Changes in appearance due to metal soap formation in a late 19th century painting <i>The Doctor</i> by Luke Fildes were investigated. Preferential saponification of zinc white in lead and zinc white containing paints was found in both light and dark areas. SEM-BSE imaging of an amorphous area of zinc soap suggests that this leads to a reduction in reflectance from the surface. Paint reconstructions demonstrate that saponification of white pigments causes a significant decrease in light reflection. The optical effects of saponification are perceptible, and depend on the colour and hiding power of both the surface and underlying paints. This explains why the appearance of the painting can be altered to differing extents in different areas.