Describes the physical properties of paint layers. The optics of the paint layers include: the refractive index, the constituents of the paint layers (pigments, dyes, extenders, binders, additives), the electronic absorption and transition, the interaction light-material, the interaction of paint layers, the influence of particle-size of extenders and pigments. The mechanics of the paint layers include thickness and hardness. Research results on Roman mural paintings from Lero, France, are given as examples.