Porous silica ceramics were obtained at low forming pressure (40–80 MPa) and low sintering temperature (850–1300 °C) for 4 h in air. Boric acid was used as a low-cost additive, in the amount of 2 wt%. Relatively high porosities of nearly 40% and 65% are obtained for the samples of clay and diatomite pressed at 40 MPa, and sintered at 1000 °C, respectively. The samples sintered at 1150 °C and 1300 °C have the average pore size diameters in the range of macroporous for clay 0.2–10 μm and for diatomite 0.2–5 μm. X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and mercury porosimetry measurements were employed to characterize of the obtained samples. Measurements of densities and open porosities by immersion technique were used, according to the Archimedes principle. The relations between mechanical characteristics of the samples formed by using different pressures and sintered at different temperature, were discussed.