(_a_) _Pasting or Saponification._--The melted fats and oils are introduced into the soap-pan and boiled by means of open steam with a caustic soda lye 14 deg. to 23.5 deg. Tw. (10 deg. to 15 deg. B.). Whether the fatty matters and alkali are run into the pan simultaneously or separately is immaterial, provided the alkali is not added in sufficient excess to retard the union. The commencement of the saponification is denoted by the formation of an emulsion. Sometimes it is difficult to start the saponification; the presence of soap will often assist this by emulsifying the fat and thus bringing it into intimate contact with the caustic soda solution. When the action has started, caustic soda lye of a greater density, 29 deg. to 33 deg. Tw. (18 deg. to 20 deg. B.), is frequently added, with continued boiling, in small quantities as long as it is being absorbed, which is ascertained by taking out samples from time to time and examining them. There should be no greasiness in the sample, but when pressed between finger and thumb it must be firm and dry. Boiling is continued until the faint caustic taste on applying the cooled sample to the tongue is permanent, when it is ready for "graining out". The pasty mass now consists of the soda salts of the fat (as imperfect soap, probably containing emulsified diglycerides and monoglycerides), together with water, in which is dissolved the glycerine formed by the union of the liberated glyceryl radicle from the fat with the hydroxyl radicle of the caustic soda, and any slight excess of caustic soda and carbonates. The object of the next operation is to separate this water (spent lye) from the soap. (_b_) _Graining Out or Separation._--This is brought about by the use of common salt, in a dry form or in solution as brine, or by caustic soda lye. Whilst the soap is boiling, the salt is spread uniformly over its surface, or brine 40 deg. Tw. (24 deg. B.) is run in, and the whole well boiled together. The soap must be thoroughly boiled after each addition of salt, and care taken that too large a quantity is not added at once. As the soap is gradually thrown out of solution, it loses its smooth transparent appearance, and becomes opaque and granular. When a sample, taken out on a wooden trowel, consists of distinct grains of soap and a liquid portion, which will easily separate, sufficient salt or brine has been added; the boiling is stopped and the spent lye allowed to settle out, whilst the soap remains on the surface as a more or less thick mass. The separated spent lye consists of a solution of common salt, glycerine, and alkaline salts; the preparation of crude glycerine therefrom is considered in chapter ix. The degree of separation of water (spent lye) depends upon the amount of precipitant used. The aim is to obtain a maximum amount of spent lye separated by the use of a minimum quantity of salt.