The craquelure on a painting is the network, or pattern, of cracks that develops across the surface as the paint layers age and shrink.
Easel paintings in most types of paint, particularly egg tempera and oil, develop cracks which join up into a complex, extensive network.
Most pictures of any age show craquelure everywhere at the surface. It is also usual for the cracking to extend into the varnish. The craquelure on a picture can be very disfiguring, particularly when cracks in light-coloured paint layers become filled in with dirt.