Paint Is Colored Glue
The polymerization takes time, though, since oxygen has to diffuse through the top, hard layers of paint before it can get to the unreacted oil underneath. They overlaid many thin layers of oil paint, which one by one chemically reacted with oxygen and hardened, building up a number of layers of semitransparent plastic, one on top of another, a complex encasing of many differently colored pigments. Analysis of Titian’s painting Resurrection reveals nine layers of oil paint, all working to create complex visual effects.
Source: delanceyplace.com