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UV Visible Luminescence Image with ultraviolet radiation from UV Systems TripleBright II LW370 Lamp, long wave, UV peak: 368 nm.

During examination with scanning electron microscope, Hitachi TM3000 SEM, both samples showed high-density, opaque particle concentrations in the opaque paint layers of white and red particles. Further analysis with Electron Dispersive Spectroscopy using Bruker Xflash MIN SVE Quantax 70 software, detected high presence of both calcium and lead in both samples. Elemental mapping of the software showed that calcium was concentrated in the ground layer, while lead was found in both ground and paint layers. The locations of lead also overlapped with mapped locations of sulfur and it was decided that point analysis would be needed to confirm pigment identification.

Technical Study

This technical study was performed in conjunction with my treatment of Saint Catherine of Alexandria and Saint John the Baptist. To understand the materials and conditions of each painting, technical analyses were performed non-invasively on the surface, and selected samples were removed for further study. This analysis was supervised by Marco Leona, David H. Koch Scientist in Charge at the Metropolitan Museum of Art.  

During ultraviolet visible luminescence imaging, both panels showed strong luminescence of their red robes. This evidence suggested a tentative identification of the red pigment as a lake dye. After a preliminary study with point analysis by XRF, two samples were removed from existing losses in each panel and embedded to view a cross-section of each paint layer. Sample 1 was removed from a loss on the right edge of Saint John’s robe. Sample 2 was removed from a loss in the dark glaze on the left side of Saint Catherine’s robe. Cross sections prepared from each sample were observed with visible and UV light, and scanning electron microscopy.

In both samples, ground layers are followed by alternating structures of opaque paint layers with varying amounts of fine white pigment and a larger red pigment and deep red translucent glazes.

Sample 1; 20X magnification 

Sample 2; 20X magnification 

Top: Sample 2 viewed in SEM at 1200X Below: EDS mapping of calcium in red and lead in blue.

Raman spectrum from sample 2 with baseline correction. Sample in Magenta and Green, UCL reference spectrum in Blue

Raman spectroscopy point analysis

Raman spectroscopic studies were performed using the Renishaw Raman System 1000, featuring a Leica DM LM microscope and 785-nm laser to analyze single particles. These analyses were able to confirm the sole presence of gypsum, calcium sulfate, in the ground layer. It also detected the presence of lead white and vermillion particles mixed into the opaque pink paint layers. Raman analysis could not identify the component of the red glaze layers.

A Surface Enhanced Raman spectroscopy (SERS) technique was performed with a loose scraping from the red glaze layer. Spectra from the test were inconclusive and lacked the strong signal expected from the purpurin of madder lake dye. The sample did not show any color change during a pre-treatment exposure to hydrogen flouride, as expected with madder lake. Though the test did not lead to clear results, it points to greater probability that a different dye compound such as carmine, was used. 

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