Niello

A soft, black alloy of sulfur with copper (copper sulphide), gold (gold sulphide), silver (silver sulphide), or lead (lead sulphide), used to ornament metal objects. Designs incised on the objects are filled with the alloy (usually as a powder mixed with flux), which is then fused with the metal by application of heat. Also, to decorate using this process, or the objects decorated with it. Chiefly a late medieval and early Renaissance process.