Clarification is the process of separating from liquids, without making use of strainers or filters, solid substances which interfere with transparency. The principal methods of clarification are: (1) by the application of heat; and (2) through the use of gelatin and other substances. Boiling facilitates the separation, since the minute bubbles of steam adhere to the particles and rise with them to form scum, which may be skimmed off. This process takes place when milk is heated and the albumin rises to the top. If albumin be added and heat applied to a turbid ("milky") liquid, the albumin will, on coagulating, envelop the particles and rise to the top with them. Acids may be used to precipitate the casein (white curd) of milk, and the precipitated casein will carry with it the insoluble particles. If a cloudy liquid be agitated with paper pulp and then allowed to stand, it will gradually become clear.