Top panel: average T_d as a function of redshift z. Middle panel: average β_d as a function of redshift z. There is a clear evolution between the low-z (z< 1.5) , high-temperature and low-β and the high-z, low-temperature and high-β QSO populations. Bottom panel: average dust (combination describing the degeneracy line) as a function of redshift z. There is no evidence for evolution of the dust properties between z = 0 and 1.5. The product increases between z = 1.5 and 4.