Filter Aids The objective of filter aid is to prevent the medium from becoming blocked and to form an open, porous cake, hence, reducing the resistance to flow of the filtrate. Filter aid forms a surface deposit which screens out the solids and also prevents the plugging of supporting filter medium. Characteristics of filter aids: Chemically inert and free from impurities. Low specific gravity, so remain suspended in liquids. Porous rather than dense, so that pervious cake can be formed. Recoverable. Disadvantages: Remove the coloured substances by absorbing them. Sometimes active principles such as alkaloids are absorbed on filter aid. Rarely, filters are source of contamination such as soluble iron salts, which can provoke degradation of sensitive ingredient.