Soap is made by blending fats and oils with sodium hydroxide and water. While it can be infinitely more complicated than this the basic science is simple.
|Fats & oils + sodium hydroxide + water = soap + glycerin|
Fats and oils include anything from common oils like olive and coconut to rich butters like shea, animal fats such as lard or specialty oils like rosehip or raspberry seed. Each different oil brings different properties to the soap, some good, some not as desirable, so a great soap recipe incorporates a range of different oils carefully balanced to promote the good and minimise any negatives.
Sodium hydroxide (also known as lye or NaOH) is an industrial chemical. Strongly alkaline and capable of causing serious burns if it contacts skin sodium hydroxide is essential to soapmaking. Sodium hydroxide is created by electrolysis of salt water. It does not exist in nature as it is very reactive and will quickly turn into sodium carbonate in the presence of air. You can sometimes see sodium carbonate (also called soda ash) on the surface of some handmade soap.
Water is needed to dissolve the sodium hydroxide. This water can be as simple as plain distilled water or may be milk, beer, tea or a hydrosol. If it can dissolve lye it can be used. Some water alternatives contribute interesting science of their own. Lactic acid from milk or dairy converts to sodium lactate which is used as a bar hardener. Acetic acid from vinegar converts to sodium acetate (another bar hardener) and citric acid from citrus fruits creates sodium citrate, a chelator which helps bind the minerals that cause soap scum! With cold process soap any ingredient used as a water substitute remains in the final bar. With industrial soap there is no benefit to expensive additives here as all water soluble ingredients are washed away in the glycerin separating process.
Mixing: When the oils and sodium hydroxide + water are combined sodium hydroxide splits up the triglycerides, turning the fatty acids into soap and freeing up glycerin. In a properly calculated and weighed recipe the sodium hydroxide is all used up in the soapmaking process. For cold process soap mixing is done at low temperatures with high shear mixers. The goal here is not to complete saponification but to properly emulsify the ingredients as saponification will complete all by itself over time. With industrially made soap the ingredients are heated to boiling and stirred until the soap is fully made.
About superfatting: In industrial soap manufacture there is a small excess of sodium hydroxide to ensure there are no unreacted oils remaining in the soap as they can reduce shelf life. For handmade soap we use an excess of oils instead as long shelf life is less important than making good safe soap. These excess oils (known as superfats) are a great skin care ingredient.
Finishing up: For handmade cold process soap the resulting soap is coloured, fragranced and poured into moulds to finish saponifying and set up hard. In the next day or so it will be cut and then left to cure for 4-8 weeks to lose excess water and become more mild. For industrial soap its more complicated. The resulting soap is mixed with salt water to separate the pure soap from the glycerin and any extra sodium hydroxide and other contaminants. The soap curd is washed, dried, milled and compressed into noodles and the leftover salty water is refined to extract the glycerin for use in lotions and other skin care. The soap noodles are then either coloured, fragranced and formed into bars or sold on to other soap finishing plants around the world.