The sulfides cause the chemical reaction of lead carbonate to black lead sulfide. Radiation scattered by molecules contains photons with the same frequency as the incident radiation, but may also contain photons with changed or shifted frequency.
Hydrogen sulfide was abundant at the British Library—where the manuscript is conserved—due to the gas lamps used at the museum during the Victorian age. This effect is very weak—approximately one photon out of a million (0.0001%) will scatter from that sample at a wavelength slightly shifted from the original wavelengths.
It shows that the red color of the gown is vermillion, which also fits, as does the lead white frame.
Examine the face of the angel, though, and the Raman spectra matches it with lead sulfide.
Raman spectroscopy is non-invasive, non-destructive, and fast.
Fiber optics can even be used for remote investigation.
Raman spectrometers are typically coupled with a microscope in order to observe the sample and can take a Raman spectrum from a small area down to 1micron, which is the usual dimension of artists’ pigments.
Spectroscopic examination is the only method that allows conservators and art historians to precisely identify the materials artists used for their works.
This information enables art historians and scholars to understand what artistic materials were available during a certain period in a particular region, and therefore illuminates trade routes and interactions among cultures.
Each line of the spectrum corresponds to a specific vibrational mode of the chemical bonds in the molecule.
Since each type of molecule has its own Raman spectrum, this can be used to characterize molecular structure and identify chemical compounds.
The spectrum of this wavelength-shifted light is called the Raman spectrum.