A panel painting which is illuminated by strong lights will show a considerable increase in temperature. As wood is a poor heat-conductor the painted face exposed to the light gets hotter than the reverse of the panel. This leads to unequal thermal expansion between front and back of the panel. However, the thermal expansion is unimportant compared with the shrinkage of the wood through evaporation of moisture. The paint layer acts as a moisture barrier and the wood shrinks faster on the back than on the front, so that the panel shows a convex warp. The opposite is true of an unpainted panel, which would show a concave warp. In addition to these tensions between the front and back of a panel painting, there are also tensions between different areas of the surface. Dark tones show a greater rise in temperature than light tones and these temperature variations only adjust themselves slowly, since wood is a poor heat-conductor. These tensions often relax only slowly once the spot-lighting is over and the tension distribution in the wood then may take on a new equilibrium which is different from the original one due to plastic deformation. If the wood is again spot-lit during this period of relaxation considerably stronger tensions may be set up and these may sometimes even result in splits in the wood. No generally applicable rule can be made as to the period required before a condition of tension set up by illumination is fully relaxed.