Why can paintings made with bitumen develop a crocodile-skin pattern of cracks?

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Paintings created with bitumen develop a characteristic crocodile-skin pattern of cracks because the pigment never dries completely.1

As the paint remains in a semi-liquid or unstable state, it fails to harden like traditional oil paints. This leads to a surface effect that resembles the texture of crocodile skin. Unfortunately, these defects are considered permanent and cannot be cured; they can only be visually improved through careful, judicious retouching.1