What Is Saponification in Soap-Making?

We asked a chemist what's happening.

homemade soap and tool on table
Photo: Betsie Van der Meer / Getty Images

Soap-making is an enjoyable and easy creative hobby that anyone can try. And while there are several different methods for making soap (like melt-and-pour or cold process), it comes down to chemistry regardless of the technique. One of the chemical reactions that happen during the soap-making process is saponification, but what is it and why is it important?

"Saponification is an age-old chemical process where triglycerides (plant or animal fats and oils) are mixed with aqueous lye (sodium hydroxide, NaOH, or potassium hydroxide, KOH, dissolved into water) and then heated to reaction," explains Christopher Fenk, Ph.D., professor and chair of the department of chemistry and biochemistry at Kent State University. "The triglycerides are thereby converted into glycerol and sodium or potassium salts of long chain fatty acids (soaps)."

Lye in Saponification

In the cold process of making soap, different oils like shea butter or argan oil are combined with lye to produce solidified soap. Lye technically refers to sodium hydroxide, which is a very caustic chemical that produces dangerous fumes so it is important to wear protective gear and work in a ventilated room. The saponification generally takes about 24 to 48 hours to complete once the lye and oils have been mixed and the raw soap has been poured into the mold. Afterwards, it should be left to air-dry for approximately four to six weeks; this is known as the curing time and it will allow for any excess water to evaporate out of the soap.

Impact of Ingredients

"The key variables are the choice of fat or oil and the type of base used (NaOH or KOH). Vegetable oils are now often used, but, animal fats may be used as well. It is critical that the amount of base be kept to a minimum otherwise the soap produced can be very harmful to the skin," says Fenk. "The type of base (NaOH or KOH) determines the kind of soap. Sodium hydroxide (NaOH) is used to produce hard soaps like bar soap. Whereas, potassium hydroxide (KOH) is used to make liquid (or soft) soaps." He notes that artificial colors and fragrances are often added to soap simply to improve its appeal.

You can add a variety of ingredients to your soap including oatmeal, fruit and herbs, and flowers. The other ingredients that you put in the soap can provide additional benefits toward your skin care, though not necessarily with lathering and cleansing abilities. How well the soap lathers and cleanses also depends on the oils used for the soap and their reaction with the lye, so you can get a variety of soaps using different ingredients.

When you are making soap, you are testing—by trial and error—with chemistry. Soap is one of those crafts that also have a scientific component and can be great for experimenting with different ingredients. Just make sure to thoroughly research the ingredients that you use, the proportions of oil to lye, and that you also follow safety precautions because lye is a volatile substance. Try it, and you will have beautiful soaps to use in your bathroom, sell at craft fairs, or give to friends.

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