Effect of NaCl on the fabrication of the Egyptian blue pigment

Abstract

The effect of the presence of NaCl on the synthesis of Egyptian blue pigment (cuprorivaite, CaCuSi4O10) was studied through experiments in which different amounts of NaCl were introduced in the initial mixture of reactants. The solids synthesized were characterized by X-ray Diffraction, Field Emission Scanning Electron Microscopy (FE-SEM) coupled to Energy-Dispersive Spectrometry (EDS) and High Resolution Transmission Electron Microscopy (HR-TEM) coupled to EDS. The main result of the experiments was that Egyptian blue formation was inhibited in the presence of NaCl, actually, almost no cuprorivaite was found in >10% NaCl experiments and the solids synthesized in NaCl were always green instead of the characteristic blue of the Egyptian blue pigment. The solids synthesized in the presence of NaCl, a mixture of wollastonite (CaSiO3) and a Cu-rich amorphous phase, probably corresponded to the composition of the pigment known as Egyptian green. Considering the usual presence of NaCl on some of the raw materials used for the pigment fabrication, Egyptian artisans had to be very careful on selecting the reactants for the fabrication of the pigment (quartz would be much more suitable than Egyptian sand, and ash plants than natron) or they incorporated a process of purification of the reactants in order to eliminate chlorides prior to the synthesis.