The Blackening of Paint Containing Egyptian Blue

On ancient Egyptian artifacts, paint containing Egyptian blue pigment is often found to have become brownish green or even black. Darkened paints from several coffins have been investigated using gas-chromatography/mass-spectrometry (GC/MS) and other methods, to identify the cause of the discoloration. Different phenomena have been seen: one type of discoloration involved the browning of gum arabic throughout the paint layer; other examples showed surface discoloration in the varnish, the paint underneath being still bright blue. Surface dirt also contributes to darkening. Artificial aging of paints demonstrated that gum arabic binder can discolour sufficiently for paint to appear black. The poor hiding power and transparency of Egyptian blue contribute to the overall dark appearance.