Let's make hand soap

Soap is a salt of fatty acids. Okay, but what does that really mean? To give these words meaning, we have to bring up some chemistry. Soap is the result of a chemical reaction called saponification. It happens when animal fats or vegetable oils are mixed with a strong base (sodium hydroxide or potassium hydroxide). The type of alkali determines the consistency of the products – sodium soaps are solid and potassium soaps are liquid or soft. When our strong base reacts with the triglycerides (the fats), it breaks the bonds in the triglycerides between the fatty acids and the glycerin. The fatty acids acquire sodium, becoming salt. So, from this chemical reaction, you get glycerin, which must be removed, and soap (salt of the fatty acid).

To get saponification in the kettle process, you have to place animal fats or vegetable oils in a kettle and add gradually the alkali metal. From a perforated coil, steam jets are used to heat the mixture and encourage the chemical reaction to produce soap and glycerin. Treating the soap with hot brine (very salty water) causes an upper layer to form with soap curd and a bottom layer where glycerin settles. Glycerin is then pumped out of the kettle and treated for other uses. To remove impurities, you have to wash the soap left in the kettle and then boil it with water (pitching step). This creates again two layers: a top layer called net soap (70% soap and 30% water), and a bottom layer called “nigre” which are impurities.

The difference between a soap bar and liquid hand soap lies in the alkali. To make the soap softer, you must use potassium hydroxide. However, to reach the liquid state you have to add water, which dilutes the concentration. Mind that water increases the chances of germs and bacteria growth, so disinfecting compounds and fungicidal solutions are added to your soap.