Émail en blanc, French for literally, white enamel. Enamelling by the use of only white enamel; it was the usual style of émail en ronde bosse. It was often applied over figures in the round placed over a gold ground or polychrome enamelling or close to gemstones. It was a style characteristic of the 14th-16th centuries, and was often used on brooches.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson