SEM–EDS and μ-XRD study of the niello inlays of a unique late Roman silver augur staff (lituus) from Brigetio, Pannonia (Hungary)

Niello, a bluish black metal sulphide, was widely used for decoration of metal objects by artisans of the Roman Empire. Our knowledge, however, concerning the exact Roman-period niello technique, the appearance of binary silver–copper sulphide niello-decorated silver objects in particular is very scarce. An extensively nielloed silver augur staff (lituus), a unique find from the territory of the Roman Empire, was analysed non-destructively with SEM–EDS and micro-X-ray diffraction techniques. Five niello types were identified, their chemical compositions range from silver sulphide (Ag2S, acanthite) to binary silver–copper sulphide of Ag:Cu ratio 1:1 (stromeyerite). The augur staff decorated with such heterogeneous niello inlays is the first object ever analysed in this manner. Mineralogical and archaeological arguments link niello heterogeneity to the primary production of the object rather than to any post-production repair. Texture-related mineral phase observations allow closer insight into Roman niello technology. The variable copper content is not to be attributed to some technological innovation. The silversmith simply employed not only silver, but in order to make up for the shortage of silver, also differently debased silver, possibly scrap materials for producing niello. Both production (260–280s AD) and burial (early fourth century AD) periods of the lituus are well-dated, and the object was excavated from an undisturbed sarcophagus. Therefore, our data imply that silver–copper sulphide niello reaching the composition of stromeyerite was used by the Roman craftsmen 200 years earlier than previously studied finds had indicated.