A study of 18th century Coptic icons of Ibrahim Al-Nasekh using Raman microscopy and gas chromatography–mass spectrometry: Indigo as an organic pigment in Egyptian panel paintings

Two Coptic icons were studied, these date from the 18th century and were painted by IbrahimAl-Nasekh and are currently to be found in Saint Mercurius Church, Saint Mercurius Monastery in Old Cairo, Egypt. The analytical techniques used were Raman microscopy, optical microscopy and gas chromatography–mass spectrometry in order to determine the stratigraphy of the artworks, to identify the pigments used and to determine the type of protective varnishes applied. Along with commonly used pigments in Egyptian artefacts such as orpiment (As2S3), vermilion (HgS), red lead (Pb3O4), white lead (2PbCO3·Pb(OH)2) and lamp black (C), the pigment indigo (C16H10N2O2) has been detected for the first time in Egyptian panel paintings. Gypsum (CaSO4·2H2O) was used as a white ground layer and the protective varnish applied is found to be Pinaceae resin.