  To unravel how the microstructure affects the fracture surface roughness in
heterogeneous brittle solids like rocks or ceramics, we characterized the
roughness statistics of post-mortem fracture surfaces in home-made materials of
adjustable microstructure length-scale and porosity, obtained by sintering
monodisperse polystyrene beads. Beyond the characteristic size of disorder, the
roughness profiles are found to exhibit self-affine scaling features evolving
with porosity. Starting from a null value and increasing the porosity, we
quantitatively modify the self-affine scaling properties from anisotropic (at
low porosity) to isotropic (for porosity larger than 10 %).
