At this step of the thought process, it appears that a two-, three- or four-step sintering mechanism occurs, depending on the initial grain shape of the powder. The two-step mechanism corresponds only to non-agglomerated spheres, whereas the three-step mechanism consist in particles forming spheres, but developing preferential interactions along many different directions. These systems can be obtained after the calcination of the particles presenting large shape factors as Lange showed. Finally, the four-step sintering mechanism is related to all other geometries which are far away from the ideal case of spheres. Then, if all other parameters are fixed such as time and temperature of sintering, the evolution of relative density and grain size can be predicted in relation with the initial grain powder morphology.