NIR Luminescence and Composition of Egyptian Blue as Markers in Archaeometric Evaluations

The unique NIR luminescence of the ancient pigments Egyptian Blue, Han Blue and Han Purple [1] has recently attracted significant interest because its existence allows fast identification these pigments even when present in minimal amounts and using non-destructive tools [2]. A more comprehensive study of this feature associated also to micro-imaging and compositional evaluations can be used as a mean to improve archaeometric studies about production technologies [3], giving also useful data to hint the provenance of the pigments and their trade routes. The three pigments, and especially Egyptian Blue (EB) were widely used in a large span of years and places. Indeed, EB was invented during the 4th millennium BCE [4] and until Middle Ages has been by far the most used blue pigment in all the Mediterranean Basin and the Near East. It was first produced probably in Egypt from where it has been heavily traded to many Mediterranean countries. However the technology of production evolved in time and spread in different places such as Mesopotamia and Italy [5]. EB invention is related to the development of close related materials such as pottery, bronze and especially glass and Egyptian faience. A large part of Egyptian faience is light blue (LBEF) and its color is to copper LBEF glaze and other glassy materials are intimately linked EB that they can show almost the same color and share a very similar elemental composition. The differences between EB and the outer layer of LBEF may relay mainly on the production technology used. However, from a chemical stand point the main difference is that EB has a structure made of CaCuSi4O10 (cuprorivaite) crystals embedded in an amorphous matrix rich in Na or K, while LBEF glaze and Cu-rich glasses are a glassy phase made of Si, Ca and O, rich in Na and/or K from the