Uniaxial hot pressing of amorphous, α- and β-rhombohedral boron was performed. Graphical data representations of hot pressing kinetics have been used to disentangle the various phenomena occuring during this operation. For the first pressing stage, while the temperature increases, various impurities -- such as the dissociation of H3BO3 for oxidized boron -- or phase changes were detected. It was found that the transformation of α-rhombohedral to β-rhombohedral boron occurs at the same temperature for all pressures, in spite of a change in volume as the theoretical density decreases from 2.45 to 2.35. Final-stage sintering occurs above 1600 °C always on the same β phase, which is the only stable polymorph of boron at those temperatures, but the sinterability varies greatly with the product origin, phase composition (α-, β-rhombohedral or amorphous) or impurity content.