Consisting of the semi-permeable paper barrier that is used with liquid or airflow to separate fine substances from either liquids or air, filter paper is available in a wide variety of porosities and grades depending on the intended application. Parameters for filter paper include wet strength, particle retention,porosity, efficiency, compatibility, and capacity. Functioning as a result of the dual mechanisms of filtration (volume and surface), particles are either caught in the bulk of the filter paper as the medium in which they are suspended passes through, or by trapping particles on the surface of the paper. Filter paper is preferred because even a small piece will absorb large volumes of liquid. Raw materials for filter paper include various paper pulps from softwood, hardwood, fiber crops, and mineral fibers. The highest quality filters use dissolving pulp and mercerised pulp for maximum efficiency. The majority of filter papers are made on small paper machines, and for producing laboratory filters the machines may be as small as 50 cm. In width.