Improving the use of microscopy in the study of weathering: Sampling issues

Many microscopic techniques are now routinely used in the study of weathering of rocks, minerals and building materials. Such techniques permit ever more detailed observations of the results of weathering, but in many cases authors using such techniques do not make clear how representative their microscope observations are. A case study on limestone tablets from the British National Materials Exposure Programme (NMEP) addresses three important sampling issues based on SEM observations, i.e. how many points need to be observed to characterise the weathering features on a millimetre to centimetre sized SEM sample?, how can such a sample of points be obtained in an unbiaised but quick way?, and finally, how many SEM samples should be studied in order to characterise the weathering experienced by the whole tablet? The results of 1130 observations show 30 points to be adequate, the operator selected co-ordinates (OSC) method to provide a quick, effective sample, and that 1 small SEM sample can adequately characterise the whole of one 50 x 50 mm face of an NMEP tablet. Such pilot studies on sampling design should be carried out at the start of all weathering projects involving up-scaling of information from microscope observations.