Cracks in oil paintings, which appear in an "alligator" design of fine lines, happen when the ‘fat over lean’ rule has not been observed. In the layering technique, the ‘fat over lean’ rule requires each paint layer to have more oil content than the previous layer. A higher oil content results in a more elastic (i.e., flexible) paint film. If the bottom layer is more elastic than the top layer, movements in the bottom layer will cause the top layer to crack. Additionally, oil paint needs oxygen and ultraviolet light to oxidise and form a paint film. A leaner top layer will dry first and block oxygen from getting through to the bottom layer. The lack of oxygen will prevent the bottom layer from drying properly, which means it will be more elastic than the top layer.
These are some of the reasons why the 'fat over lean' rule might have been offended. First, too much oil has been added to the bottom layer during painting giving it a higher oil content than the top layer. Second, too much solvent has been used to dilute the paint on the top layer making it leaner than the bottom layer. Third, too much drier has been added to the top layer, making it dry faster and form a paint film before the bottom layer. Fourth, the bottom layer has sucked oil from the top layer, causing the latter to dry faster. In this regard, it’s worth noting that some full-strength oil colours like lampblack, ivory black, umber, and viridian have higher oil absorption than other oil colours. The more absorbent oil colours should not be used as a bottom layer without dilution or increasing the oil content in the top layer to counter absorption during drying.
Artists are not advised to use materials (either on their own or mixed with other ingredients) that are inherently prone to cracking. For example, the natural resin Copal was once a popular ingredient for making high gloss mediums (i.e., enamel-like glazes). The popularity of Copal declined when it was found that it aged poorly and the paint film was brittle and prone to cracking. These days, manufactured Copal mediums have been formulated to address this concern.
Variations in temperature can cause cracking in a concentric circle that goes all the way through to the ground. This usually happens when paintings are kept or stored in places where they are subjected to extreme cold. Again, paintings, where Copal has been used in varnishes, are more susceptible to cracking due to its rigidity. Hard supports like wood and panel suffer more than flexible supports such as canvas when it comes to variation in temperature.
A canvas that is flexed too much will result in cracking of the paint film. This happens when the auxiliary support such as stretcher bars warp; it may also happen when paintings made on an unstretched canvas is rolled or improperly stretched. A painting on an unstretched canvas should not subsequently be stretched tightly. Further, a painting made on unstretched canvas should (whenever practicable) be stored flat. If rolled, the painted surface should face out and the diameter of the roll should be as large as possible. Finally, a faulty ground (either one that is not properly sized or gessoed) will also result in cracking.