Certain filter aids may be used to aid filtration. These are often incompressible diatomaceous earth or kieselguhr, which is composed primarily of silica. Also used are wood cellulose and other inert porous solids. These filter aids can be used in two different ways. They can be used as a pre coat before the slurry is filtered. This will prevent gelatinous-type solids from plugging the filter medium and also give a clearer filtrate. They can also be added to the slurry before filtration. This increases the porosity of the cake and reduces resistance of the cake during filtration. In a rotary filter, the filter aid may be applied as a pre coat; subsequently, thin slices of this layer are sliced off with the cake. The use of filter aids is usually limited to cases where the cake is discarded or where the precipitate can be separated chemically from the filter. Filtration is a more efficient method for the separation of mixtures than decantation, but is much more time consuming. If very small amounts of solution are involved, most of the solution may be soaked up by the filter medium. An alternative to filtration is centrifugation - instead of filtering the mixture of solid and liquid particles, the mixture is centrifuged to force the (usually) denser solid to the bottom, where it often forms a firm cake. The liquid above can then be decanted. This method is especially useful for separating solids which do not filter well, such as gelatinous or fine particles. These solids can clog or pass through the filter, respectively. Filter aids are used to assist filtration of colloids, extra fine particles, gel like, and highly compact materials, which are difficult to be filtered due to low filtrate rate, unacceptable filtrate clarity, high cake moisture content, or serious filter medium clogging. They can be used as "precoat" on the surface of filter media or in conjunction with feeding suspension as "body feed" (or "admix"), or a combination of "precoat" and "body feed".