calcium carbonate is birefringent and the main colours are blue and yellow under retardation (although in the selected particle (1) blue does not show). Note the size distribution with an emphasis on small particles that under the chosen enlargements show as dark ‘dust’. The streaks in the image are due to the fact that the particles are a smear on the slide. Sample courtesy Hugo Geerdinck, Coldenhoven papier, Eerbeek [micrographs and z-stacks BvV].