The properties determined by these tests are inherent in the brush materials themselves as distinct from performance characteristics in operation on electrical equipment. Values of physical properties published in catalogues, etc., shall be based upon test methods, given in these recommendations. The purpose of the different sections in these recommendations is to provide a uniform procedure for determining the following properties of brush materials: Density and porosity; Hardness; Resistivity; Transverse strength; — Ash content. Some test methods are suitable for use in production quality control, others only for more thorough investigations, using precise laboratory techniques. It should be kept in mind that since brush materials are generally brittle, porous materials, it is reasonable that their properties vary much more than the same properties in metals. Samples used for these tests must be representative of the batch of material involved using a recognized and/or agreed sampling procedure or technique.