the espresso crema effect is an analogue model for superficial material alteration. Alteration processes such as weathering can influence the physical character and chemical composition of matter near the surface, without affecting the bulk medium beneath. Increases in porosity can increase light refraction, diffuse reflection and scattering, resulting in a brightening of the material's surface. Besides the physical changes, chemical differences compared to the bulk medium may be involved. Because of increased gas–solid interfaces which result, for example due to selective leaching processes during burial, surfaces of ancient ceramics can appear brighter than they originally were. This has to be taken into account when categorizing and discussing archaeological ceramics.