Abstract Rheological properties of two ceramic clays (Sındırgı and Küre) having different chemical and mineral compositions were determined. The variation in viscosity for suspensions having 50%, 55% and 60% (w/w) clay was measured as a function of initial loading of sodium silicate. The relationship between shear rate (0–120 s −1) and shear stress (0–350 Pa) was determined for these suspensions in the presence of various amounts of sodium silicate. The effect of the amount of clay on the suspension thixotropy properties was investigated for original and treated suspensions. Slip viscosities of 1500, 500 and 100 cP were evaluated and the suspensions were filtered to obtain cakes. From rheological measurements, thixotropy properties changes were correlated with increased solid and electrolyte in the suspension and the clay mineral type. In addition, results showed that the permeability, the porosity and the casting rate increase with the viscosity in the clay suspensions and the permeability decreases with decreasing porosity. When the clay ratio in the suspensions is 50% (w/w) and the viscosity 1500 cP, the porosity and the permeability values obtained in each sample are higher than the others.