Saponification: The Science Behind Soap-Making

woman making soap — A woman pours the soap mixture into molds to harden into bar soap. RUSS ROHDE/Getty Images

Great-grandma Betty didn't have a drugstore on every corner, so she typically used simple ingredients to make her own soap from scratch. Old-fashioned recipes typically involve using lard or pork fat. Today, even though you can easily buy commercial soap, many people like to buy handmade soap as it smells better and may be gentler on the skin. You might even want to try making soap yourself. Modern recipes have evolved to include supplies most of us have lying around the house. Still, the process hasn't changed all that much from centuries past and is pretty fun. First, let's find out how the ingredients in a typical bar of soap come together to create the product. For centuries, the process was actually not understood.

Saponification: The Chemistry of Soap-Making

Soap is made by mixing fats and oils with an alkaline solution (i.e. a base mixed with water). Fats may include ingredients like lard or shortening. Oils could be cooking oils like olive, peanut, corn or coconut. The base could be something like lye (sodium hydroxide) which is readily available at hardware stores. Since water and oil don't mix naturally, the ingredients must be heated and stirred to come together.

When the fats come into contact with the lye, a chemical reaction called saponification takes place. This occurs when the triglycerides (fat) and lye are combined and react to form fatty acid metal salts (the soap) and a soap byproduct (glycerol). The base solution forces the soap to coagulate without dissolving in the water. Once this happens, the concoction can cool and then harden to form soap. The soap's appearance and performance (lather, aroma, moisturizing capabilities and so on) are affected by how well the unsaturated and saturated fatty acids are distributed.

Soap cleans by acting as an agent between water and dirt. It soaks into the surface being cleaned, allowing water to wash away the dirt. Lye soap is popular for two major reasons: It's simple and it works. The soap will be creamy, oily and gentle on your skin. And the best part is you can make it right at home.

Just follow these instructions, adapted from a recipe in "The Soap Book." Be sure to wear safety gear, such as goggles and gloves.

3-Oil Lye Soap Recipe

Ingredients

*Makes a 2-pound (1-kg) batch.

3 ounces lye
7 ounces purified or distilled cold water
6 ounces olive oil
9.4 ounces Crisco vegetable shortening
6 ounces coconut oil
1 ounce fragrance or essential oil

Weigh all ingredients using a scale. Be sure to zero out the scale. To do so, put the container(s) you're going to pour the ingredients in on the scale while empty. Then press "tare." Then add ingredients.

The Step-by-step

Put on your safety gear. Make sure there are no children around, because of the fumes that could be generated when making this.
Measure the cold water in a large container. Then add the lye to the water (never do this the other way around). Mix them slowly, carefully and thoroughly. The mixture will start to heat up or even steam. Make sure the room you are working in is well ventilated, because the mixture becomes very hot and can emit fumes. Set the solution aside to cool.
Measure, then place the olive oil, coconut oil and Crisco in a pot and heat it over a medium low flame until completely melted.
Once the oil mixture and the lye mixture are both between 100 and 110 degrees F (37 and 43 C), turn the heat off and slowly add the lye and water solution to the oil mixture. Mix for five minutes, alternating between a wooden spoon and a spatula, until the mixture begins to harden. You can also use a stick blender and alternate stirring with blasts. Make sure that nothing splashes on you.
Mix until it reaches a light trace, which means that no oil streaks are visible. The mixture should have the consistency of cake batter.
Using a whisk, stir in desired fragrance or colorants you want to use.
Pour the mixture into soap molds. Let them harden 24 to 36 hours. They need to harden enough to cut before removal.
Turn the bars out of the molds, slice into bars and wrap them in cellophane. Allow to cure at least two to four weeks before use.