Relative densities between 0.93 and 0.96 were found. The increasing relative density from 2 to 101C min−1 is due to the surface crystallization of the frits. However, the decreasing relative density from 10 to 251C min−1 may be due to the thermal gradient formed between the surface and the bulk of the sample. This gradient is higher at 251C min−1, resulting in greater porosity in the bulk. This effect is more significant when the holding time is decreased. The higher relative densities observed for 2 and 101C min−1 at 120 min in relation to 360 min might be caused by the appearance of porosity induced by crystallization. Other authors also observed the reduction of the relative density due to the crystallization of a wollastonite system[14,27].