Egyptian Blue in the Polychromy of the Acropolis Monuments: An Analytical Investigation

ABSTRACT Egyptian blue (EB), considered the first artificial pigment in history, exhibits an intense blue color and chemical stability over time. Throughout antiquity, EB was widely used to decorate architectural structures, sculptures, and artifacts. The aim of this research is to identify and characterize the EB paint layers found on the Parthenon and the Propylaea, used for the painting of entire areas and/or ornamental patterns (e.g., double meander). The paint layers are largely concealed under decay crusts and orange‐brown patina layers, developed over the centuries. For the EB identification, a visible‐induced luminescence imaging technique was used in situ, while μ‐Raman spectroscopy, stereomicroscopy, polarized microscopy, and SEM/EDX analysis were applied in lab scale. The investigation revealed that the EB from both the Parthenon and the Propylaea consists primarily of cuprorivaite, quartz, and calcite, with no evidence of a glassy phase. The pigment's grain size ranges from 15 to 110 μm, with the most common size observed around 50 to 80 μm, which typically provides the dark blue hue on EB. The examined EB demonstrated remarkable durability over millennia, with only a few isolated grains showing slight discoloration. At the Propylaea, the EB paint layer was found to have a thickness ranging from 500 to 750 μm. A thick orange‐brown patina layer, approximately 1.60 mm in thickness, was observed above the EB paint layer on the Parthenon. The patina is composed primarily of Ca, Si, Al, Mg, K, Na, and Fe—elements typically associated with surface deposits and weathering products.