The raw materials are different paper pulps. The pulp may be from softwood, hardwood, fiber crops, mineral fibers. For high quality filters,  dissolving pulp and mercerised pulp are used. Most filter papers are made using small paper machines. For laboratory filters, the machines may be as small as 50 cm in width. The paper is often crêped to improve porosity. The filter papers may also be treated with reagents or impregnation to get the right properties.