The clinker brick had been imaged previously at the microtomography facility in 1996, at a voxel size of 6.67 µm per pixel . At this resolution, it was difficult to properly resolve many of the slit-like pores present in the material. However, as shown in Figure 10, at a resolution of 0.95 µm, these (dark) pores are clearly visible. A simple thresholding (segmentation) could be used to isolate the porosity in the three-dimensional microstructure and examine percolation and transport properties such as conductivity  and permeability, as was performed previously for the microstructures obtained at the lower resolution . According to physical measurements, the clinker brick has an average porosity of 20 % , so in this case, the threshold greylevel value used to separate solids from pores could be selected so as to closely match this porosity value in the segmented image. While the porosity appears as discontinuous isolated pores in two dimensions (Figure 10), in three dimensions it is percolated. This serves to illustrate the importance of obtaining and analyzing three-dimensional images of a material's microstructure, as opposed to attempting to infer three-dimensional information from two-dimensional images, in agreement with previous conclusions .