Screening is a process that uses a media, such as mesh or a perforated plate, to separate a material based on particle size. Other techniques are used to separate particles based on characteristics such as density or shape. While screening is often the most accurate, economical, and common separation technique, size separation can also be achieved using other processes such as air classification or flotation. These latter techniques exploit differences in aerodynamic or hydrodynamic characteristics of different particle sizes to effect a separation. They are good choices for separating very fine materials (<40 microns) that present problems (e.g., screen blinding, high screen area requirements) for conventional mechanical screening. They cannot, however, provide a sharp separation, so serious compromises on product quality and separation efficiency must be made. Hence conventional dry screening is often the preferred method of separation.