We often used this figure's face as a teaching tool to explain the degraded varnish and why the coating needs to be removed and replaced with a more stable coating.
The proper left side of the woman's face appears to suffer major paint loss, but it is actually a degraded coating that is obscuring the paint layer underneath.
When wet up with mineral spirits or a similar non-polar solvent that acts like a temporary varnish, the cracks in the degraded coating are filled and reflect light so that the paint layer is visible.