- Filter paper is used to filter separate particles out of a fluid or. It is made out of a flat sheet of material (usually, but not always, wood or vegetable pulp) with tiny pores in it. The pores allow a liquid or gas to flow through, but are too small for larger particles to traverse.
- One of the most important properties of filter paper is porosity. The filter paper needs to have small enough holes to catch the smallest particles it is rated for, while letting fluid flow in and out. A tea bag filter will be extremely porous. It only needs to stop tea leaves from flowing through, so it is made with large holes to allow as much water circulation as possible. A piece of filter paper used in a chemistry lab, on the other hand, might have to catch extremely small particles. As a result, it would have much smaller holes to stop them from flowing through.
- Another important quality of filter paper is capacity. As fluids flow through filter paper, dissolved particles tend to block up the holes, slowing the flow and eventually using up the filter paper. There are many different strategies used to maximize capacity. Some air filters, for example, use a large sheet of paper that fans back and forth in an accordion fold. This increases the surface level, allowing much more space for air to flow through. There are also some paper filters that use fairly large pores scattered through many layers in a sponge-like arrangement. As fluid flows through, turbulence makes most dissolved particles collide and stick to one part of the filter or another. Because there are so many paths through the filter, it can absorb a lot of dissolved particles before it is exhausted.
- Some filter papers are used in highly specialized circumstances, different materials or designs. Oil filters, for example, are specially treated so that they can filter hot oil without breaking down. Some high-quality laboratory filters are designed so that they will not react with chemicals that pass through them. They are made out of glass, carbon or quartz fiber because these materials are inert while ordinary paper is not.
