Raman mapping of the S 3 − chromophore in degraded ultramarine blue paints

A specific case of degradation was observed in synthetic ultramarine paint within 3 paintings from the early 20th century, manifesting as an intricate pattern of white lines criss‐crossing the blue paint surface. Raman spectroscopy can be performed directly on untreated sample surfaces and is sensitive to the colour components in ultramarine (S 3 − and S 2 − chromophores). This method was chosen to map the chromophore intensity distribution and to relate it to the surface degradation pattern. Raman signal intensity, however, decreases when the laser is out of focus, providing a challenge when mapping signal intensity across rough or undulating surfaces. To account for this, a series of experiments was conducted to determine the laser and objective combination least sensitive to changes in surface topography. The optimal settings were found to be the 785‐nm excitation wavelength with the 50× long working distance objective (numerical aperture 0.50). This combination gave the smallest focus‐dependent signal decrease on test samples: When shifted 5 μm out of focus above or below the sample surface, the signal from the same spot showed a decrease of 7% only. Maps of the blue (S 3 − ) chromophore signal at 548 cm −1 taken from 3 samples showed a clear decrease in intensity on the degraded white lines. Patterns of signal intensity distribution matched well with the optical degradation pattern. From this observation, in conjunction with previous surface characterisation reported elsewhere, it was concluded that the surface phenomenon was indeed a discolouration of the ultramarine pigment, caused by a reduction in the concentration of the chromophore.