niello

Black substance made by melting powdered silver, copper, sulphur, and often borax. It is used as a filling for incised decoration on silver and fixed by the application of heat.

Niello was used to decorate objects in ancient Egypt, in the Bronze Age Aegean, in the European Middle Ages, and in much Anglo-Saxon metalwork. It reached its height of technical and artistic excellence in the early Renaissance in Italy, especially in Florence. It was much used in 19th-century Russia, where it is known as tula work.