Buchner filter funnels have relatively large holes. When working with powerful vacuum pumps, the pressure can punch holes in even double layered heavy duty filter paper. Filter fabric is used to underlie the filter paper, providing both support and the ability to catch particles of precious metal that pass either through or around the filter paper. Shor filter fabrics are plastic based and hold up well under acids for repeated use and washings. Comes standard in 25 micron size but is also available in several different micron sizes. See below for sizes and prices. Filter fabrics are cut to order and, as such, are not returnable.