically challenging and the skill hasn't been transferred between generations of jewelers. A couple of some luxury houses (Tiffany in jewelery, Bulushoff in jewelery and tableware), Wikipedia says, produce limited numbers of products in plique-à-jour technique.