The filtration of oils may be effected in a very great variety of ways, either with or without the assistance of artificial pressure derived from (a) a "head" of the liquor to be filtered, (A) one of the many forms of filter-press in use, or (c) atmospheric pressure by the production of a vacuum under the filter-bed. For example, olive oil is mostly subjected to no process of purification beyond what is attained by allowing it to deposit impurities and repeatedly decanting. But, for the best qualities, further purification is necessary not only to secure limpidity, but a capacity for lengthened preservation by eliminating the water, mucilage, and parenchymatous matters. Various devices are employed in different localities, one and all being filters. In France, the oil to be purified is received into perforated boxes carpeted with carded cotton (wadding); elsewhere, cotton tissue interposed between beds of granular and washed animal charcoal form the filter; a bed of dry moss, on the "Grouvelle et Jaunez" system; layers of sand, gypsum, and coke; alternate beds of sand and vegetable charcoal, according to Denis de Montfort's plan; carbonised schist and peat, by Cossus' method; clay heated to 200° (?F.), as proposed by Wright; by introducing china-clay and allowing to stand at a moderate temperature, then filtering through cotton, as adopted by A. Pizarri. Perhaps the best mode is that of Pietro Isnardi, of Livornia, Tuscany, which received an award at the Vienna Exhibition. This apparatus, Fig. 173, consists of a boiler full of water, serving as a water bath for 2 turned-iron cylinders b, receiving the oil from the reservoir c, a suction- and force-pump d, and a filter e, containing perforated trays whose holes are filled with wadding.
This apparatus enables the oil to be filtered without coming into contact with the air, and at an elevated temperature which can be regularly maintained. Coco-nut oil is another example of purification by simple subsidence and filtration.