describes the flow of liquid through a porous medium. It was formulated by French engineer Henry Darcy in 1856 on the basis of experiments into the passage of water through sand. In a brewing context it is applicable to wort separation and beer filtration, where liquid collection will be fastest through devices of high surface area, small depth, and high hydraulic head (i.e., differential pressure). However, the filter medium is critical—factors influencing permeability include particle size and porosity. Larger particles allow more rapid flow of liquid (less liquid “hold-up”). Less viscous liquids will flow more rapidly.

The designs of the lauter tun and mash filter are very much in accord with Darcy’s law. Lauter tuns are very broad with shallow bed depth and depend on husk with minimal damage (c.f., large porosity) and worts that are not excessively viscous. The mash filter does not depend on malt husks, whose function is replaced by the sheets, but the latter cumulatively equate to a very large surface area and the distance between the plates (equivalent to bed depth) is exceedingly small. Furthermore, in a mash filter there is the facility to compress the bed by applying pressure and squeezing out the wort.

See also filtration, lauter tun, and mash filter.