Multi-analytical approach for identification of early artificial ultramarine, made by Raimondo di Sangro, in the blue frame on the altar of the Sansevero Chapel Museum in Naples

The history of ultramarine, a vivid blue pigment derived from lapis lazuli, spans thousands of years. During the 16th to 18th centuries the demand for ultramarine remained high, but its production was limited to specific regions and was extremely expensive. It was used sparingly by artists due to its cost, with some opting for cheaper alternatives such as azurite. In the first half of the 19th century, advancements in chemical knowledge allowed the production of artificial ultramarine, which then spread throughout Europe. The identification of natural versus artificial ultramarine has become a challenge. Here we present analytical results signifying that Raimondo di Sangro (Prince of Sansevero) produced artificial ultramarine before the well-known registered invention by Jean Baptiste Guimet. A detailed study of the chemical composition by SEM-EDS and TEM EDS, along with the absence of accessory minerals commonly found in natural pigment and the presence of uncommon phases, revealed the synthetic origin of the pigment found in the blue frame on the high altar in Sansevero Chapel Museum (Naples, Italy). Moreover, the reflectance spectra disclose an uncommon shift in the main band of the pigment, distinguishing it from both natural ultramarine and more recent synthetic versions. Further, a deep comparison with previous literature data also strengthens our experimental evidence. These results open a new perspective about the first production of this artificial blue pigment.