How did Joshua Reynolds's use of bitumen damage some of his paintings?

Joshua Reynolds’s use of bitumen—a brown pigment derived from coal substances—significantly damaged many of his paintings because the material never dries completely.12

This chemical instability caused the paint layers to shrink unevenly, resulting in a surface effect that resembles crocodile skin. Because these bituminous paints remain permanently soft, the resulting damage is considered ineradicable and cannot be cured; it can only be visually improved through careful retouching. This practice was part of a broader trend of individual experimentation in the 18th and 19th centuries that often led to faulty techniques, such as superimposing layers that dried at different rates, further contributing to widespread cracking, or craquelure.1