Manufacture of soap The following row materials are used for the manufacture of soap.

i) Oils or fats:- coconut oil, mustard oil, mahua oil, cotton seed oil, tallow, lard, etc., are generally used.

ii) Caustic soda for hard soaps and potash for soft soaps is employed for saponification.

iii)Filter:-Certain substances, called filters, are usually added to increase the weight and volume of the soap. These substances also help in solidification. Sodium silicate, soap stone, rosin, starch, sodium carbonate, etc., are generally added to washing soaps.

The cold process:- the oil or fat is taken in a big vessel and heated to about 50degree C. A calculated quality of caustic soda is added with constant stirring. Some amount of filler like starch or sodium silicate is then mixed. The whole mass is left as such for 12 to 24 hours. It sets to a solid mass. It is cut into pieces of desired shape. The soap obtained by this process always consist glycerol, alkali and free oil or fat. It is used for washing purposes.

The hot process:- the process is superior to the cold process. The oil or fat is taken in a big steel tank called kettle and heated with open steam. The alkali solution is slowly run in and the mass is kept boiling with steam. After  several hours of continuous boiling the saponification is completed to about 80%. Sodium chloride is then added. The soap gets precipitated and being lighter floats to the surface as curdy mass. This is called salting out hof soap. The lower aqueous layer containing sodium chloride, glycerol and free alkali is drawn out. This is called spent lye and used for the recovery of glycerol. The soap left in the kettle still contains some unreacted fat. It is boiled with fresh supply of  NaOH solution. More soap is formed. The lower layer is drawn off. The soap layer is boiled with water to wash away excess of NaOH. The soap, while still molten, is pumped into the crutcher. Here the soap is mixed with colour, perfumes, germicides, etc., and stirred. In case of laundry soap some fillers like rosin, sodium silicate, sodium carbonate, etc. are cadded. These have some detergent values and are cheaper than soap.

The crulched soap is poured into frames and on solidification cut into cakes.

Modern Process:- In this process the oil or fat is hydrolysed with super heated steam under pressure in the presence of a catalyst like zinc oxide. The fatty acids produced as a result of hydrolysis float on water as these are lighter than water. These are separated and neutralized by calculated quantity of alkali solution when soap is obtained. This method is simple and cheap. The aqueous solution is treated for the recovery of glycerol.