Soap by the Cold Process. Although the commoner kinds of soap are usually made by boiling, they can be made by the cold process if desired; and the fatty substances employed are substantially the same in both methods. The cold or little-pan process is, however, almost exclusively adopted in the manufacture of fancy or toilet soaps, and for these purposes the fat requires to be purified and deodorized, especially where any delicate scent is to be used in perfuming it. (-See Nos. 533 (To Purify Tallow and Other Fats) and 530.) The lye employed for saponification without boiling must be much stronger than that used in the boiling process, and should be entirely clear and colorless; a strength of about 36° Baum'e is usually necessary.

583. To Make Soap by the Cold Process

583.    To Make Soap by the Cold Process. Incorporate by degrees 50 pounds concentrated caustic lye of 36° Baum'e, into 100 pounds fat at a temperature not higher than 104° Fahr. (see No. 523 (Beef Tallow)); continue to stir thoroughly with a broad wooden paddle, until a complete ring can be drawn on its surface with the paddle. In making scented soap, the perfuming ingredients must now bo stirred in. The paste is then run into frames lined with linen, flaps of which should be left above the edges of each frame, wide enough to admit of their being laid over the surface of the paste, with which the frame must be entirely filled. The paste being thus completely confined by the linen, the frames are closed with a wooden cover and left for 12 hours. During this interval the temperature of the paste in the frames rises spontaneously to a much higher degree, producing complete saponification. The soap is afterwards taken out of the frames, cut, and dried. The hardness of the soap will depend on the description of fats and lyes used. (See No. 521 (Hard and Soft Soap).)

584. Method of Testing Caustic Alkali

584.      Method of Testing Caustic Alkali. The strength and practical value of commercial caustic soda or potash can only be ascertained by analysis. The methods given below are simple, and will determine, with sufficient accuracy, the percentage of water, caustic alkali, and carbonated alkali contained in a given sample; and hence the quantity of impurity, if any.

585. To Find the Percentage of Water in a Caustic Soda or Potash

585.    To Find the Percentage of Water in a Caustic Soda or Potash. Weigh carefully 100 grains of the alkali into a capsule (a flat evaporating dish of suitable size, a watch glass is a small capsule), and dry them by heating over a flame; a cold glass held over the contents of the capsule will show the slightest evaporation of water. When no more moisture can be detected, allow them to cool; then weigh the residue in the capsule, and the difference of the weights before and after drying will be the number of grains of water contained in 100 grains of the alkali; that is, the percentage of water.

586. To Estimate the Percentage of Caustic Alkali in a Caustic Soda or Potash

586.    To Estimate the Percentage of Caustic Alkali in a Caustic Soda or Potash. Powder 100 grains of the alkali to be tested; put it into a flask containing an ounce of 95° alcohol, and shake thoroughly ; the alcohol dissolves the caustic alkali perfectly, but will not take up any other ingredients. After standing for a few hours to settle, decant the clear liquid, and evaporate on a porcelain capsule until thoroughly dry; the weight of the dry residue will be the number of grains, i. e., the percentage, of caustic alkali in 100 grains of the soda or potash.