This grade is used for the filtration of coarse particles and gelatinous precipitates such as ferric aluminum and magnesium hydroxides. Ahlstrom 631 is used in air monitoring and gas filtration when high flow rate is needed. This grade is also found in routine cleanup analysis of insoluble liquids such as biological fluids and organic elements.

Standard qualitative filter papers are suitable for quadrant folded or gravity flow applications. These grades contain a high level of alpha-cellulose that provides high purity but will weaken when wet. The inherent strength of standard (untreated) grades does not pose a problem when they are used in routine quadrant folded applications. We do not recommend these grades for use in vacuum work or when wet handling is required.