  Cadmium sulfide (CdS) pigments have degraded in several well-known paintings,
but the mechanisms of degradation have yet to be fully understood. Traditional
non-destructive analysis techniques primarily focus on macroscopic degradation,
whereas microscopic information is typically obtained with invasive techniques
that require sample removal. Here, we demonstrate the use of pump-probe
microscopy to nondestructively visualize the three-dimensional structure and
degradation progress of CdS pigments in oil paints. CdS pigments, reproduced
following historical synthesis methods, were artificially aged by exposure to
high relative humidity (RH) and ultraviolet (UV) light. Pump-probe microscopy
was applied to track the degradation progress in single grains, and volumetric
imaging revealed early CdS degradation of small particles and on the surface of
large particles. This indicates that the particle dimension influences the
extent and evolution of degradation of historical CdS. In addition, the
pump-probe signal decrease in degraded CdS is observable before visible changes
to the eye, demonstrating that pump-probe microscopy is a promising tool to
detect early-stage degradation in artworks. The observed degradation by
pump-probe microscopy occurred through the conversion from CdS into CdSO4.xH2O,
verified by both FTIR (Fourier-transform infrared) and XPS (X-ray photoelectron
spectroscopy) experiment.
