Characterization of an unusual coating on funerary portraits from Roman Egypt circa 100-300AD

Abstract

This paper details the investigation of a discrete coating observed on a group of Egyptian panel paintings, six mummy portraits and one funerary panel, dating from first-third century CE. Six mummy portraits in this group are encaustic, and the funerary panel is tempera using an animal glue binder. An accretion or coating has been observed on the surface and recesses of the paint layers on these panels. Examination of the portraits using ultraviolet radiation revealed an irregular visible fluorescence on the surface. On the mummy portraits, the fluorescence often extends only as far as where the linen wrappings would have secured the portrait to its mummy. Under magnification, the coating appears as a crizzled encrustation. Material exhibiting these characteristics was sampled from the surface of all seven panels. Initial analysis of samples from four panels by gas chromatography mass spectrometry (GC/MS) and enzyme-linked immunoassay (ELISA) revealed the presence of egg. Subsequent analysis of the coating from all seven portraits by peptide mass fingerprinting (PMF) and liquid chromatography with tandem mass spectrometry (LCMSMS) confirmed egg and further characterized the coating as highly deamidated, whole hen egg, or hen egg white in one instance. Importantly, the 14C date of the coating from two portraits indicates the time of application as approximately 2000 years ago, implying that the coating, at least in those cases, is not a modern addition. This report summarizes the examination and analytical characterization of this unusual coating. Possibly applied as an aesthetic or protective layer, or a symbolic and ritual unguent, the principal function of this coating remains unknown.