Varnish removal also revealed fingerprints along the edges, probably left when the canvas was moved while the paint was still wet (fig. 2). After old retouching was removed, other impressions in the paint became more clearly visible. These were probably left when the painting accidentally leant against something.
The paint surface itself contains numerous dust particles that became embedded before the paint had dried. All these traces shed light on Sargent’s working process, suggesting he worked quickly and handled the painting before allowing it to dry.
Research
The dyers depicted in the painting used organic indigo to dye textiles. Obtaining this dye was a time-consuming process requiring specialist knowledge. In the last quarter of the 19th century, synthetic indigo was also developed and subsequently found its way into the paint industry.
During treatment, this raised the question of which blue paint Sargent had used for the colour that plays such an important role in the composition. Could he have used synthetic indigo? And if not, which blue did he use?
Tubes labelled ‘indigo’ have been found in one of Sargent’s paint boxes, but it is not known what they actually contain or whether he took this colour with him on his trip to Egypt. To analyse the blue in Egyptian Indigo Dyers, the painting was examined using scanning macro X-Ray fluorescence (MA-XRF).
This technique detects a number of chemical elements that can provide an indication of the pigments present in the paint. The results show that Sargent at least used cobalt blue and ultramarine blue in this painting.
The ground was found to consist primarily of white lead. Two types of organic red were also used, as well as umber, ochres, bone or ivory black, an unidentified purple and chromium oxide green. This range of pigments is consistent with previous research into Sargent’s use of colour but may not provide a complete picture. Purple pigment grains were also observed in the painting using a stereomicroscope.
Other analytical methods would be needed to identify these and to confirm or rule out the presence of other possible blue pigments, such as indigo and Prussian blue. For the time being, it was decided not to pursue these further analyses.
The MA-XRF scan did, however, yield another remarkable discovery. The image produced by the scanner revealed an unusual shape in the distribution of mercury (Hg) (fig. 3). The only pigment containing mercury is the red pigment vermilion, but this colour does not occur in the composition of Egyptian Indigo Dyers. Rotating the scan by 180 degrees revealed that the unusual shape consists of two legs and feet, with part of a hand visible at the top (fig. 4).