Application example: Revealing colour compositions in ancient Egyptian polychromatic wall paintings

Foto: Thutmosis III Grab ©Copyright: public domain

Thutmosis III tomb

Source: public domain

Ancient Egyptians decorated the monuments for members of their ruling classes with colourful wall paintings. The oldest of these wall paintings are more than 4000 years old and have been subject to degradation and discoloration. Therefore, optical inspection is often insufficient to assess the original choice of colours for purposes of art history or conservation and restoration. However, by means of the ion beam analysis method PIXE (proton-induced X-ray emission) the pigments used in the paints can be identified unambiguously. Due to mechanical deterioration of the wall painting over time numerous fragments are available which can be transported for examination at ion beam facilities. The investigation of paint fragments from the tomb of Tuthmosis III and from the mortuary temple of Ramesses III yielded the identification of pigments and further instructive insights on the processes connected with wall painting in ancient Egypt:

The yellow paint in fragments from the tomb of Thutmosis III contained mostly the common pigment yellow ochre (goethite). However, it was augmented by adding a thin top layer of paint containing the pigment orpiment (yellow arsenic blende), which provided a shiny golden glaze. Orpiment had to be imported to Egypt and therefore was a rare and expensive material, its use being a royal privilege.
Blue paint samples originate from the great statue of Ramesses III in his mortuary temple near modern day Luxor. Based on the silicon content of the sample, the blue pigment was identified as Egyptian Blue (cuprorivaite) – the oldest known synthetic pigment. Surprisingly, the sample also contained a significant amount of haematite. This hints at some improvisation on the part of the artist who painted the statue: Using too high temperatures in the synthesis of Egyptian Blue results in a lighter blue-greenish colour. In order to make use of the pigment nevertheless, the painter apparently added haematite to achieve the desired dark blue hue.
Green paint fragments were collected from the tomb of Thutmosis III. In his dynasty, the most commonly used green pigment was Green Frit (wollastonite). This pigment was also used in these paint fragments, but they contained haematite in addition. The synthesis of Green Frit requires precise control of temperature and of the ratio of components. In this case, too, the painter was apparently not satisfied with the resulting pigment hue and added haematite to darken it.

Due to difficulties in controlling the synthesis conditions, synthetic pigment colours were not absolutely reproducible, leading painters to blend them with other pigments for the intended effect. Other effects were obtained by layering different paints. This produced complex material compositions, which are not necessarily apparent to the naked eye, but can be revealed by ion beam analysis.