The porosity was determined using the most common Archimedian method (Schopper, 1982). It is based on the following three weight determinations of the sample: (1) The samples were dried for approximately 100 hours at 80 °C in an oven until the sample weight remains constant. The pore space is filled with air. (2) The samples were saturated, i.e. the pore space was completely filled with destilled water. Therefore, at room temperature and under vacuum the samples were submerged in destilled water for at least 24 hours. Afterwards atmospheric pressure was applied for at least 24 hours to let the water invade into the sample. The sample was taken from the water and the fluid film on the sample surface was removed. (3) At least, the saturated samples were submerged again in destilled water and the third measurement was done. An apparent mass due to buoyancy was determined.