It’s not clear who first invented soap. There are documents suggesting that it was used by ancient Phoenicians over 5,000 years ago. Substances believed to be soaps have been found in ancient Egyptian ruins. It might have been invented independently in several regions at different times. An intriguing story about how the Romans learned to make soap involves the tradition of sacrificing animals on Mount Sapo. Parts of the sacrificed animals were burned as offerings to the gods. Fats from the burnt animal flesh mixed with ashes from the fires. When it rained, the Roman’s noticed that a substance formed in the pools of water that ran from the ashes that had been mixed with the animal fats. Upon experimentation, they learned that this new substance, later called soap, had useful properties, including the ability to clean surfaces. Chemists now refer to the chemical reaction for making soap as saponification, in honor of the discovery on Mount Sapo.
Soap is formed by mixing fats or oils with strong bases, such as sodium hydroxide. Sodium hydroxide is also called lye. The traditional way to make lye is to leach ashes with water. The ashes contain substantial amounts of sodium hydroxide, which dissolves in the water, forming a solution of sodium hydroxide. Before soap became available from large companies, people made their own by mixing animal fats with lye in a pot and boiling it. You could tell when it was “done” by taking a small amount of the mixture and adding it to some clean water. If there were droplets of fat on the surface of the water, the reaction was incomplete. More lye was added and the reaction continued. It was later discovered that the soap could be purified by adding salt to it. The addition of salt caused the soap to form a solid that excluded impurities, such as the sodium hydroxide. This soap was milder and suitable not just for washing clothes or pots, but also for use on skin. The figure shows the chemical reaction that is the basis for soap synthesis.
Figure 1. Saponification of a fat molecule. The bonds that connect the long chains of the fat molecule to the “backbone” are broken by the reaction of sodium hydroxide (and heat), yielding glycerol and three fatty-acid molecules (soap). The “acid” part of the fatty acid is the side with the oxygen (O) atoms. This end mixes well with water. The fatty part is the long chain of carbons, shown here as the crooked lines. This end mixes well with fats and oils. In the second step, the fatty acids are converted into relatively pure fatty-acid salts by the addition of sodium chloride.
Now to explain, chemically, how soap works to clean things. Fats mixed with strong bases are hydrolyzed into fatty acids. Fatty acids have the very useful property of having one end that mixes well with water (it is hydrophilic, or “water-loving”) and another end that mixes well with oils and fats (it is hydrophobic, or “water-hating”). The part that mixes well with water is the “acid” part. The part that mixes well with fats is the “fatty” part. This dual nature allows soaps to dissolve fat, grease, and dirt in water. Without soap, oil and water don’t mix. With soap, they do.
Making Soap: A Basic Chemical Reaction
Soap is the result of a basic chemical reaction between fats or oils and lye. The process of achieving the chemical reaction is called saponification
. By carefully choosing a combination of quality oils, adding your favorite fragrance or essential oils
, and swirling in a lively colorant, your handmade soap suddenly takes on a charming, rustic character.
There are two basic methods for making soap at home.
- Melt and pour: This easy process involves melting pre-made blocks of soap and adding your own fragrance.
- Cold process: The cold process is the most common method of making soap from scratch using oils and lye.
Each method has pros, cons, and variations. Review the two most popular methods to select your method.
To get started with melt and pour soap making, you’ll need a few tools after you purchase a soap base.
- A microwave or double boiler
- A heat-resistant bowl for the microwave
- Measuring spoons and whisks
- Fragrance, color, or additives, as desired
- A mold
The most popular soap bases are white or clear glycerin. For a more luxurious soap, try a base made with goat’s milk, olive oil, or Shea butter. You’ll cut the soap base up into chunks to help it melt faster. If you use a microwave to melt the chunks, put the base in a microwave-safe bowl and stir at 30-second intervals until the chunks are liquid and smooth. Or melt in a double boiler over low heat, stirring until liquid and smooth. Then, allow the base to cool to 120 degrees Fahrenheit, then stir in colorants, fragrances, and additives of your choice. Finally, pour the mixture into your soap mold, wait a day until the soap is hardened and dry, remove from the mold, and your creation is ready to use.
There are a few tricks to know about when making melt and pour soap. The melted base will be thin, which means additives may sink to the bottom unless you wait until the base cools a bit before adding in. Melt and pour soap cools and hardens quickly so you’ll have to learn to time it right when using additives. If the base is too hot, it can burn and become gloppy and tough to work into a mold.
Some additives work better than others in melt and pour soaps. Try sandalwood powder or dried calendula flower petals for best results. Many herbs tend to change color in the soap. Other additives include exfoliants, fruit seeds, and milk powders.
Cold Process Soap Making Method
The cold process method is a little more complicated and takes longer than melt and pour soap. It also involves using lye, which is a caustic substance. To make cold process soap, you’ll heat your choice of oils
in a soap pot until they reach approximately 100 degrees Fahrenheit. Then, you’ll slowly add a lye-water mixture
and blend the soap until it thickens to trace
. After the mixture reaches trace, add fragrance, color, and additives, then pour it into a mold. The raw soap takes about 24 hours to harden and a few weeks to cure before it’s ready to use.
To get started making cold process soap
, be prepared to need more equipment and clean-up
time than you would with melt and pour soap. Work where there is a heat source and access to water. There are several tools
you’ll want to have on hand for this method of soap making, but begin with the basics:
- Animal fats or vegetable oils
- A pitcher of lye-water
- A soap pot
- Fragrance or essential oil, as desired
- Natural or synthetic colorant, as desired
- A mold to pour the raw soap into
- Safety gear
You’ll need to have a cool, dry place where the soap can cure. Since this method of soap making includes the saponification process, you’re able to use fresh additives such as milk and fruit. Fresh additives can be included because of the high pH environment of the saponification process preserves the ingredients and prevents the formation of bacteria or mold. The texture of cold process soap is also thicker, which means you can use heavier additives that won’t sink to the bottom.
Take note that any vanilla ingredient might not be a reliable additive in cold process soap making because of the potential alcohol content, and it may turn your soap brown.
Safely Working With Lye
Lye is a caustic ingredient. When working with lye
, wear protective gear including eye goggles, gloves, long sleeves, and pants to fully cover any exposed skin from spillage.
- What are other oils, besides coconut, that are used to make bath soaps?
- Based on your research, why is potassium hydroxide used instead of sodium hydroxide to make certain kinds of soaps?
- Based on your research, what is a micelle?
- What is the chemical name of the most common fatty-acid molecule found in coconut oil?
- How does the “split-personality” of a soap molecule make it a good cleaning agent?
- What is a triglyceride?
- The procedure for this science project uses 3-molar (3M) sodium hydroxide. What does the term molar mean?
David B. Whyte, PhD, Science Buddies